Fabricating a String Potentiometer

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Bespoke string potentiometer

String potentiometers are fast becoming the mainstay for those wanting to obtain accurate outputs when a lever is moved, such as the: ailerons, elevators, rudder, flaps, speedbrake, or thrust levers in the throttle quadrant.  This is because the slightest movement of a the string is registered by the potentiometer.

While commercial made string potentiometers can be purchased, they are not inexpensive, and if used will deliver an order of accuracy that isn’t necessary for the assigned tasks.

In the example discussed I I am not using a commercial string potentiometer.  Rather, a standard Bourne 3500-3501 rotary potentiometer has been adapted to use a string.

Requirements

You will need the following items:

  1. Bourne rotary potentiometer;

  2. Plastic housing box;

  3. 6 screws (to secure box lid and to fasten spool);

  4. One nut and washer (to secure potentiometer to box)

  5. A retractable spring-loaded spool and stainless steel ot nylon string;

  6. A cylindrical piece if moulded ABS plastic (or wood); and,

  7. A small dog leash type clip or other fastening device (not pictured).

It’s best to use a CNC machine to fabricate the correctly shaped box, however, any box can be used.  Boxes can be purchased from electronic shops that are used as a housing for interface cards.  These are suitable.

It’s difficult to document exactly how the process is done, but by carefully studying the pictures, you should be able to replicate the process.

Fabrication

  1. Make a small hole in the side of the box for the string.  Ensure the hole enables the string to have some lateral movement.  You may need to attach some type of protection to the inside or outside of the hole so that the string doesn’t rub the plastic.  I have used a soft piece of plastic for this task (Figure 1).  Equally suitable is piece of cork (wine bottle)

  2. Drill a circular hole in the top of the box to enable the shaft from the potentiometer to be inserted.  Secure the shaft with a nut and washer (Figure 2).  The main body of the potentiometer will be outside the box.

  3. Glue a piece of solid ABS plastic (or wood) to the lid of the box.  Make a small drill hole that enables a screw to be attached.  This screw is used to help secure the retractable spool (Figure 2, 4 & 6).

  4. You must fabricate, from a piece of ABS plastic or similar, a cylindrical attachment that is glued to the retractable spring-loaded spool.  This piece if plastic must a hole drilled that is the same circumference as the shard of the potentiometer (Figure 3).

  5. The retractable spring-loaded spool is glued to the bottom of the box in direct line with the shaft of the potentiometer.  The shaft must align with the hole in the spool (Figure 1).

  6. Drill a small hole into the side of the shaft of the potentiometer and the retractable spool.  The hole should be large enough to enable a small screw to secure the retractable spring-loaded spool to the shaft.  This is done to stop the spool from freely spooling.  When it’s secured, the string when pulled in or out will turn the shaft of the potentiometer (Figure 2, 3 & 6).

  7. When everything is complete, the string should move the shaft of the potentiometer as it is pulled out of the retractable spring-loaded spool.  To secure the spring to the control device, a small clip can be used which is attached to the end of the string.  I have used a small dog leash style clip, but any clip will work.

Photographs

The fabrication as seen in the gallery photographs appear quite rough.  This is because the example photographed was a prototype.  If you are carefully, work methodically, and have an eye for detail, then there is no reason why the end product will not look semi-professional.


String Potentiometers - Are They Worthwhile

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Custom-made box housing Bourns 3500-3501 rotary potentiometer.  Note cable, dog lead clip, and JR Servo connection wires

A flight simulator enables us to fly a virtual aircraft in an endless number of differing scenarios.  The accuracy of the flight controls, especially when the aircraft is flown manually (hand flown) comes down to how well the aircraft’s flight controls are calibrated, and what type of potentiometer is being used to enable each control surface to be calibrated.

This article will examine the most common potentiometers used.  It will also outline the advantages in using string potentiometers in contrast to inexpensive linear and rotary potentiometers.

What is a Potentiometer

A potentiometer (pot for short) is a small sized electronic component (variable resister) whose resistance can be adjusted manually, either by increasing or decreasing the amount of current flowing in a circuit.

The most important part of the potentiometer is the conductive/resistance layer that is attached (printed) on what is called the phenolic strip. This layer of material, often called a track, is usually made from carbon, but can be made from ceramic, conductive plastic, wire, or a composite material.  

The phenolic strip has two metal ends that connect with the three connectors on the potentiometer.  It’s these connectors that the wires from a control device are soldered to.  The strip has a wiper-style mechanism (called a slider) that slides along the surface of the track and connects with two of the potentiometer’s connectors. 

The strip enables the potentiometer to transport current into the circuit in accordance with the resistance as set by the position of the potentiometer on the phenolic strip. 

As the potentiometer moves from one position to another, the slider moves across the carbon layer printed to the phenolic strip.  The movement alters the current (electrical signal) which is read by the calibration software.

Inexpensive rotary potentiometer.  This pot previously controlled the calibration of the ailerons.  The pot was inserted into the base of the control column (removed for picture) and held in place by the fabricated bracket.  It worked, but accurate calibration was time consuming

Types of Potentiometers (linear, rotary and string)

Potentiometers are used in a number of industries including manufacturing, robotics, aerospace and medical.  Basically, a potentiometer is used whenever the movement of a part needs to be accurately calibrated. 

For the most part, flight simulators use adjustable type potentiometers which, broadly speaking, are either linear or rotational potentiometers.  Both do exactly the same thing, however, they are constructed differently.  Another type of rotary potentiometer is the string potentiometer.

A linear potentiometer (often called a slider) measures changes in variance along the track in a straight line (linear) as the potentiometer's slider moves either in a left or right direction.  A linear potentiometer is more suitable in areas where there is space available to install the potentiometer. 

A rotary potentiometer uses a rotary motion to move the slider around a track that is almost circular. Because the potentiometer's track is circular, the size of a rotary potentiometer can be quite small and does not require a lot of space to install.

A very inexpensive linear potentiometer ($3.00).  The tracks on this pot are made from carbon and the body is open to dust and grime.  They work quite well, but expect their life to be limited once they begin to get dirty

Potentiometer Accuracy

The ability of the potentiometer to accurately read the position of the slider as it moves along the track is vital if the attached control device is to perform in an accurate and repeatable way. 

The performance, accuracy, and how long that accuracy is maintained, is governed by the internal construction of the potentiometer; in particular the material used for the track (carbon, cermet, composite, etc).  Of particular importance, is the coarseness of the signal and the noise generated (electrical interference). when the potentiometer has power running through it.

For example, cermet which is composite of metal and plastic produces a very clean low noise signal, where as carbon often exhibits higher noise characteristics and can generate a course output.  It’s the coarseness of the signal that makes a control device easy or difficult to calibrate.  It also defines how accurately the potentiometer will read any small movement.

Potentiometers that use carbon form the mainstay of the less expensive types, such as those used in the gaming industry, while higher-end applications that requite more exacting accuracy use cermet or other materials. 

Essentially, higher end potentiometers generate less noise and produce a cleaner output that is less course.  This translates to more accurate calibration.  This is seen when you trim the aircraft. 

A quality mid to high-end potentiometer, when calibrated correctly, will enable you to easily trim the aircraft, insofar that the trim conditions can be replicated time and time again (assuming the same flight conditions, aircraft weight, engine output, etc).

Simulators, Dust, Grime and Other Foreign Bodies

Flight simulators to control a number of moving parts, generally use a combination of linear and rotary potentiometers.  For example, a rotary potentiometer may be used to control the flight controls (ailerons, elevator and rudder) while a linear potentiometer may be used to control the movement of the flaps lever, speedbrake and steering tiller. 

Any component that has a current running through it will attract dust, and the location of the potentiometer will often determine how much dust is attracted to the unit.  A potentiometer positioned beneath a platform is likely to attract more dust than one located behind the MIP or enclosed in the throttle quadrant.

A rotary potentiometer is an enclosed unit;  it is impervious to dust, grime and whatever else lurks beneath a flight simulator platform.  In comparison, a linear potentiometer is open to the environment and its carbon track can easily be contaminated.  Once the track has become contaminated, the potentiometer will become difficult to calibrate, and its output will become inaccurate.

Sometime ago, I had a linear potentiometer that was difficult to calibrate, and when calibrated produced spurious outputs.  The potentiometer was positioned beneath the platform adjacent to the rod that links the two control columns.  When I removed the potentiometer, I discovered part of the body of a dead cockroach on the carbon track. 

This is not to say that linear potentiometers do not have a place – they do.  But, if they are to be used in a dusty environment, they must have some type of cover fitted.  A cover will minimise the chance of dust adhering to the potentiometer’s track. 

I use linear potentiometers mounted to the inside of the throttle quadrant to control the flaps and speedbrake.  The two potentiometers are mounted vertically on the quadrant’s sidewall.  This area is relatively clean, and the vertical position of the mounted potentiometers is not conducive to dust accumulation.

Ease of Installation

Both linear and rotary potentiometers are straightforward to install, however, they must be installed relatively close to the item they control.  Often a lever or connecting rod must be fabricated to enable the potentiometer to be connected with the control device.

String Potentiometers (strings)

Cross section diagram showing internals of string potentiometer. Diagram © TE Connectivity.

A string potentiometer (also called a string position transducer) is a rotary potentiometer that has a stainless steel cable connected to a spring-loaded spool. 

The string potentiometer is mounted to a fixed surface and the cable attached to a moveable part (such as a control device).  As the control device moves, the potentiometer produces an electrical signal (by the slider moving across the track) that is proportional to the cable’s extension or velocity.  This signal is then read by the calibration software. 

The advantages of using string potentiometers over a standard-issue rotary potentiometer are many:

(i)        Quick and easy installation;

(ii)       Greater accuracy as you are measuring the linear pull along a cable;

(iii)      Greater flexibility in mounting and positioning relative to the control device;

(iv)      No dust problems as the potentiometer is enclosed;

(v)       No fabrication is needed to connect the potentiometer to the control device (only cable and dog clip) and,

(vi)       Greater time span before calibration is required (compared to a linear potentiometer).

The importance of point (iii) cannot be underestimated.  The string can be extended from the potentiometer within a arc of roughly 60-70 degrees, meaning that the unit can be mounted more or less anywhere.  The only proviso is that the cable must have unimpeded movement. 

Attachment of the string to the control device can be by whatever method you choose.  I have used a small dog lead clip.  As the potentiometer is completely enclosed dust is not an issue, which is a clear advantage in that once the potentiometer calibrated, the calibration does not alter (as dust does not settle on the track).

I have used string potentiometers to calibrate the axis on the ailerons, elevators and rudder (one potentiometer per item), in addition to using  a dual-string potentiometer in the throttle quadrant to calibrate the two thrust levers.  Another single-string potentiometer controls the position of the flaps lever.

Cost

High-end commercial string potentiometers are not inexpensive.  Many are used in the medical industry where extremely tight tolerances must be met at all times.  The more accurate the potentiometer the more the potentiometer will cost.  But you have to look at the end product in use and the level of positional accuracy that's required.  While a high-end potentiometer can definitely be used, the accuracy you are paying for probably won't be needed or used by ProSim-AR.  Put another way, it's like buying a high tensile strength dog lead, when a piece of rope will do the same job.

If you search the Internet, you will find average priced string potentiometers, and these are the ones that will suit your application perfectly.

rotary String potentiometer.  This pot connects to the ailerons.  The stainless cable can be seen leaving the casing that connects with the aileron controls.  An advantage of string pots is that they can installed more or less anywhere, as long as there is unimpeded access for the cable to move

Fabricate Your Own String Potentiometer

As mentioned, whilst you can purchase ready-made string potentiometers, their cost is not inexpensive.  As a trial, a friend and I decided to fabricate our own string potentiometers.

The potentiometers used are manufactured by Bourns (3590S series precision potentiometer).  These units are a sealed, wire-wound potentiometer with a stainless steel shaft.  According to the Bourns specification sheet these potentiometers have a tolerance +-5%. 

Diagram showing spring-loaded spool. Diagram © TE Connectivity.

The potentiometer is mounted in a custom-made acrylic box in which a hole the size of the potentiometer's end, has been drilled into the lid.   Similar boxes can be purchased in pre-cut sizes, but making your own custom-sized box enables the potentiometer to be mounted inside the box in a position most advantageous to your set-up. It also enables you to place the mounting holes on the box in strategic positions.

Another small hole has been drilled in the side of the box to enable the stainless steel cable to move freely (see image at beginning of article).  If you want to allow the cable to move through an arc, this hole must be elongated to enable the cable to extend at an angle and move unimpeded. 

The cable (string) is part of a self-ratcheting spool (also called a retractor clip) which is glued to the inside of the box and connected directly with the stainless steel shaft of the potentiometer.  To stop the shaft of the potentiometer from spinning freely, a hole was drilled into the shaft.  A small screw secures the shaft to the inside the ratchet spool mechanism. 

The cable when attached to a solid point is kept taught by the tension of the self-ratchet spool (an internal spring controls the tension).   Ratchet spools are easily obtainable and come in many sizes and tensions.   Three standard JR servo wires connect the potentiometer to a Leo Bodnar BU0836A 12 bit Joystick Controller card.  A mini dog lead clip is used at attach the cable to the control device.

One of the major advantages when using string potentiometers is that the actual potentiometer does not have to be mounted adjacent to, or even close to the device it controls.  The line of pull on the cable can be anything within roughly a 70 degree arc. 

A string potentiometer that connects to the two thrust levers in the throttle quadrant

Applications

A string potentiometer can be used in the following applications: ailerons, elevators, thrust levers, speedbrake and flaps.  The string potentiometer can also be used for the rudder, however, as the input to the rudder is course, there probably is little advantage in using a string potentiometer in this application - a normal rotary potentiometer is suitable.

By far the most important of the above-mentioned applications are the ailerons, elevators and the thrust levers on the throttle quadrant.

Additional Information

Final Call

Previously, I used inexpensive linear and rotary potentiometers to control the main flight controls.  I was continually plagued with calibration issues, and when calibrated, the calibration was not maintained for more than few months.  Furthermore, manual flight was problematic as the output from each of the  (cheap) potentiometers was very course, which translated to less accuracy when using the ailerons and elevators.  Trimming the aircraft in any condition other than level flight was difficult.

Without doubt, the use of quality string potentiometers have resolved all the earlier calibration and accuracy issues I had been experiencing.  With the replacement potentiometers, the aircraft is easily hand-flown and can be trimmed more accurately.

Perhaps in the future I will ‘up the anti’ and purchase two commercial high-end string potentiometers (or use hall sensors), but for the time being the Bourns potentiometers suit my requirements.

Throttle Quadrant Rebuild - Clutch, Motors, and Potentiometers

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Captain-side of throttle quadrant showing an overview of the new design.  The clutch assembly, motors, and  string potentiometer can be seen, in addition to a portion of the revised parking brake mechanism

An earlier article, Throttle Quadrant Rebuild – Evolution Has Led to Major Changes has outlined the main changes that have been made to the throttle quadrant during the rebuild process. 

This article will add detail and explain the decision making process behind the changes and the advantages they provide.  As such, a very brief overview of the earlier system will be made followed by an examination of the replacement system.

Limitation

It is not my intent to become bogged down in infinite detail; this would only serve to make the posts very long, complicated and difficult to understand, as the conversion of a throttle unit is not simplistic.

This said, the provided information should be enough to enable you to assimilate ideas that can be used in your project.  I hope you understand the reasoning for this decision.

The process of documenting the throttle quadrant rebuild will be recorded in a number of articles.  In his article I will discuss the clutch assembly, motors, and potentiometers. 

Why Rebuild The Throttle Quadrant

Put bluntly, the earlier conversion had several problems; there were shortfalls that needed improvement, and when work commenced to rectify these problems, it became apparent that it would be easier to begin again rather than retrofit. Moreover, the alterations spurred the design and development of two additional interface modules that control how the throttle quadrant was to be connected with the simulator.

TIM houses the interface cards responsible for the throttle operation while the TCM provides a communication gateway between TIM and the throttle.

Motor and Clutch Assembly - Poor Design (in previous conversion)

The previous throttle conversion used an inexpensive 12 volt motor to power the thrust lever handles forward and aft.  Prior to being used in the simulator, the motors were used to power electric automobile windows.  To move the thrust lever handles, an automobile fan belt was used to connect to a home-made clutch assembly.

This system was sourly lacking in that the fan belt continually slipped.  Likewise, the nut on the clutch assembly, designed to loosen or tighten the control on the fan belt, was either too tight or too loose - a happy medium was not possible.   Furthermore, the operation of the throttle caused the clutch nut to continually become loose requiring frequent adjustment.

The 12 volt motors, although suitable, were not designed to entertain the precision needed to synchronize the movement of the thrust levers; they were designed to push a window either up or down at a predefined speed on an automobile.

The torque produced from these motors was too great, and the physical backlash when the drive shaft moved was unacceptable.  The backlash transferred to the thrust levers causing the levers to jerk (jump) when the automation took control (google motor backlash).

This system was removed from the throttle.  Its replacement incorporated two commercial motors professionally attached to a clutch system using slipper clutches.

Close up image of the aluminium bar and ninety degree flange attachment.  The long-threaded screw connects with the tail of the respective thrust lever handle. An identical attachment at the end of the screw connects the screw to the large cog wheel that the thrust lever handles are attached

Clutch Assembly, Connection Bars and Slipper Clutches - New Design

Mounted to the floor of the throttle quadrant are two clutch assemblies (mounted beside each other) – one clutch assembly controls the Captain-side thrust lever handle while the other controls the First officer-side. 

Each assembly connects to the drive shaft of a respective motor and includes in its design a slipper clutch.  Each clutch assembly then connects to the respective thrust lever handle.  A wiring lumen connects the clutch assembly with each motor and a dedicated 12 volt power supply (mounted forward of the throttle quadrant).  See above image.

Connection Bars

diagram 1: crossection and a cut-away of a slipper clutch

To connect each clutch assembly to the respective thrust lever handle, two pieces of aluminium bar were engineered to fit over and attach to the shaft of each clutch assembly. 

Each metal bar connects to one of two long-threaded screws, which in turn connect directly with the tail of each thrust lever handle mounted to the main cog wheel in the throttle quadrant. 

Slipper Clutches

close up of slipper clutch showing precision ball bearings

A slipper clutch is a small mechanical device made from tempered steel, brass and aluminum.  The clutch consists of tensioned springs sandwiched between brass plates and interfaced with stainless-steel bearings.  The bearings enable ease of movement and ensure a long trouble-free life.

The adjustable springs are used to maintain constant pressure on the friction plates assuring constant torque is always applied to the clutch.  This controls any intermittent, continuous or overload slip.

A major advantage, other than their small size, is the ease at which the slipper clutches can be sandwiched into a clutch assembly.

Anatomy and Key Advantages of a Slipper Clutch

A number of manufacturers produce slipper clutches that are specific to a particular industry application, and while it's possible that a particular clutch will suit the purpose required, it's probably a better idea to have a slipper clutch engineered that is specific to your application. 

The benefit of having a clutch engineered is that you do not have to redesign the drive mechanism used with the clutch motors.

Key advantages in using slipper clutches are:

  • Variable torque;

  • Long life (on average 30 million cycles with torque applied);

  • Consistent, smooth and reliable operation with no lubrication required;

  • Bi-directional rotation; and,

  • Compact size.

The clutch assembly as seen from the First Officer side of the throttle quadrant.  Note the slipper clutch that is sandwiched between the assembly and the connection rods.  Each thrust lever handle has a dedicated motor, slipper clutch and connection rod.  The motor that powers the F/O side can be seen in the foreground

Clutch Motors

The two 12 Volt commercial-grade motors that provide the torque to drive the clutch assembly and movement of the thrust lever handles, have been specifically designed to be used with drives that incorporate slipper clutches.

In the real world, these motors are used in the railway and marine industry to drive high speed components.  As such, their design and build quality is excellent. The motors are designed and made in South Korea.

Each motor is manufactured from stainless steel parts and has a gearhead actuator that enables the motor to be operated in either forward or reverse.  Although the torque generated by the motor (18Nm stall torque) exceeds that required to move the thrust lever handles forward and aft, the high quality design of the motor removes all the backlash evident when using other commercial-grade motors.  The end result is an extraordinary smooth, and consistent operation when the thrust lever handles move.

A further benefit using this type of motor is its size.  Each motor can easily be mounted to the floor of the throttle quadrant; one motor on the Captain-side and the second motor on the First Officer-side.  This enables a more streamlined build without using the traditional approach of mounting the motors on the forward firewall of the throttle quadrant.

captain-side 12 Volt motor, wiring lumen and dual string potentiometer that control thrust levers

String Potentiometers - Thrust Levers 1/2

Two Bourns dual-string potentiometers have been mounted in the aft section of the throttle unit.  The two potentiometers are used to accurately calibrate the position of each thrust lever handle to a defined %N1 value.  The potentiometers are also used to calibrate differential reverse thrust.

The benefit of using Bourns potentiometers is that they are designed and constructed to military specification, are very durable, and are sealed.  The last point is important as sealed potentiometers will not, unlike a standard potentiometer, ingest dust and dirt.  This translates to zero maintenance.

Traditionally, string potentiometers have been mounted either forward or rear of the throttle quadrant; the downside being that considerable room is needed for the operational of the strings.  

In this build, the potentiometers were mounted on the floor of the throttle housing (adjacent to the motors) and the dual strings connected vertically, rather than horizontally.  This allowed maximum usage of the minimal space available inside the throttle unit.

Automation, Calibration and Movement

The automation of the throttle remains as it was.  However, the use of motors that generate no backlash, and the improved calibration gained from using string potentiometers, has enabled a synchronised movement of both thrust lever handles which is more consistent than previously experienced.

Reverse Thrust 1/2

Micro-buttons were used in the previous conversion to enable enable reverse thrust - reverse thrust was either on or off, and it was not possible to calibrate differential reverse thrust. 

Dual Bourns string potentiometer that enables accurate calibration of thrust lever handles and enables differential thrust when reversers are engaged

In the new design, the buttons have been replaced by two string potentiometers (mentioned earlier).  This enables each reverse thrust lever to be accurately calibrated to provide differential reverse thrust.  Additionally, because a string potentiometer has been used, the full range of movement that the reverse thrust is capable of can be used.

The video below demonstrates differential reverse thrust using theProSim737 avionics suite. The first segment displays equal reverse thrust while the second part of the video displays differential thrust.

 
 

Calibration

To correctly position the thrust lever handles in relation to %N1, calibration is done within the ProSim737 avionics software  In calibration/levers, the position of each thrust lever handle is accurately ‘registered’ by moving the tab and selecting minimum and maximum.  Unfortunately, this registration is rather arbitrary in that to obtain a correct setting, to ensure that both thrust lever handles are in the same position with identical %N1 outputs, the tab control must be tweaked left or right (followed by flight testing).

When tweaked correctly, the two thrust lever handles should, when the aircraft is hand-flown (manual flight), read an identical %N1 setting with both thrust levers positioned beside each other.  In automated flight the %N1 is controlled by the interface card settings (Polulu JRK cards or Alpha Quadrant cards).

Have The Changes Been Worthwhile

Comparing the new system with the old is 'chalk and cheese'.  

One of the main reasons for the improvement has been the benefits had from using high-end commercial-grade components.  In the previous conversion, I had used inexpensive potentiometers, unbalanced motors, and hobby-grade material.  Whilst this worked, the finesse needed was not there.

One of the main shortcomings in the previous conversion, was the backlash of the motors on the thrust lever handles.  When the handles were positioned in the aft position and automation was engaged, the handles would jump forward out of sync.  Furthermore, calibration with any degree of accuracy was very difficult, if not impossible. 

The replacement motors have completely removed this backlash, while the use of string potentiometers have enabled the position of each thrust lever handle to be finely calibrated, in so far, as each lever will creep slowly forward or aft in almost perfect harmony with the other.

An additional improvement not anticipated was with the installation of the two slipper clutches.  Previously, when hand-flying there was a binding feeling felt as the thrust lever handles were moved forward or aft.  Traditionally, this binding has been difficult to remove with older-style clutch systems, and in its worst case, has felt as if the thrust lever handles were attached to the ratchet of a bicycle chain.

The use of high-end slipper clutches has removed much of these feeling, and the result is a more or less smooth feeling as the thrust lever handles transition across the throttle arc.

Future Articles

Future articles will address the alterations made to the speedbrake, parking brake lever, and internal wiring, interfacing and calibration.  The rotation of the stab trim wheels and movement of the stab trim indicator tabs will be discussed.

This article is one of several that pertain to the conversion of the OEM throttle quadrant. A summary page with links can be viewed here: OEM Throttle Quadrant. 

Update

on 2018-04-11 01:08 by FLAPS 2 APPROACH

This article was not able to be published at an earlier time because of issues with confidentiality and potential patents.  The article has been re-written (March 2018).