[ahmedrehan]

I am using 2000 pulse encoder but UHU seems to not handel these pulses so i used a divided by 4 counter to reduce the pulses to 500 using CD4520 but servo run away to one side using the counter. any sujestion.

Ahmed

[H.O]

Hi,
It's not as easy as it first seems to divide the quadrature signal from an encoder like that. The number of pulses on the encoder isn't really the limit per se, it's the frequency at which the chip can count the pulses. So the faster you want to go the lower resolution you'll have to use. Are you saying that it doesn't work at all without the divider either or are you loosing position and getting O-counts as the speed increases.

Start slow and work your way upwards and see where the problem starts. I've found that 130kHz is about max what I can achive with my HP-UHU. So if your setup is about equal to mine you should be able to reach 975rpm with 2000 lines encoders (8000ppr in quadraure).

USDigital sells a quadrature divider called EDIVIDE and there's also some open source project available that uses a small Atmel uC to do the quadrature dividing. Even so I say it's better to either live with the reduced speed or buy new encoders.

[ahmedrehan]

Any link for microcontroller based divider.

Will check for the o count. which line driver you are using?

Ahmed

[H.O]

This is the site I was thinking about, there's a couple of different versions available there.

The RENCO encoders that I'm using has the AM26C31 line driver in them - so that's what I'm using.

[ahmedrehan]

i have checked o count it is giving some time 10 sometime 100 random could it be powersupply problem? also on forward movement it gives more travel and reverse it give less travel. any sujestion.

Regards

Ahmed

[H.O]

Do you see any relationship between the amount of O-counts and the speed at which you run the motor. IDo you get less O-count if you run the motor at 100rpm for a minute compared to if you run it at 1000rpm?

It could be a problem with PCB layout so switching noice from the powerstage upsets the controller. I've only used the HP-UHU so I have no experience with the other versions available but I've read numerous times that some are very sensitive to noice.

If you have a scope, look at the signals and verify the phase shift and signal levels etc. Try feeding the encoder with a separate +5V supply (common ground with the drive of course) and then run the motor very slow. Still getting O-counts?

/Henrik.

PS. I had HUGE problems with invalid counts and lost postion on my system and I worked on it for several weeks trying to figure out what was wrong. I had brand new USDigital E7P encoders with linedrivers and I tried just about every trick in the book (and some not in the book) regarding grounding, shielding, powersupply filtering, separate powersupply for the encoder, cable length, cable type, filtering - you name it. It simply refused to work. In the end I was convinced it was the encoders that was the cause. I threw the brand new USDigital encoder in the spare parts bin and got some RENCO encoders and the problem went away for good.

[contactirfu]

a very interesting reading as usual - continue folks - very informative too.

[ahmedrehan]

Hi Henrik,

Should i isolate the computer ground from UHU ground and machine ground? as again there are some Ocounts also do i need to reduce the voltage of servo motor? or increase the capacitor Bank.
Ahmed

[H.O]

It's really hard to say as each system is different but you should aim for a single ground point - called star ground.

Take the -ive of you motor powersupply to this point, take the -ive of the logic supply (12v) to this point. Take the the two GND connections on the UHU to this point. Connect the motorcable shield (if you have) to this point as well. Use shielded cable for the encoder and connect the shield to the same ground-point.

The UHU has opto-isolators on the inputs so you shouldn't need to do anything in particular with the PC GND.

And don't forget to check the capacitors on the SN75115 that I mentioned earlier. If you have them mounted, try removing them or replacing them with 22pF or something like that.

[ahmedrehan]

It's really hard to say as each system is different but you should aim for a single ground point - called star ground.

Take the -ive of you motor powersupply to this point, take the -ive of the logic supply (12v) to this point. Take the the two GND connections on the UHU to this point. Connect the motorcable shield (if you have) to this point as well. Use shielded cable for the encoder and connect the shield to the same ground-point.

The UHU has opto-isolators on the inputs so you shouldn't need to do anything in particular with the PC GND.

And don't forget to check the capacitors on the SN75115 that I mentioned earlier. If you have them mounted, try removing them or replacing them with 22pF or something like that.

Hi Henrik,

Today i made the the single ground for 12volt uhu motor supply and encoder sheild. Thank GOD the problem for o counts was solved. But another problem in x drive started as i gave command to x servo the uhu chip strarts to reset many times any sugestion but the z drive works fine.

Ahmed