Divided by 4 Counter for 2000 pulse encoder

[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.

[ahmedrehan]

Originally Posted by 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.

Encoder are FANUC 2000pulse made by fijitsu.

If i run fast it gives more o count if run slow it give less o counts.

It is hard to find new encoder here in Pakistan. I have to import them and will atleast takes 2-3weeks.

Ahmed Rehan

[H.O]

Can you run at any speed without getting O-counts?

I initally started with 3600lines (14400ppr) encoders and it worked fine up to ~130kHz (540rpm). Based on that number and my ballscrew pitch and gearing I ordered the USDigital encoders with 625 lines (2500ppr) which will give me a theoretical resoultion of 0.001mm. However, with the USDigital encoders it was impossible to get it to run reliably at ANY speed no matter what. I then got the Renco encoders, also 625 lines and it works perfectly. I got both the USDigital and the Renco encoder directly from the US. Renco had 6 weeks leadtime.... :-(

I'm sorry I can't be of any more help... I know my setup works up to 130kHz so the UHU chip is good for at least that. If your FANUC encoders are 2000lines (8000ppr) then you should be able to reach 975rpm if all else is equal.

Again, can you get it to run at any speed without getting O-counts? What version of the UHU hardware (PCB etc) are you using?

/Henrik.

[ahmedrehan]

1. attach please find the version of pcb but the important thing is i am using external mosfet module not the on board mosfet. may be they are picking up some noise.

2. Any check points
3. can you send me the screen shot of analyzer software how it looks when the servo is properly tuned.

Ahmed

[ahmedrehan]

Henrik,

Do you think i should put inductor in series with motor? also i have seen in other post about load dump should i use that?

Ahmed

[H.O]

Hi Ahmed,
The powerdump circuit is only needed if there's a risk that the voltage of the powersupply goes beyond the maximum voltage for the drive while deccelerating. It's not a part of this particular problem so leave it for now ;-)

It's really hard to guess what the problem is here.... You still haven't told me if it works at any speed or if you always get O-counts, no matter what the speed is. Try running at 100rpm or something for a minute, then check the O-count.

Also, you should be aware that the max frequency the chip can keep up with drops when you use the analyse function in the software so don't have that turned on while trying to figure out this problem.

Are you using shielded cable for the encoder? Shield grounded at one end only?

Don't worry about tuning just yet - you have to figure out this problem first.... I'll grab a screenshot next time I fire mine up.

/Henrik.

[ahmedrehan]

When run at very slow speed it dose not give o count at higher speed it gives more ocount and in one direction it give more ocout than in other direction with same setting.

what is the max value for o count for drive or it should always be zero?

the encoder is with original wiring for fanuc 6t control, encoder are fitted with line driver and i have used 74sn115 for line reciver.

Thanks for your help.

Regards

Ahmed