Build it custom or buy it?

Aug 05, 2005 4 Replies
C

Hi:



I need to do this:


  1. Count quadrature shaft encoder signal with max frequency (for a single phase) of about 90kHz.
  2. Output a 16 bit word where each of the 16 bits changes state at some specific angle of the shaft encoder. There could potentially be an arbitrary number of state changes per bit per revolution, so there would be a list of angles at which to go high or low for each bit. But the list is unlikely to be more than 3-4 in length.
  3. However, the word pattern from one revolution to the next might not be the same, but would repeat after N revolutions. So there is not only a list of angles to flip bits within each revolution, but a new list for each revolution.
  4. On 1-4 of the 16 output bits, the angular event should trigger a time-domain pulse width sequence, with up to at least 4 pulses with adjustable widths and separations. At least 1us resolution and jitter, preferrably
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J

For recent projects I was in the exact same situation as yours. The application was to controll fuel injection magnetics with high accuracy to crancshaft angle. The only diffrence: I had only one output (not 16) per device and additional PWM for pressure regulation. Totally number of about

20 pcs for pump test required. Angle Parameters has now to move with the pressure error and crancshaft speed becouse magnet delay is constant.

I believe we all agree, those things are impossible with PLC. As far I am familiar with HW and SW, the first attempt was a PLD solution using several Lattice CPLD. Somewhat later I remembered a recent desgin what was a servo controller for DC brushless motor using a Texas Instruments F240 DSP. That Device is about $800 usable from stock without any HW modification and I was lucky to have the complete firmware sources.

1) The CAN Bus links to a PC for parameter setup and test result statistics 2) The power stage can drive the magnets directly 3) Interface to both incremental encoder and resolver are available 4) SW updates in field possible to meet new requirements

To mention in my case, a software comparision of the angle position was not accurate enough. The servo motor controllers most time have several HW comparators for commutation control what can be used to switch the outputs by HW and generate a interrupt to enable the SW to load next event distance

In the meantime there are 5 years over and it was a good desicion not to use a CPLD solution with less flexibility. The SW had about a dozen of changes and the magnets changed for more speed. Formerly they were simply on-off and today they have low inductance with start pulse and further PWM current limiting to reduce the heat...

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R

#3. Me too. Build it once, with a fair number of spare IOs, produce a stack and forget about them.

Rene

Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
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G

Hi Chris:

The following is a link to a magazine article describing a hardware solution.

formatting link

I think is also lays a foundation for a software solution.

gm

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