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DIY Control Loader Flight Column

DIY Force Feeback Flight Column

 DIY Force Feedback Flight Column Test Rig (not pretty, but functional)

 

UPDATE July '10 - The flight control loading software developed for the project is now available for free.

 

PAGE 1    PAGE 2

 

(FOR MORE RECENT DEVELOPMENTS SEE HERE)

 

DIY CONTROL LOADER COLUMN PROJECT

(Video Clips on Page 2)

 

In the DIY force feedback yoke pages I described an ongoing project to build DIY force feedback flight sim controls. Although the overall system seemed to work reasonably well one big issue was with the smoothness of the force feel, and this was due to the cogging or commutation ripple in shaft torque produced by the electric motors used to power the controls. I felt that the ripple spoiled the feel of the force feedback.

 

I've now been able to try good quality disc armature motors1 in the system to take the project a step further. With these motors I think the quality of the loading on the controls is greatly improved over the skewed rotor servo motors I tried last time; generally the forces are smoother and the motors offer little motion resistance when operating at light force levels. The force feedback isn't perfect however as some commutation disturbance in the motor torque output is still discernable especially at higher force levels - how significant individual users might think this is is an interesting question! If I'm honest with myself I think it is noticeable.

 

The motors I've used are Mavilor MSS-4 DC servo motors (below left). They are quite large and this has prevented their use in the flight yoke setup so I've built a simple flight column test rig to further work on the system. The smaller MSS-2 motor in the same range might be a more suitable choice for a yoke build - however not being sure of the levels of torque performance needed I went for these bigger motors for testing.

 

 

THE TEST FLIGHT COLUMN

 

The control loader flight column test rig is shown in the image above right. You can see I've not spent much time trying to make it look pretty - however it does function. There are several features of the design that have turned out to be important in getting the force feedback system working and these are -

  • Rigid structural elements to minimise flexure of the drive mechanism under load.

  • Low friction, well fitting pivots for both elevator and aileron axis movements. The aileron axis is needle roller bearing mounted, the column is pivoted at the base on a steel pin mounted in close fitting steel housings. I think ball bearings might further improve things.

  • Toothed belt transmissions connecting the servo motors to the column and yoke with belt tension adjustment to ensure there is no backlash (slack) in the transmission.

  • Transmission shafts bearing mounted in rigid housings to minimise friction and ensure constraint of any belt pulley displacements which might lead to slackened belts.

  • Precision position feedback potentiometers geared without backlash to the movement axes. (The pots are geared to utilise as much of their full electrical travel as possible and are used with a 12bit joystick card to generate the necessary resolution of position feedback.)

Disc Armature Servo Motor

 Disc armature DC servo motor

It looks like all the above mechanical elements of the design are necessary for acceptable performance of the force feedback system. Any kind of lag, deadband, backlash or softness anywhere between the accurate reporting of column movement to the PC and the application of the force response from the motors to the column will compromise the performance of the system.

 

The need for this mechanical design/build performance is matched by the need for similar levels of performance from the electrical/control system and the force feedback software. It is helpful to think of the whole system as a chain of elements that needs to be designed to minimse the time taken and maximise the accuracy with which the column movement is detected and the force response to this movement is applied to the column.

 

For information the elevator movement range of the test flight column is roughly 300mm and the aileron axis rotation is just short of +/- 90 Deg. The aileron drive has a 4:1 speed reduction between the MSS-4 motor and the yoke axis, and the elevator drive has a 16:1 reduction made in two 4:1 stages. The elevator drive motor works harder than the aileron drive and I think it might be possible to use a smaller MSS-2 motor for the latter which would ease the mechanical layout problems.

 

CONTROL SYSTEM & SOFTWARE

 

DIY Control Loading System

 Improved DIY force feedback system

I've managed to improve both the electrical and control system and the PC force feedback software. In particular following my own experiments with the system and also suggestions made by several emailers (thanks to all the helpful suggestions emailed in) I've worked to increase the refresh rate of the main force feedback to loop to 500 Hz. Speeds higher than this are used in many commercial control loader systems however an aim of the project was to try and get reasonable levels of performance using low cost components. This has not been possible regarding the motors because of their critical role in the system, however I have managed to keep the cost of the rest of the system down.

 

The force feedback software now does its load calculations at a target 500 Hz. This is the limit of the 12 bit joystick card (Leo Bodnar's) used to report control axis positions. Note the feedback pots on the flight column are the same ones FSX uses, but they need to be precision pots geared to utilise their full electrical travel. Flight data from FSX/9 providing airspeed, engine thrust, acceleration data etc is brought into the calculations at the frame rate of the flight sim.  The calculated force demands are exported by the software using 500K baud serial comms and are processed by a new signal processor unit based on the PICAXE 20X2 micro controller running at 64 MHZ. The 500K baud comms is supported by the standard FTDI chip based picaxe USB download cable (with some serial port setting adjustments), and 64 MHz is a standard speed for the 20X2 chip. The result is that the voltage demands to the MD03 motor controllers on the I2C data bus are refreshed at a target 500Hz.

 

The MD03 motor controllers use 8 bit resolution in their motor voltage outputs, with their separate direction control this gives torque demand over a range of nominally +/-255 steps. This could be an issue if the system was set up to produce a maximum force output that was much higher than the usual working forces experienced during normal flight conditions. So the 8 bit resolution does pose a limitation. I have the test rig set up to try and match the maximum 255 demand output to what I think are the max feedback forces associated with flying at the normal airspeed limit of the aircraft. This ensures that flight in the normal speed ranges produces forces that use a good proportion of the available 8 bit resolution and do not feel rough because of the minimum step change in motor torque that can be achieved. This is done by selecting the right voltage supply for the MD03. The clear implication though is that trying to set up a force range that includes very high failure mode forces for example would probably make the feel at more normal lower forces rougher.

 

The actual refresh speeds achieved by the system will be PC dependent. On my oldish dual core AMD Athlon 64 6000+ machine I get slower refresh speeds when running with FSX on the same PC when the force feedback software is set not to hog CPU (I get about 350 Hz). I have also written a LAN version of the software and using this to run the force feedback system on an old single core machine I get the full 500Hz as there is no FSX to compete with. I suspect that a newer 4 or higher core machine might be quite happy running both FSX and the force feedback software - something to test.

 

DIY Control Loading System 64 MHz signal processor unit

 Motor drive test set up with 64MHz signal processor unit on breadboard

The higher refresh speeds are required because they are important for some of the components of the force feedback, but not all. The main benefit is to reduce system lag when processing the main control surface deflection related aerodynamic force component. At lower refresh rates a greater delay is present in how quickly the force responds to stick movement. If the stick is moved fast enough the forcing can get sufficiently out of phase with the motion to start to induce unstable oscillations - this is not good! At the higher refresh rates this does not happen and the stick response is stable even if it is accidentally let go when in flight. See - the video clips on the next page.

 

Secondary, but still potentially important effects of the higher refresh rates are to improve the effectiveness with which speed of motion related damping can be added to the stick response, and also how friction (or -ve friction) can be simulated. I am finding that there are limits here however, and although this low cost system seems to be able to add moderate damping and friction effects the force feel deteriorates if I overdo it and try to very heavily damp the column movement through the dynamic model. One problem is that small commutation based torque ripples from the motor result in small speed fluctuations in the column motion which are then magnified by a high speed related damping force gain in the closed loop. The frequency of these fluctuations is much higher than the stick movements used for flight control and would probably need a significant further uplift in the force feedback loop refresh speed to remove the small system lag causing the magnification. An alternative would be not to use DC commutated motors - but both solutions would take us further towards commercial control loader system specs and the price tags associated with them.

 

So I think the system performs well for the price, but it looks like there is a firm limit to the capabilities of such low cost electrical/control components.

 

Continued on page 2...... Performance and Video Clips

 

1. Many thanks are due to Don McKellow who has kindly lent me the Mavilor disc armature motors to work with. They are destined for Dons' full size B777 sim once I've made enough progress on the system (assuming I don't break them first). Don is a B777 training captain.

 

 

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