Encoder problem

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25 Mar 2010 16:50 #2441 by faandevos
Encoder problem was created by faandevos

I am new to the retro-fit game and have very limited electronic knowledge.

Am busy with a Hermle PF 1000, it is very similar to Deckel FP4. I used mesa 5i20, 7i37 and 7129 hardware. Had no DC drives for the gettys 220volt servo, so I fitted 5 volt TTL AMD 102 V encoders from CUI inc. and hooked them up to the Hbridge drivers. The problem is that the encoder will randomly start running when I jog a axis or switch on the main spindle on high speed. I used shielded, twisted pair (0.25mm) wire for the encoders.
Followed all the instructions in the manual for shielding the encoders.
I do not understand electrical noise and would love some help.
Will the 7i29 Hbridge handle the 220 volt servo motor, if there is no brake circuit?

What do I need to do ?

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25 Mar 2010 18:37 #2445 by BigJohnT
Replied by BigJohnT on topic Re:Encoder problem
The shield wire for your encoder should be only grounded one one end. That's all I know about your questions.


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26 Mar 2010 14:50 - 26 Mar 2010 15:03 #2454 by PCW
Replied by PCW on topic Re:Encoder problem
TTL encoders are probably not the best choice if you have a larger machine or long cable runs.
Differential encoders will give you much better noise resistance. Depending on your encoder resolution and maximum
servo speed you may be able to use the encoder counters filter to reject the noise. Unfortunately the HostMot2 driver does not
have support for this so you must use the drivers raw-write capability (man hostmot2) to setup the encoders filterRate register:

The default encoder input filter on the 5I20 (with filter=on) is 15 counts of the 33 MHz clock = 30 nSX15 = 450 ns
so noise pulses shorter than 450 nS will be rejected. The sample clock (33 MHz ) of the encoders can be changed
by writing the QfilterRate register:

0x3400 QfilterRate 12 bit Quadrature counter filter rate
count rate is ClockLow/(QFilterRate+2).
Any divisor with MSb set = divide by 1

Default QfilterRate is 0x800h = divide by one so 33 MHz on 5I20
So for example if you use raw-write and set QfilterRate to say 0x0008 (divide by 10)
the basic encoder filter will run at 3.3 MHz instead of the default 33 MHz This means that input noise
pulses less than 4.5 uS will be rejected (but you will have reduced the maximum count rate to to about 440 KHz)

As far as the brake goes this depends on your power supply voltage, and the output capacitance of your power supply
if you are close to the 7I29s maximum motor voltage, you will need a brake circuit to limit the power supply voltage
rise to a safe level during decceleration.
Last edit: 26 Mar 2010 15:03 by PCW. Reason: clarity

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