On Wed, 8 Apr 2009, Jon Elson wrote:
> Date: Wed, 08 Apr 2009 01:12:46 -0500
> From: Jon Elson <[email protected]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <[email protected]>
> To: "Enhanced Machine Controller (EMC)" <[email protected]>
> Subject: Re: [Emc-users] ilowpass filter
>
> Sebastian Kuzminsky wrote:
>>
>>
>> If the encoder counter provides only the number of edges seen since
>> startup (or since some reset event), then it's really hard to estimate
>> velocity well - it'll be quantised and crunchy (though as you say, maybe
>> filtering could help).
>>
> Yup, that is what my boards do right now. They do have a fast clock (1
> MHz for digital filtering) but
> the FPGAs run at 10 MHz (old boards) and 40 MHz (newest ones). But, I
> hate to make a major modification if there is a decent way to do what we
> need in the PC. Maybe we can't, though. Once the information is lost,
> it can't be reconstructed. It just seems like 1000 samples/second OUGHT
> to be fast enough. My Allen-Bradley 7320 control had a 100 Hz servo
> cycle, and it worked fine, although clearly not as high a bandwidth as
> EMC can now provide.
>
> Jon
As you mentioned before, raising the sample rate alleviates some of these
problems. Especially for torque or voltage mode amps. 100 Hz might be OK for a
velocity mode amplifier where most or all of the D term is handled in the
motor drive, but would create to much phase shift in the D term (where you can
least afford it) with voltage or current mode amplifiers.
I rank motor drive systems like this. simplest to most complex:
1. Voltage mode, bare HBridges. Our Hbridges are like this.
Voltage mode requires high sample rates, probably greater then 1 KHz
Voltage more also _requires_ FF1 to compensate for BEMF. The lower output
impedance of votage mode amplifiers means less D term is needed than
current mode amplifiers. The PID output controls acceleration, but
acceleration is not a linear function of PID output.
2. Current (torque) mode. Our 8I20 is like this. Like Voltage mode, a high
sample rate is needed. The PID output controls acceleration.
3. Velocity mode. These amplifiers have a built in velocity loop, so normally
very little D term is required. Properly tuned, a quite slow sample rate
will work. PID output controls Velocity.
>
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Peter Wallace
Mesa Electronics
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