Phew. Finally got the servo drive working again. In the end, I merely had to replace the burnt out resistors in the step and direction optos. I previously desoldered the buggered 75ALS174 line driver device. As I don't anticipate any need to use the encoder feedback outputs from the drive back to the Acorn, I left the device position unpopulated on the PCB. There is no means of monitoring whether it's there, so the drive works fine without it. All is well now unless you remove the housing and see the abortionate bodge I left behind. Best not to do that.
So, having finally repaired the trail of destruction left by The Stupid Fat bloke's ham fisted attempts at connecting up the servo drive to the Acorn controller, I can now get to the bottom of the myriad parameters that need to be set up for correct operation. Some of these reside in the servo amp itself and others within the Acorn, which are best configured using the Acorn Wizard.
I have to take account of a few factors:
- The reduction ratio due to the timing belt drive, with 34t driver and 60t driven pulleys.
- The harmonic drive is a nominal 50:1 - but in the arrangement I've chosen, the effective ratio is actually 51:1. That's the sort of trap you could easily fall foul of.
- It's possible to set the number of steps per rev within the servo drive, also taking account of both the belt and harmonic drive. The manual is cryptic in places - nothing as powerful as the Chinglish I encountered in the Newker "manual" but still lacking explanation or examples to clarify your understanding. Some experimentation is required, unfortunately.
So the overall reduction due to the belt and harmonic drive is
- Pn203 = 36000
- Pn202 = 5898240
- Pn203 = 4000
- Pn202 = 65535
With effectively 4 pulses per clock cycle, you can see how the a "1000 PPR" pulse frequency from the Acorn would work with this.
Good to have a sanity / cross check giving results that make sense.
So here are the final parameter values, as reported by the parameter editor in the SigmaWin+ software: