April 09 - On the Enhanced Speed SPU page I described a development of the servo system Signal Processor Unit which  runs at higher serial communications rates than the Basic SPU. And on the MA3 Encoder page I described the use of absolute position encoders to obtain smoother position feedback signals.

 

With the recent release of the PICAXE 28X2 micro-controller chip I've had another look at the SPU design and have been able to take advantage of the 28X2's higher performance to further improve the motion platform servo control system. The improvements stem from the 28X2's faster speed - with an external 10 MHz resonator fitted the 28X2 runs at 40 MHz - that's 2.5 times faster than the 28X1 used in the Enhanced SPU. This faster chip speed increases the processing speed of the servo system, but it also allows PWM pulse length measurements down to 1 μs which allows the US Digital MA3 position encoder to be used at it's full 12 bit resolution (effectively 4096 counts per rev).

 

 

The net result is consistent servo loop refresh speeds of about 50Hz on my PC and clean high resolution position feedback data. The faster refresh speeds generally improve the system performance by allowing the control system to react quicker to the position errors as they develop, and the higher resolution feedback helps by allowing the KD term of the PID controller to be used more effectively. Generally the KP (proportional) term can also be strengthened so that the system reacts more quickly to position errors. The overall effect is to improve the higher frequency response whilst maintaining stability and smoothness of motion - the servo system can be run with higher "gains".

 

The improvement is mainly noticeable in effects such as touch-down bumps, runway vibrations and in reacting to lower levels of heave "bobbing" motions and to the higher frequency components of the heave cues - most pitch and roll cues are of low frequency.

 

To use the 28X2 chip the PID Servo Controller software needed to be revised to handle the 12bit resolution on the feedback data, and the Motion Driver software revised to export its position demands in better than 12 bit resolution to match the feedback. The 28X2 off course also requires a new flash program. There are no wiring changes needed to implement the High-Speed SPU other than to replace the 16 MHz resonator on the Enhanced SPU with a 10 MHz one and to fit a 28X2 chip.

 

Off course this is still a low-cost DIY servo system and can't be expected to match the best industrial controllers used in commercial motion platforms, however it does perform fairly well and costs a fraction of an industrial servo system.

 

One draw-back of improving the control system performance in a DIY system is that it starts to show up the limitations in the other elements of the low cost motion platform. Although the low-level amplitude motion is better than before it is now clear that further improvement in this area would probably need lower stiction drives and in particular better quality drive motors and gearboxes. Although the servo control can improve things it's difficult for it to compensate for friction in the drives when driving low amplitude oscillations.

 

Hmm.... yet another improvement on the horizon!

 

UPDATE - The enhanced features of the new 28X2 chip described above have now been built into the new 40SPU-1 signal processor card which is now available to buy as a ready-built card. See the main 40SPU-1 page for more details....

 

 

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