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Enhanced Signal Processor Unit

Motion Platform Signal Processor


Comms Speed Enhanced SPU

Summer 09 - A ready built higher speed Signal Processor card is now available to buy - this does away with all the breadboard electronics and fiddly wiring - see the 40SPU-1 card page for up-to-date details.


April 09 - I've completed work on a High-Speed SPU based on the newly released PICAXE 28X2 chip - faster response and smoother control.


February 09 - Added real-time speed controller condition monitoring to allow you to keep an eye on your drive system loading.


January 09 - With the prototype Mini-Motion Platform (Platform 3) now built and running I've had time to take a look at some improvements to the control system.

I've already described my experience with the use of US Digital's MA3 position encoders on the MA3 Encoder page.


These improved the position feedback signals available to the Signal Processor Unit by reducing the noise levels on the signals. A further improvement that can be made is to increase the data transfer speeds between the SPU and the PID Servo Controller software running on the driving PC.


The serial communications between the PICAXE 28X1 chip and the PC using the basic SPU run at 9600 baud. This is the speed limit of the serin/serout commands on the 28X1 when it runs with its internal 8MHz clock speed. The 28X1 however has "hardware" serial comms capabilities which are capable of running at 115200 baud. The much faster comms speed has the effect of minimising the small delay that occurs between the sampling of the instantaneous position of the platform by the SPU and a correcting set of motor speed demands being received by the motor speed controllers. The time delays here are very small but they are significant in affecting the smoothness of the drive as felt by the pilot in the motion platform. The delay is reduced because the time taken to send and receive data is reduced at the higher baud.



Motion Platform RS232 TTL Converter


RS232 to TTL Converter Module

Implementation of the 115200 baud serial comms requires an additional bit of signal processing hardware on the SPU. This is because the "hardware" serial-in pin on the 28X1 chip requires true RS232 voltage polarities at TTL (5V) levels. For serial comms with a PC this needs either a RS232/TTL Converter Module or a separate MAX232 converter chip to be incorporated into the data line.


Either approach will work although I've chosen to use an RS232/TTL Converter as this is neater for me and also allows a standard serial cable to be plugged in directly without having to strip wires etc. The converter module I used has its TTL connector pins physically inverted and so can be pressed directly into the breadboard which allows very simple further connection to the 28X1 chip and the available 5V board supply - see image above.


I've recently come across this AVITResearch USB to TTL converter cable. This has the MAX232 type level shifter circuitry built into it and can be connected directly to the serial read and write pins on the PICAXE chip. More expensive than a MAX232 chip and some capacitors but much less hassle. If you do use it connect cable blue to 28X1 pin17 (Position Out) and red to pin 18 (Speed In).




Combined with the MA3 Encoder position feedback and V2.02 driver and PID software running on a LAN PC I've found the 115200 baud SPU does improve the quality of the Mini Motion Platform motion. On my LAN PC I get refresh times on the PID Servo Controller of around 0.03s (33 loops/s - see performance tips below) and a smoother drive. There now seems more scope for adjustment of the PID control terms and in particular both the Proportional gain and Derivative term can be strengthened to improve the higher frequency response for on-ground operations. This improves touch-down bumps and runway rumble effects. Generally the in-air motion also seems smoother.


Overall I think it is a worthwhile enhancement and when the PICAXE 28X2 chip becomes available the full resolution of the MA3 feedback encoders can be realised to take the picaxe based SPU to the probable limit of its capabilities. - UPDATE April '09 - the 28X2 chips have now been released and I've completed work on a High-Speed SPU - click on the link.




PID Servo Controller Software for Motion PlatformsThe faster communication speeds available with this enhancement can be used to add additional capabilities to the system. One very useful addition is to read real-time current and temperature data from the MD03 speed controllers and to send this back to the PID Servo Controller software for display. This allows you to keep an eye on the real-time loading on the MD03 speed controllers. You can confirm how heavily loaded they are generally and can watch out for potentially damaging current flow or controller temperature rise. This will be particularly useful during the initial motion platform setup and commissioning activities when the patterns of the controller loading might not yet be known.



I've put together some additional wiring diagrams etc for the Enhanced SPU set up and there are also new chip flash programs to handle the faster serial communications (I've included a .bas program which adds the 115200 serial comms to the existing SPU without the MA3 encoders - a 16MHz resonator will still be needed).


28X1 Pin Designations

(with MA3 Encoders)

Enhanced SPU Wiring

(with MA3 Encoders)


Enhanced SPU

System Wiring

Enhanced SPU

.bas Flash Progs


Three versions are

included - for normal

potentiometer feed-

back and for MA3

encoder feedback and

a version for speed

controller monitoring.


Note the SPU is built on the PICAXE Experimenter Kit breadboard (this kit has been discontinued by PICAXE - this is the replacement). If you use the AVITResearch USB to TTL converter cable then connect cable blue to 28X1 pin17 (Position Out) and red to pin 18 (Speed In).


To get the best performance out of this low-cost drive system there are a few recommendations worth considering.

  • If possible run the V2 Motion Driver and SPU Servo Controller on a LAN PC separate from the main flight sim PC. This will reduce the demand on the main PC and also give the best fresh speeds from the motion software. The V2 software is more CPU demanding and is likely to run faster when not fighting the flight sim for CPU time.

  • On the LAN PC try setting the process priority of the Motion Driver slightly lower than the PID Servo Controller (for example at BelowNormal) - be very careful however and stand by the emergency stop button! On my set up the controller remains stable and runs at a consistently faster refresh speed, however this may not be the case on all systems and it will also depend on the overall demand on the LAN PC - be careful if you try this.

  • If you do not want the Articulated Projector drive output from the V2 Motion Driver set the output mode to NONE in the vis.cfg configuration file (in the line near the bottom of the file - change "Pololu" to "NONE"). This will reduce the CPU demand.

  • Use the Enhanced SPU hardware with the US Digital MA3 position encoders - this will improve the quality of the motor control and give more flexibility in your PID configuration. For the PID Servo Controller to run at 115200 baud the PID2.cfg file must be modified - line 28 should be changed from "9600" to "115200" to set the correct baud. If you find that line 28 is blank in your PID2.cfg file then add a new line with "115200" (without the inverted commas).

  • Move the main supply voltage for the MD03 motor speed controllers up to 36V - ie use 3 x 12V batteries in series rather than 2. Although the motors I use are nominally rated at 24V increasing the voltage supply to 36V has no detrimental effect on them in my set up. The higher voltage increases the torque available when the motors are accelerating or decelerating. Given the short actuator stroke length it is not likely that the motors will over-speed, it is the added current flow and so higher torques that are of interest. If you try this be sure to monitor carefully the working temperatures of the motors and the speed controllers for signs of overheating - a sign of excessive current flow.

Overall the aim is drive smoothness and I think the speed enhanced SPU takes us a further step in the right direction. My experience is that if the motor drive is slightly rough then it does not feel right to the pilot in the motion platform. When some roughness is expected - for example when on the runway then that's okay but for normal flight motion you want the platform motion to be a smooth as you can possibly get it.



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