Wednesday, 22 of May of 2013

## Effects of Input Impedance on Potentiometer Output

This article will demonstrate the effects of the input impedance of you amplifier or controller will effect the linearity of a potentiometer.

As illustrated in the image left (click image to see full size) – The resistor in parallel with the volt meter represents the value of the input impedance for the amplifier device.

I have simulated other hypothetical amplifiers with the same connections, but with higher values.

Normally you would expect 5V to be shown at 50% of the travel. But because output of the potentiometer sees a load, you will get a slightly skewed result. (As shown by the different values on the volt meters.) I have simulated several hypothetical situations where the input impedance is different. As you can see the result shows that the higher the input impedance is the less linearity error you will have in your output. Below are the results of the lowest and highest impedance values used in the experiment.

Here is a table with the results for a 30Kohm Input Impedance Amplifier

 Displacement Expected Real Error 0 10 10 0.000% 10 9 8.841 1.767% 20 8 7.752 3.100% 30 7 6.718 4.029% 40 6 5.725 4.583% 50 5 4.762 4.760% 60 4 3.817 4.575% 70 3 2.879 4.033% 80 2 1.938 3.100% 90 1 0.982 1.800% 100 0 0 0.000%

Now here is the same circuit but with a 1Mohm (1000Kohm) Input impedance.

 Displacement Expected Real Error 0 10 10 0.000% 10 9 8.995 0.056% 20 8 7.992 0.100% 30 7 6.991 0.129% 40 6 5.991 0.150% 50 5 4.992 0.160% 60 4 3.994 0.150% 70 3 2.996 0.133% 80 2 1.998 0.100% 90 1 0.9995 0.050% 100 0 0 0.000%

As you can see the error is much smaller than the one of the lower impedance. IF the input impedance cannot be raised there are several things you can do to solve the problem.

If the potentiometer has a center tap, (a terminal that is connected at 50% of the stroke), you can send a reference centre voltage to that point which is 1/2 the value of the applied voltage. This is generally implemented into joystick controls, where the floating of the centre voltage would be detrimental to the opeartion of the joystick. More information on this topic can be found HERE.

The other alternative is to add a buffer amplifier in between the joystick and the controller, we have been busy creating various buffer boards to solve these problems for our customer. We have links to these products on our products page. HERE

## High Current Switched Multi-Axis Joystick Controller (5A)

### Product Part Number: CDG-GEN-0220

Product Description: JC6000 Multi Axis Joystick with switched function.

The joystick is a 4 way joystick, (NSEW) the contacts activate at approximately 5°from centre. Multiple contacts can be engaged at the same time (NW, SW, NE, SE). The joystick allows for custom handles to be fitted.It can be used to control solenoid coils directly, providing that the current rating is within the switching capability of the joystick. Considering that higher current will reduce the life of the switches, we have made the switches a replaceable item, available ex-stock from Control Devices or you can order them from Farnell.

 ELECTRICAL Dielectric Strength 2500V Insulation Resistance 10 M Ohm Contact Rating 5@30V DC Max Current Type OMRON D2VW5 (Available as spare part) N/C N/O Number of Analogue Input/Outputs 4 (NSEW) Life 100,000 operations @5A, 1,000,000 @0.1A

 MECHANICAL Dimension 43mm x 28mm x 92mm Life 15 million operations ENVIRONMENTAL Shock Destruction: 1,000 m/s2 {approx. 100G} max. Malfunction: 300 m/s2 {approx. 30G} max. Vibration Malfunction: 10 to 55 Hz, 1.5-mm double amplitude Ingress Protection IEC IP67 (No Function Grip)* Operating Temperature Range -40 + 85 C Weight Approx 700 g EMC AS/NZS CISPR 22: 2009 CLASS B

*IP Rating is determined by the control grip used on top of the joystick, various models exist from IP67 to IP50

## 4-20mA Joystick Buffer Board

### Product Part Number: CDG-GEN-0181

Product Description: Electronic buffer board with 4-20mA current output.

This device is designed to regulate the power supply to the buffer any Penny and Giles joystick. It has a regulated 5V supply to the joystick, has an additional 2.5V reference voltage for center tap stabilization. Its wiper current draw will be in a magnitude of nano-amps offering very high input impedance. The device has over current, polarity and over voltage protection. The device is provided with a 16 Way harness which allows easy interconnection between the byffer board, joystick and the rest of the system.

 ELECTRICAL Dielectric Strength 2500V Insulation Resistance 10 M Ohm Supply Voltage +30V DC Current Consumption 40mA Excitation Voltage to Joystick +5V DC Input Impedance of Input from Joystick >1M Ohm Over Current Protection Yes – Self Recovery Fuse Over Voltage Protection Yes – TVS Diode Polarity protection Yes  – Diode Output Type Current Number of Analogue Inputs/Outputs 2 Output Range 4-20mA (Factory set – others available on request) Life 1000 Hours

 MECHANICAL Dimension 30mm x 15mm x 80mm Life 1000 Hours

 ENVIRONMENTAL Shock TBA Vibration TBA Ingress Protection IP50 Operating Temperature Range -40 + 85 C Weight 100 g EMC TBA

## Joystick Protection Buffer Boards.

Our buffer board project is now reach field testing – we have prvided several customers with 4-20mA, 0-10V boards to see how well they will work in the field.

We have redesigned the package that the board sits in – making it more aesthetic. for more information please see the previous article here. ( http://controldeviceseng.com/2010/07/22/buffer-pcb-to-ensure-low-current-draw-and-protect-joystick-controller/ )

## Mini Thumb Joystick

He have recently commenced work on a miniature joystick designed for auxiliary control on standard control grips. The device is a simple potentiometer joystick with a similar feel to that of a standard gaming joystick. We have created a couple of packages which show the body size as well as the connector attached to the base.

We should have a working prototype in a few weeks, and we are currently seeking a local company that would be willing to perform field tests.

We are also taking into consideration the Ingress Protection for the device. It appears that we should be able to achieve at least IP65 above the panel if not or perhaps even diving depth above the panel with the appropriate rubber gaiter.

The current size of the device is 33mm (Dia) x 35mm full length.  16mm above the panel.

## RS485 Field Test Version Completed

After a few problems with the PCB we have finally completed the first working prototype of the RS485 communications board PCB. The final package has been assembled and is ready for shipping for field testing by our customer. I have provided some photos of the finished product. A technical drawing of the functions and mechanical design is available for download here. DW-0205-CDG-GEN-0143

## More Progress on the RS485 Board

Case prototyping was finished today – next stage is painting the device. Picture of painted prototype to follow soon.

## UPDATE – Industrial Joysticks with RS485 and (Eventually) USB

We have made significant progress with the RS485 communications board project. Currenty the PCB is being manufactured and all components for the first electronic assemblies have been ordered.

We anticipate the delivery of all items later this week for assembly and bench testing. We have also completed the mechanical assembly drawings of the device and it’s enclosure. We will be making the first prototype of the enclosure early tomorrow.

Some extra functions have been added to the board.

• 5V excitation voltage for energising the joystick or sensor.
• Error detection with LED indication
• RS422 connection

## JC6000 Micro-switched Joystick Update (New Test Rig)

Continues on for the original article which can be found HERE.

Today we have completed a rough desing and model of a test rig which we will use to test the selected micro switches for the new JC6000 Micro-switched prototype.

We will be testing the device under load and log the information electronically. We will be testing contact resistance during the test. This should allow us to plot the changes in the device and determine it’s actual useful life.

The tests will be performed at 4 Hz  switching cycle.  By my calculations we should be able to test well past a million cycles within 3 days.

## Industrial Joysticks with RS485 and (Eventually) USB

We have recently commenced work on a communications board project that which accommodate an analogue and digital joystick signal and transmit this on a RS485 bus.

RS485 Prototype PCB Layout

Essentially the devices will be designed to function with the JC6000 joystick, hence the board will accomodate all of the possible digital and analogue functions that the joystick can provide. Essentially the board will be able to receive signals from the joystick as well as provide excitation power to the joystick device, eventually allowing it to be used with more than just the JC6000 series joystick. Considering that the JC6000 can provide many function we have anticipated this and are providing the following inputs.

• 6 Analogue Inputs
• 20 Digital Inputs
• 5 x 5 Voltage supply
• 5 Analogue GND Terminals
• 4 Digital GND Terminals

We plan to house the PCB in a housing providing screw terminals for the external power and RS485 signal. the other side of the PBC will be soldered terminated directly to the harness which will connect to the joystick.

The Microprocessor we are using is capable of many different function. The device is capable of communications standards such as…

• SSI
• CAN
• RS485
• RS232
• RS422
• USB

The plan is to complete the RS485 communications protocol first and then move on to completing the USB output option. Currently we are finishing the PCB design layout for the first RS485 board we hope to have the first version assembled and working within a couple of weeks.

Mark (Maciej) Dzidowski
Engineering Manager
Control Devices Engineering
Control Devices Group
Australia – New Zealand – Singapore – Thailand – China
02 9368 7100
0408 011 606
www.controldevices.net
mark@controldevices.net

## JC6000 Industrial Joystick for High Current Switching

For quite some time now, we have been asked for a basic, switch output, robust industrial joystick. Although the Penny and Giles JC6000 does already have a switched option, unfortunately it cannot be used in all application and the joystick does come with proportional output as standard.

So why cant we use this device on it’s own. Well the maximum rating is only 2A, however customers have asked for a device that can handle up to 5A with peaks as high as 15A. Also if you do damage the directional switch there is no cost effective repair path, since the existing switches are connected to a PCB board which is permanently fixed to the joystick base. We also have the proportional output which these particular customers did not need, essentially a function that is not used but paid for.

The JC6000 has an excellent mechanical gimble mechanism and is designed to last over 15 million operations. This is why we are currently in development for a modification to the joystick. We plan to utilise the mechanical strength of the gimble, and finding a solution for a high electrical life in high current applications.

JC6000 High Current Micro-switched Prototype Concept

The biggest hurdle we face is finding a switch that has a high enough operational life in a high current scenario. We are looking at a peak current of 15A, extensive research has shown that when a switch is available to handle the high current, the electrical operational life is 10,000 operations at best. Naturally lower currents yield longer life spans, some in the order of 100,000. I have found one that claimed a 2,000,000 operational like, but somehow I am sceptical of this specification, but we will do an independent test to verify this.

So what’s the solution? Well for now, the best solution would be to make the switches very easy and very inexpensive to replace. We are planning to release the replacement switches as a spare part in two configurations.

1. The switch as a stand alone replacement
2. The switch block with 2 or 4 switches attached (Depending on which configuration is required)

These switches will be field replaceable, no specialised knowledge or tooling will be required to change these devices out. We are also considering extending the cam mechanism in order to provide multiple outputs per direction, meaning more then one contact and possibly activating in different positions.

Mark (Maciej) Dzidowski
Engineering Manager
Control Devices Engineering
Control Devices Group
Australia – New Zealand – Singapore – Thailand - China
02 9368 7100
0408 011 606
www.controldevices.net
mark@controldevices.net

## JC6000 Industrial Joystick Upgrades IP66 to possibly IP67

This article discussed a new project undertake by Control Devices Engineering for a solution for extreme dust and moisture. The scope of work is to taje a standard JC6000 Base and improve it’s sealing in order to cope worth the environments mentioned earlier.

JC6000-Sealed Base Prototype Close Up

There are many misconceptions as to the IP rating of a device.

“The IP Code (or International Protection Rating, sometimes also interpreted as Ingress Protection Rating) consists of the letters IPfollowed by two digits and an optional letter. As defined in international standard IEC 60529, it classifies the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in electrical enclosures. The standard aims to provide users more detailed information than vague marketing terms such as waterproof.” WIKIPEDIA

We find that people generally request a rating far beyond what they actually require. Hence spend their budget on devices that are over rated in relation to their actual application. When considering a device such as a joystick, you must first consider the environment the device will operate in. Things you would want to consider are the following.

• Temperature
• Humidity
• Exposure to the Sun
• Proximity to the Sea
• Dust
• Moisture
• Indoor or Outdoor Operation
• Operator
• Electrical Noise
• Shock and Vibration

The IP rating will address the following of the list above

• Moisure
• Dust
• Indoor or Outdoor (Partially)

There is a full and comprehensive list of the IP rating codes available on this site (IP CODE) From this list you could probably ascertain the correct code for your application. There will be times that the device that meets you electrical criteria may not meet your environmental criteria. We have recently embarked on an improvement project for the Penny and Giles JC6000 industrial joystick. The device is an excellent device, it have a long life capability, and is very reasonably priced.

JC6000 Prototype with sealed IP base

The device is already rated to IP65, which should be sufficient for most applications, however this rating only applies to the part of the device that is designed to be above the panel. Unfortunately leaving the base exposed to the elements below the panel. Normally this is not a problem, however we have come across applications where high humidity situations and extreme dust situations occur. Applications such as a timber drying kiln or a cement manufacturing plant, respectively.

The amounts of stem vapour in the timber mill is so high that you cannot see a foot in front of your nose, and the dust inside a cement facility gets into all sorts of gaps and crevices.

Faced with such a tough environment we made the decision to improve the base enclose for the JC6000. We essentially extended the base, and added some IP68 Glands to lock around the 16, 12 and 8 way harnesses, that still plug into the base of the joystick.  We have provided some images as to the progress of the project.

We are now in the testing stage of our design, results soon to follow.

## Buffer PCB – to ensure low current draw and protect joystick controller.

News – We have commenced a new project which will provide our customers with electrical buffer boards which will protect the joystick from over current on the proportional output of the joystick. We are also working on buffering the directional switches.

Buffer Board for Joystick Harness with Loom Attached

We are considering a relay board that will isolate the relatively high currents required to drive solenoids. For now we are working on a 4-20mA and a voltage output module which can take various voltages and buffers them before the signal returns to the amplifier. We have created the first space prototype and we will be building the first working PCB which had a 4-20mA output.

## Using 2 Joystick to control 1 Valve with proportional flow control. Master-Slave Setup

Introduction

In Early 2009 Control Devices Engineering was approached by Bosch Rexroth in Queensland to provide a custom solution for an application that involved two single axis joysticks which would control a single Bosch Rexroth RA2 Amplifier. The amplifier was to control a canting keel on a racing yacht, the keel was designed by Simon Flitcroft of DDC (Design Drafting Consultancy) in Queensland Australia. (Video of Sanity Clause in Action)

We have been lucky enough to receive great support from all the parties involved, and we do highly recommend that you contact these parties in regards to the relevant sections of this project. They are all an excellent source of information, and provide quality product and service to all of their customers.

I rarely give personal endorsements. I would only put forward people who are excel at their work and provide nothing less than exemplary service. My working relationship with the parties mentioned in this article has always been a pleasant and productive one. (Author – Mark Dzidowski)

Bosch Rexroth Proportional Valves and Amplifiers

The majority of the hydraulic control system was provided by Bosch Rexroth in Queensland Australia, the contact for this project section is Mr. Geoff Kirton, his details are provided at the end of this section of the article as well as at the end in the sources section.

Mr. Curtin has been kind enough to provide us with insight into the hydraulic system and it’s component parts.  The information provides an overview of the product and technology provided by Bosch Rexroth. The following are extracts of information provided by Bosch Rexroth for this article.

Proportional Valve Technology

Acting as the linking element between switching and closed-loop technology, proportional valve technology has today become an established component part of hydraulic systems. Industry has been quick to implement the advantages offered by this technology.

How Does a Proportional Hydraulic System Work?

Please see the the diagram labelled  Proportional “Valve Technology” in reference to the points below. Please click on the image to enlarge.

• An electrical input signal in the form of a voltage (mostly between 0 and ±9 V) is converted into an electrical current in an electronic amplifier corresponding to the voltage level, e.g. 1mV = 1mA.
• Proportionally to this electrical current as the input variable, the proportional solenoid produces the output variable – force or travel
• These variables, i.e. force or travel, acting as the input signal for the hydraulic valve, signify proportionally a certain flow or pressure.
• For the actuator and therefore also for the working element of the machine this means, in addition to variable direction, infinitely variable control of speed and force.
• Simultaneously, acceleration or deceleration can be infinitely varied, e.g change in flow with respect to time.

Proportional Valve Technology

Proportional valves and pumps with their proportional solenoids provide perfect interface for electronic control, thereby facilitating increased flexibility in the operating cycles of production machines as well as freely programmable control systems and drives.

The technical benefits of proportional devices can primarily be found in the controlled transfer during valve change-over, i.e. the infinitely variable control of command signals and the reduction of hydraulic equipment requirements for certain control applications. This therefore also represents an effective contribution to reducing material requirements in hydraulic circuits.

Proportional directional valve, type 4 WRZ, electronic controls type VT3000

Proportional valves permit faster, simpler, and more precise movement cycles while at the same time improving the reversal process. As a result of controlled spool cross-over, pressure peaks are avoided- resulting in a longer service life of the mechanical and hydraulic components.

The fact that the signals for direction and flow or hydraulic pressure are provided by electrical means has made it possible to arrange the proportional devices directly on the loads, thereby greatly improving the dynamic characteristics of the hydraulic control system.

Proportional devices in hydraulic systems found mote widespread use when effective devices of simplified design were offered on the hydraulics market. These devices do not greatly differ from those of the standard hydraulic range. It has also been possible to adopt a great number of parts or assemblies from the standard hydraulic range of equipment.

Proportional Solenoids

Force-controlled proportional solenoid

Proportional solenoids represent the linking element between electronics and hydraulics. The proportional solenoids are a form of DC linear solenoids. Proportional to the electrical current as the input variable, they produce force and travel as the output variable.

Corresponding to the practical application, a differentiation is made between:

• Solenoids with comparatively linear stroke/current relationship over a reasonably long stroke length, the so-called “stroke-controlled solenoids” and
• Solenoids with particularly defined force/current relationship over a very short stroke, the so-called “force-controlled solenoids”

Only DC linear solenoids can be used for the current-proportional change in the output variables force and stroke. Due to their stroke-dependent current consumption, AC solenoids must assume their final stroke position as soon as possible.

The solenoid force is controlled by the change in current I in the force-controlled solenoid without the armature of the solenoid performing a measurable stroke.  Due to current feedback in the electrical amplifier, the solenoid current and therefore the solenoid force are kept constant even if the resistance changes.

• The main feature of the force-controlled proportional solenoids is the characteristic force-stroke curve.
• The solenoid force remains constant over a defined stroke range at constant current.
• The stroke for the solenoid shown in this example is approx. 1.5 mm. The solenoid is used in this range.

Characteristic force-stroke curve of force controlled solenoid

The force-controlled solenoid is of compact design due to the short-stroke. In view of this short stroke, the force-controlled solenoid is used particularly for pilot-operated proportional directional and pressure control valves with the solenoid force being converted into hydraulic force. The proportional solenoid is a controllable “wet pin” DC linear solenoid contained in an oil bath.

The information above was provided by:

Geoff Kirton
General Manager QLD
Hydraulics

Bosch Rexroth Pty Ltd ”The Drive & Control Company”
Acacia Ridge (BRISBANE) QLD 4110

Phone : +61 (0)7 3272 3555
Mobile : +61 0408 734 947
Fax : +61 (0)7 3272 3999
email : geoff.kirton@boschrexroth.com.au

Web : www.boschrexroth.com.au

The products supplied for this project by Bosch Rexroth are:

We tend to use the RA2 amplifier where possible due to its competitive price and its compatibility with the full range of Rexroth proportional directional, pressure and flow control valves, as well as our range of electronically controlled variable displacement pumps. (Geoff Kirton – Bosch Rexroth)

Sanity Clause

Canting Keel Design and Setup

The design required for a joystick to be mounted on either side of the yacht so that during the sailing event, access to the joystick was easily reachable. Both joysticks performed the same function, which was to swing the keel between port and starboard. We have added an extra function which would assign a master and slave arrangement, whereby the master signal took precedence over the the slave signal, ensuring that only one joystick would take command at one time.

The original concept was created as a hand etched PCB which used relays to determine the mater slave arrangement. We borrowed an RA2 amplifier from Bosch Rexroth in order to bench test the design.

We originally intended to power the system from from the RA2 amplifier 8V excitation voltage, however the output was not interned to power relay coils and it’s maximum current capability on the control side was not sufficient enough to power the unit. We had decided to add a linear voltage regulator and draw the current from the main 24V supply.

Hand Etched PCB prototype for Master/Slave command module.

We have also discovered that the current capabilities of the 8V excitation was not sufficient enough to keep good linearity of the control signal back to the amplifier. Essentially we were asking too much of the excitation voltage supply, so we decided to provide our own excitation.

Naturally we had a temperature problem because of the linear voltage regulator was stepping down from 24V to 8V to supply the control voltage for the relays and the excitation voltage to the joysticks. To solve the problem we provided a heat-sink on the prototype PCB to cope with the heat of the regulator.

We were also asked to provide a function to control a relay that provided power to the hydraulic pump. The function was to turn on whenever the joystick was to be displaced. We used a simple transistor to provide a path to ground for the relay to achieve this function. The function was triggered by the same directional switches that control the master/slave functions.

The prototype coped well in the environment however we discovered that the RA2 amplifier shows an error if the excitation output is not connected to a load. Even though the output amplifier is working correctly, it still displayed a red LED error state. The solution was to tie the output to ground through a 5K resistor.

The final version of the board was encased in a watertight housing that incorporated the slight short falls of the original prototype. We have removed the linear power supply and replaced it with a switch-mode power supply thus eliminating the heat issue. We have also provided terminals for all the cabling in the system and placed the 5K resistor that is used to tie the RA2 output to eliminate the error state on the PCB and added a cable terminal.

Final Production Version of the Master/Slave Module

The joystick that was paired with this device is the Penny and Giles JC120. It was selected because it can be supplied with a rubber boot that covers the whole joystick and is suitable for outdoor marine application. We would like to also point out that the board will work with any of the Penny and Giles potentiometer joysticks, essentially is uses a voltage divider circuit and the directional switches within the joystick to control the PCB functionality.

An example of the wiring used to connect the RA2 amplifier, PCB and 2 joysticks is provided here (Connection Drawing)

Earlier this year we were approached by Robert Jordan  (R.J. & D.E. Jordan Ltd.) for a solution similar to the Sanity Clause project. Although the main function of the master/slave signal was required, additional functionality needed to be added to the system in order for it to work.

The system had one proportional flow control valve, a directional valve and a on/off valve. The system required that the joysticks controlled not just the proportional control RA2 amplifier but also the directional and  on/off valves. The directional valve was controlling the anchor winch and the on off valve controlled the pot winch. The joystick requirement was for one centre spring return joystick for the anchor winch control and a friction joystick for the pot winch.

Rough Design Flow Map

We were provided with a rough sketch of what was required, we have recreated the original drawing on a digital format for the readers viewing pleasure.

The solution which was originally provided to the customer as a prototype consisted of three boxes. One contained the Master/Slave board, the second contained a patch filed and wire distribution, and the third contained the mosfet control for the directional solenoid and the on/off solenoid. Considering the joystick only had a directional switch which worked from the centre position, we also needed to include a function that monitored the output of the friction, stay put joystick, and activated a switch signal that controlled the solenoid associated with the friction joystick at the beginning of the joystick stroke. In this scenario we were using the friction joystick in a manner that used a single direction joystick that worked from one end to the other. Essentially its a speed lever accross the whole stroke of the joystick instead of the centre being the neutral voltage. Prototype Drawing

The problem with the original prototype was the complicated wiring set up where we were using three different types of PCB’s in order to achieve the desired result. We also decided to remove the mosfets and replaced them with relays that can handle far more current than the mosfets could. We also increased the size of the PCB and enclosed it in a sealed box and included fuses and polarity protection in the design. The three boards were amalgamated into one PCB thus reducing the amount of wires which required termination.

Joystick Master Slave PCB with Control Relays

The final version of the board was installed on the vessel. It has now been in operation for a few months and is working without a problem. The final Assembly and Electrical drawings can be seen here. Click on the link provided to view the drawing (Final Drawing)

Recently we have had another request for a Master/Slave scenario, where by the system had a requirement for two friction stay put joysticks which were to control  one RA2 amplifier. However, we once again would have to revise the electronic design since we have discovered a problem where by the slave joystick, being a friction unit, could cause the following situation.

If, for some reason, the slave joystick is left in an active position and they begin to operate the master joystick, the master joystick takes command. This is ok, but when the master joystick is returned to the center position then the signal will return straight to the position that the slave joystick was left in.

We are currently working on a design that will use a logic that determines the desired signal by ensuring that the joystick is returned to the centre position after the use of it’s function is completed. The function will ensure that the joystick is returned to it’s neutral position before it will allow the use of the other joystick.

## Solving Drift in Proportional (Potentiometer) Joysticks

This article will discuss a common problem with proportional (Potentiometer) joysticks controls. These devices can be found in Industrial, Military, Marine and Gaming applications. We will describe the causes and effects of the different problems a user may experience and provide practical solutions to solve these issues. We are only addressing potentiometer type joysticks in this article, Inductive, Hall Effect, and Force joysticks will be discussed in further articles.

Joystick drift is a situation where it appears that a joystick is sending a command signal to the controller without the actuation of the joystick by the user. Essentially this manifests itself as the controlled device moving on its own; this can potentially be a dangerous situation, especially in heavy industry applications. We can classify the causes of “Drift” under the following categories.

• Supply Voltage Drift
• Wear on resistive track
• Electrical Noise
• Mechanical Wear

Supply Voltage Drift

Potentiometric joysticks rely on an excitation voltage; this can be a positive or negative voltage supply or both. There are four basic examples of how a joystick can be connected, there are other possibilities, however these are the most common wiring examples.

The first example is where one side of the resistive element is connected to the excitation voltage; the other end of the potentiometer is connected to ground or 0V reference.

Three wire joystick connection

The wiper is connected to the controller (represented by the resistor) which is then connected to the common 0V reference. In this example the center voltage is determined to be the neutral position of the joystick and is usually 50% of the applied voltage. In this scenario a 10V excitation voltage will have a neutral voltage of 5V.

The following example uses a positive and negative voltage source at either end of the resistive element of the potentiometer.

Three wire connection with ±ve voltage supply

The center voltage is derived as a sum of the two excitation voltages, in this case the sum of +10V and -10V which is 0V, the wiper is connected to controller, which determines the value of the load. As the wiper moves it either has a positive or negative voltage which is determined by the displacement direction of the wiper.

The above examples determine their neutral position voltage value from the sum of the applied voltages divided by two. Thus it can be determined that any variation in the excitation voltages will have a direct effect on the value of the center voltage as well as the output voltage of the wiper signal. Knowing this, it is now easy to see that if the supply voltage to the joystick is not stable. It is not possible to expect the center voltage to remain stable either, and when the center voltage moves the wiper will have a different voltage at the neutral position than expected. This new value can be determined to be a valid signal by the controller since the controller is expecting a predetermined value as neutral, so its response would be to move the machine.

Wear on resistive track

As seen in the examples above, we also have to consider the wear on the resistive potentiometer track. When the joystick is new, both the section of the resistive track between the center position and each side of the potentiometer are very close to equal value. And given that the supply voltages are stable you should expect not drift in the system.

But problems may arise when a joystick is used in an uneven manner, for example, pushed forward more than it is pushed pack. Since the wiper is moving more on frequently from center forwards then from center backwards, it is reasonable to expect that the resistive section in the forward position will be worn more than the backwards position. Wear in one direction can have an effect on the resistive value of the track. More wear can mean less actual resistive material; hence the value of that section would now be higher than the less worn section. And since the device is a voltage divider, naturally the resistive balance is no longer in equilibrium. In this scenario the center voltage will naturally shift off mechanical center (neutral position).

As a side note: The linearity of the system will also be affected.

Electrical Noise

The location of the joystick and its wiring loom can also manifest themselves as drift in your system. Poorly screened and grounded cabling, can allow interference to be induced into a control signal, giving erratic or skewed signal commands from the joystick. This type of interference generally happens when the controller is too close to electric motors or in applications where large electro-magnetic interference is present (Such as an aluminum smelter).

Electrical noise is not always constant; it can be an intermittent problem, especially on mobile machinery. You also have to consider whether the rest of your system has an electrical situation that can induce noise into your system. Defining and fault finding can be difficult and is beyond the scope of this article hence it is omitted.

Mechanical Wear

Mechanical components within an electro-mechanical device suck as a joystick will suffer from inevitable wear. Unfortunately there is not much we can do but design better mechanical systems that have better wear results. This is where most brands of joystick manufacturers will differ from one another. Good quality products will last longer simply because their mechanical interfaces are well built.

Care should be taken in selecting the appropriate joystick for the appropriate application. When selecting a joystick consider the following.

• Operator strength
• Mode of operation (Fingertip or Hand Control)
• Owner operator or company owned machine
• Number of daily operations.

Choosing the right joystick for the job is an essential step in system design. More information is provided in this article (Insert Article Here).

Solving drift problems

So far we have described common joystick drift problems. It is also our task to give examples of preventing these issues from appearing in your system. So let’s look at some solutions to the problems described above.

For supply voltage drifts ensure that you have a proper conditioned supply voltage, these are available as commercial products. But there is a better way to ensure that the “floating” voltage derived by the excitation voltages is far more stable.

The first step is to obtain a joystick which has a potentiometer with a center tap. A center tap is a terminal on the potentiometer that sits mid way between the two sides of the resistive element of the potentiometer. As it stands the center tap will be at the contact point of the wiper when the joystick is in neutral.

For the first example we have given we had an excitation voltage on one side of the potentiometer and 0V on the other end. In order for this system to have a solid center voltage you would need to add a voltage conditioner that would feed 50% of the applied voltage to the center tap.

Four wire Joystick with Ref. voltage and directional switch

This can be either a simple amplifier or a LM series voltage regulator. By applying the correct reference voltage you can ensure that the center voltage remains at the correct voltage value. For a  joystick that derives it’s centre voltage from a positive and negative voltage supply and expect a 0V centre voltage, It is best to tie the centre tap to the 0V reference as in the example shown across. Tying 0V to this point will ensure a stable centre voltage reference even though the supply voltage may vary. Your maximum and minimum voltage values at the end of the potentiometer might vary when your error state occurs. This can be identified as a “slow to respond” or “power loss” in your machine, but it will ensure that your neutral voltage remains the same.

Four wire joystick connected to ± supply and centre tap to 0V

Both of the example state above force the centre voltage to remain at the desired level, thus preventing the centre voltage from drifting outside of the desired value.

At this point it would be prudent to mention that solving the following problems is essential to address safe operation of machinery through an electro-mechanical joystick. In our experience we have seen many applications where the implementation of a joystick has been far from safe. In order to help with this issue we have provided some guide lines in another article to help you design a safe operating system. (Article Link Here)

The next step is to solve any possible drift due to wear on the resistive track; you can use the above solution on a slight variation which could be useful for differential amplifiers/controllers.

Four wire joystick with input monitoring amplifier

The following circuit will send 50% of the applied voltage, where the applied voltage is monitored but the amplifier and controller hence always looking for 50% of the applied voltage value.

The next problem we mentioned was electrical noise, this can be prevented by using the proper grounding and shielding, alternatively you can use a voltage to current convertor and change the signal to a 4-20mA signal for longer a nosier transmission.

The final problem is mechanical wear. There is no after-market solution for this problem. But if you have a joystick that has directional switches you can use then to route the output signal though the wiper. Usually they will activate at about 1.5 to 5 degrees. Care should be taken at this point since the wiper would already have been displaced a small amount, and would already be measuring some of the output prior to engaging the switch. So the output at the center voltage would step up from the center voltage to the voltage at the switch position instantly.

Four wire joystick with ref. voltage and directional switches

This could cause a “thump” in the system, but this may be unlikely if not undetectable for several reasons. The controller could have a soft start built into it, which acts like a buffer for erratic signals. The solenoid would require time to open or close the valve, thus physically smoothing the anomaly. Finally the fluid or air or drive in the system would require energy to build up creating another buffer.

We have a schematic circuit sample of a joystick with a mechanical drift problem, but if the mechanical wear causes the joystick to go past the point of the directional switch it would be time to replace the joystick.

Written by Mark Dzidowski for Control Devices
Sources:
Control Devices – Mark Dzidowski
(Engineering Manager)

## Popular Hydraulic Valves With Proprietary Control Amplifiers – Controlled by Custom Joystick

Introduction

This article will discuss a recent colaboration between Control Devices and Hydraquip in Tasmania Australia on such a project. This project included a popular valve accompanied by it’s proprietary amplifier. In this article we briefly explain the requirements of the amplifier and provide you with recommendations for the type of controller required to operate the device.

The Valve and Amplifier

This amplifier is specifically designed to work with the valves. Its basic concept is that the amplifier receives a voltage control signal from a variable voltage supply. The variable voltage is usually a potentiometer, however this is can also be another device that can output a limited voltage span on a 12V or 24V system.

The neutral position of the amplifier is 50% of the applied voltage, I.e. the 12V version = 6V and the 24V = 12V.  The amplifier also has a fault finding function built into the design, it works by monitoring the input voltage and only considering that 25% to 75% of the applied voltage is the true signal, any signal outside of this range triggers the amplifier into an error state. This is an excellent safely mechanism for the following reasons. Consider the voltage values on an open or short circuit. A short circuit is the full applied voltage and an open circuit is 0V. If the amplifier accepted the control signal from 0V to maximum applied voltage, the circuit would be susceptible to a critical error signal that could cause a dangerous situation.

If for some reason a signal wire would have been severed or the maximum applied voltage would reach the amplifier, then the valve would be either fully opened in the forward of backward direction respectively. And depending on what the valve is controlling, lets say a rotation of a boom crane, then you would have a situation where the operator is no longer in control of the machine. (And your crane is assuming a new life as an amusement ride.)

With the amplifier monitoring the output and only accepting 25% to 75% of the applied voltage, it has effectively solved the above scenario for short and open circuits. For all other errors states, a person present switch (Dead-Man Switch) can be used, or an Emergency Stop Button. (The final design for this project had both fail-safes)

Taking into consideration the above, it is prudent that a correct controlling device is selected for the amplifier. You need to ensure that the device matches the specifications and outputs the correct voltage span in order to work correctly with the device.

The first thing to consider is whether the controller can output the correct voltage span, 25% to 75% of the applied voltage, and that it can work with a 12V or 24V supply.

The third party joystick

At this stage I would like to introduce the range of Penny and Giles Joystick controls. We have used these devices to control the amplifier successfully in the field. The following models are recomended.

JC6000 Industrial Joysticks

Penny and Giles Fingertip Joysticks

The following joysticks have a 0%-100% voltage span, however Penny and Giles do also offer 25%-75% resistor padded devices. But for this project we were creating custom electronics that would deal with the required voltage span internally. A further note: All of the Penny and Giles potentiometer joystick require at least 1MOhm load on the input device. Why is this necessary? All resistors and especially potentiometers degrade over time, this is accelerated when a larger current is passed through the resistive track. Hence having a very large input impedance ensures the current through the potentiometer’s wiper  is very low.

The Project

Proportional control of a common hydraulic valve, accompanied by it’s proprietary amplifier. The customer was looking for a pendant joystick to control the hydraulic valve though the amplifier. The vavle was to control a hydraulic system that moved a heavy load along a track. Most of the operation is performed at the systems full speed. For this situation a standard switched function would have been sufficient. However the system required that the final fraction of the movement be at a far lower speed and with proportional control.

The summary of requirements.

• Proportional control of amplifier with HI/LO speed setting
• 2 push buttons that controlled the full forward and full reverse signals
• ON/OFF witch
• All items contained in a pendant control
• 5m control cable

The Solution

Considering the above, the Control Devices Engineering team has created a small conditioning buffer board, which has the following functions.

Custom Amplifier PCB First Version

Left pushbutton when pressed = 25% of the applied voltage (This is the minimum the amplifier will accept without an error state)Right pushbutton when pressed = 75% of the applied voltage (This is the maximum the amplifier will accept without an error state)

• Potentiometer full left is 42% of the applied voltage
• Potentiometer full right is 58% of the applied voltage

When the toggle is activated the range potentiometer range shifts to the following…

• Potentiometer full left is 33% of the applied voltage
• Potentiometer full right is 66% of the applied voltage

As far as the voltages versus the speed the following would be true

• Left Button = 100% reverse
• Right Button = 100% forward
• Potentiometer left with switch in slow state = 33% reverse
• Potentiometer right with switch in slow state = 33% forward
• Potentiometer left with switch in fast state = 66% reverse
• Potentiometer left with switch in fast state = 66% reverse

Custom Control Grip for Amplifier

The board develoment happened in 2 stages. Firstly Control Devices Engineering created a hand etched dual layer board with small adjustable potentiometers to prove the concept.

The board took several hours to manufacture but proved the concept viable. Further revisions of the design were circuit protection with recoverable fuses.

Special consideration was given to the neutral voltage level to ensure that potentiometer drift was minimised. Over time potentiometers will wear and create uneven resistance values along the resistive track, this usually will shift the centre voltage. This is the “floating” voltage in the middle of the potentiometer. This voltage is determined by the two voltage supply values at each end of the potentiometer and the resistance value of each half of the potentiometer track. Good quality potentiometer joysticks such as the Penny and Giles range, come with an additional centre tap on the potentiometer, allowing for an additional reference voltage to be provided at this point. If the reader has experienced “drift” in their system before, that revealed itself as a machine moving without an actual command from the operator, chances are that you might be experiencing the “float” problem described above. You can either implement the solution we have just mentioned or use directional switching to create a dead-band for you joystick. A dead-band is a mechanical area in which the joystick can move without it sending an actual command signal. More information on solving joystick drift will be provided in further articles. (Click here for the article.)

The Control Devices Engineering team determined that it is essential to supply 50% of the applied voltage to this centre tap in order to prevent “float” from the 50% value. This function was added the PCB.

The second revision of the board was made smaller in order for the board to fit within the control grip.

Custom Amplifier PCB Second Version

Considering that we only had to buffer the device, we no longer required the switch-mode power supply. We also determined the speed condition voltage values and permanently set those values with resistor networks.

We now moved on to testing the device for voltage output performance and ESD testing to 2.5kV. The final version was produced and installed in the control grip.

Custom Amplifier PCB Final Version

The device has now been in operation for several months and is working very well and the customer is very happy with the product. Final design and connection drawings can be downloaded at this link.  CDG-GEN-106 Custom Grip

Further Development

Soon after this product was completed we had considered other possibilities for the boards application and its uses. As it happens the board has a very small profile and we are currently working on a “Harness” Version. Essentially it will be a cable harness designed to work specifically with the amplifier, it will connect in-line with the cable so that the board becomes part of the cable assembly. This way the harness will not only connect the amplifier with the joystick but it will also act as a buffer and a signal conditioner. This way the amplifier receives the correct voltage signal and the joystick potentiometer wiper current kept very low. The image provided is a test version of the harness that plugs in line, between the amplifier and a potentiometer joystick. Planned revisions of the board will include improvements to the way the harness is presented and reinforcements to the cable connection. More information can be found in the article on the Buffer PCB Project.

Amplifier buffer PCB in custom harness