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.

 

 

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).

 

RESULTS

 

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.

 

SPEED CONTROLLER CONDITION MONITORING

 

The 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).

 

 

 

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