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12. PID Servo Controller

12.1 Configuration File Settings

12.2 Operation

12.3 PID Control Settings

12.4 PID Control Settings - Setup Procedures

The BFF PID Servo Controller and 64SPU-1 hardware card operate together to form the core of a motion platform closed loop electric motor servo drive. The software and hardware support up to three output channels.

The PID Servo Controller software converts the motion cue output from the BFF Motion Driver software into motor speed and direction demands which are used to drive the electric motors via motor speed controllers. The software will not operate unless it is connected to an active and properly configured 64SPU-1 signal processor card (or other valid SPU). For full details of the 64SPU-1 read the card data sheet.

The PID Servo Controller is installed as part of the motion driver software package. It is run using the Start_PID_Controller button on the BFF Motion Driver window.

The software communicates with the 64SPU-1 hardware using serial comms via either a physical or virtual COM port. If the PICAXE AXE027 download cable is used speed can be improved by adjusting the Latency Timer setting for the virtual COM port to 1 ms.

12.1 Configuration File Settings

Motion Driver Settings (in .bff config file)

To use the PID Servo Controller the data output mode of the motion driver must be set to Mode=MEM. This instructs the motion driver to make the cue data and other settings available to the PID Servo Controller through a shared memory area.

The Port=COM* parameter should be set to specify the com port the PID Servo Controller should use to communicate with the external 64SPU-1 card.

PID Servo Controller Settings (in PID26.cfg file)

The controller software's local settings are held in the PID26.cfg file. A few settings must be made in the PID26.cfg manually using a text editor such as NotePad. In normal use the .cfg file does not need to be accessed manually - the software reads and writes settings as required.

The PID26.cfg contents are shown below (comments in blue are NOT present in the file):

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 1. 30                 ;Start of PID Settings - do not adjust in file
 2. 1.0
 3. 0.05
 4. 0
 5. 30
 6. 1.0
 7. 0.05
 8. 0
 9. 30
10. 1.0
11. 0.05
12. 0
13. 20
14. 1.0
15. 0.05
16. 0
17. 20
18. 1.0
19. 0.05
20. 0
21. 20
22. 1.0
23. 0.05
24. 0                 ;End of PID Settings
25. 0                 ;Feedback noise filter on/off - it is best to use good quality feedback devices rather than the software filter
26. 0.05            ;Noise filer Time Constant
27. 0                 ;Data trace
28. 115200        ;64SPU-1 comms baud rate
29. 00111011     ;64SPU-1 bit settings - see 64SPU-1 data sheet Section 3
30. Built-For-Fun EVs - 3-DOF Cockpit Motion Driver v2            ;Title of Motion Driver window
31. I2C              ;Data output format identifier
32. COM2          ;Dimension Engineering Sabertooth controller comms COM port
33. 38400          ;Dimension Engineering Sabertooth controller comms baud rate
34. 128-1          ;Sabertooth channel 1 address
35. 128-2          ;Sabertooth channel 2 address
36. 129-1          ;Sabertooth channel 3 address
37. Normal        ;PID Servo Controller Sabertooth comms process priority
38. Yes             ;Enable polynomial cue data use

39. Normal        ;PID Servo Controller process priority

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Lines 10-24 contain the individual PID settings for each channel - these should not be adjusted in the file. The software reads and writes these directly.

Lines 24 & 25 define the software noise filtering of the position feedback data. This is retained as a legacy setting - it is better just to use good quality feedback devices.

Line 27 is data trace output setting, =0 for off, =1, 2 or 3 to output data for numbered channel. Data is written to file PID_Output.txt each data loop so the output will build up very quickly if left on - use for short bursts only. The data items are: Position feedback, Demanded position, Position error, P, I & D speed demand terms, Total speed demand, Controller current, controller temp. The data can be inspected and charted in a spreadsheet application.

Line 28, the 64SPU-1 baud rate must be left at 115200

Line 29, 8 bit control settings for the 64SPU-1 - see the 64SPU-1 data sheet Section 3.

Line 30, motion driver window title - leave as is.

Line 31, data output mode, = "I2C" for MD03 drive from 64SPU-1, = "SERIAL2" or "SERIAL2x25" for Sabertooth controller output - see Sabertooth Quick Start Guide for details.

Lines 32-36, Sabertooth setup settings - see Sabertooth Quick Start Guide for details.

Line 37, Sabertooth comms process priority, = Normal, High etc. This can be used to give the Sabertooth comms process better access to system resources if it is running on the same sim PC as the flight sim and is struggling to produce smooth output.

Line 38, best to leave as is - this enables the servo controller to use position demands sent from the motion driver as polynomial curve fits. This gives smoother interpolation and smoother D term behaviour.

12.2 Operation

The PID Servo Controller will only run properly once your 64SPU-1 card has been connected and configured. See the 64SPU-1 data sheet for details for the card setup.

To run the PID Servo Controller start the BFF Motion Driver and once it is running and in Hold mode select the Start_PID_Controller button.

The PID Servo Controller will start and try to communicate with the 64SPU-1 card - check the messages in the text area of the window. If the comms are established successfully the software will report the current actuator positions, servo loop refresh speed etc. The 64SPU-1 LEDs will also change to running status if the comms are operational - see the 64SPU-1 data sheet for details.

PLATFORM START PROCEDURE

1. With Motion Driver on Hold, select servo controller  HOLD_AT_STRT_POS - to move the platform to its park/hold position.

2. Select ENGAGE_DRIVE to make the servo drive active for cue following.

3. Now use the Hold_Start_Pos and Drive buttons on the motion driver to move the platform smoothly between Hold and Drive modes as required.

PLATFORM STOP PROCEDURE

1. With the servo drive still engaged use the motion driver Hold button to move the platform smoothly to park/hold position.

2. Select the servo controller HOLD_AT_STRT_POS to fix the platform at park/hold position, and when required select STOP_MOTION to stop the drive.

In STOP_MOTION mode the servo drive will request zero speed from the motors. When CLOSE_and_RELEASE is selected the software stops sending speed demands and closes. Note that the motor speed controllers may not release the zero speed demand on the speed controllers until they are powered-down.

The Show/Hide button is used to minimise the display - this reduces CPU load and lifts refresh speed slightly.

Scope* buttons to show/hide the built-in oscilloscope for the chosen channel.

The Speed Controller Condition area shows live feedback of MD03 motor controller current and temperature. It is only active if MD03's are present and readback is enabled in the PID26.cfg file. The values are relative - use the Zero_Set button to zero them at startup. They provide a useful visual monitor of controller condition.

12.3 PID Control Settings

The Show_Act* buttons are used to display the control details for each channel and to set the PID control parameters.

The individual components of the speed demand output are displayed in the upper window area. In the lower area the PID settings are shown. 

There are two sets of PID settings - one for in-air operations and one for on-ground operations.

WARNING - changing the PID settings can make the drive unstable - take great care when making changes and do so by making small adjustments only. DO NOT make changes to the settings whilst the platform is in motion, stop the motion first using the HOLD and STOP buttons. Test changes by first using the HOLD_AT_START_POS button to instruct the system to hold position before moving on to full drive with the new settings.

Position Error for full proportional demand

The position error that gives full speed demand. If the instantaneous error is below this value the speed demand is scaled down if it is above this value the speed demand holds at maximum. The smaller the value is the faster the platform movement will respond to changes in the demanded position. However the smaller values will also mean it has more trouble slowing to a stop once the rig has moved to the demanded position. So small values give fast response with likely position overshoot, and large values give slow smooth response with no overshoot.

Values between 20 and 100 as a guide only - set with reference to the specified feedback range for the actuator.

Multiplier for Integral Term

Also know as the re-set term, this calculates a contribution to the speed demand based on how long a position error is sustained. It is mainly of use for making sure that the position comes back to that demanded when the proportional term has lost its effect on small position changes.

Guide values would be from 0 to 4 or 5.

Multiplier for Derivative Term

The trickiest term to get right. This speed demand component is calculated from the rate at which the error is changing. If the rate is high it means that the actual and demanded positions are either moving apart quickly or coming together quickly. If they are moving apart quickly the rig isn't moving fast enough and the D term gives a boost to the speed, if they are coming together quickly the rig needs to slow down faster and the D term acts as a brake on the motion. The practical effect can be as a brake and to help to reduce the extent of any overshoot there may be if the proportional term is set to respond strongly to position errors. Its use is therefore likely to be able to allow faster response Proportional terms to be used by reducing the extent of the associated overshoot. It can also be used simply to enhance the response to events such as touch-down bumps when a slow proportional response is being used.

Guide values would be from 0.01 to 0.2 - depends strongly on feedback resolution and quality, use 12bit feedback from the 12ADC-1 card for best effects.

Minimum Speed Demand

This is an additional term which allows a small fixed speed demand to be added in to the total to help overcome any friction based movement threshold in the system. Setting this just below the speed demand that actually initiates movement may help improve the speed of response.

Guide values, 0 to 20.

12.4 PID Control Settings - Setup Procedures

Unfortunately there is no set of settings that will be correct for every motion platform. Many of the above variables need to be set experimentally. Start by setting the Integral and Derivative terms to zero and work with the Proportional term first. This should be set at the higher end of the guide scale and then reduced gradually until the rig motion in response to step position inputs starts to overshoot or oscillate.

If you wish to make use of the Minimum Speed Demand setting, set this next. With the Servo Controller holding mid position increase the setting from zero until the movement starts to oscillate slightly – do it in small steps. A setting which holds the motion steady but just “ready” to move is what you are looking for. If this is set too high you will feel a “clunk” each time the platform drive changes direction.

The Integral term can also be set with the controller holding position. Increase the values in small steps until you start to see small oscillations occurring. Watch the progress bars – you will see the integral term building until a small movement adjustment takes place and the term reduces again.

The Derivative term will be the hardest to get right. If you feel it is not needed then don't use it – set it to zero. Again increase the setting in small steps from zero and watch the response of the rig before going on to see if you can then further reduce the Proportional term error setting. You will see the D term progress bar start to jitter as the D term is increased, this will give an indication of the limit of applicability with your feedback quality.

ALWAYS TAKE CARE – ADJUSTING THE CONTROL SETTINGS CAN CAUSE RAPID MOVEMENT OF THE PLATFORM. ENSURE SAFETY PRECAUTIONS ARE TAKEN AND ENSURE THE PLATFORM TRAVEL LIMIT SWITCHES AND ACCESSIBLE EMERGENCY STOP BUTTON ARE FITTED AND FULLY OPERATIONAL.

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