Quadrature encoder multiplicator from 2000ppr to 2500ppr? - Page 1
Forum section: electronics / projects

author: Kjelt
author_rank: Super Contributor
timestamp: March 20, 2022, 09:24:50 am
content: I have some nice 200W AC servos from Omron that give out 2000ppr quadrature encoder signal.Unfortunately the matching servo driver is no longer produced and second hand the prices are sky high probably aimed for businesses that need one asap to continue business. (The price for the driver is higher than brand new chinese servo plus driver.)I have found some alternatives from chinese and taiwanese manufacturers. Unfortunately both have their drivers only to support 2500ppr encoding and they do not support 2000ppr.My alternatives are to replace the encoder on the servo, costing >$80 per servo and unsure if the mechanics will match.Or to somehow multiply the 2000ppr to 2500ppr.The question is how ? Doubling would be easier.I probably need a small FPGA ? Is there any digital designer here that can explain if this is feasible?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2022, 09:35:00 am
content: Digitally translating from one decoder to another is not that simple. There is just no information for the higher resolution.The possibly easiest solution would be to jet use the encoder as is and accept that the speed of the servo is 20% lower. So the translation would be at the point were commands are send to the driver, calling for and extra 25% in speed or position.

author: langwadt
author_rank: Super Contributor
timestamp: March 20, 2022, 11:54:23 am
content: Quote from: Kleinstein on March 20, 2022, 09:35:00 amI doubt that will work, It is AC servos so the encoder is probably also used as reference for generating the 3 phase drive

author: eugene
author_rank: Frequent Contributor
timestamp: March 20, 2022, 12:22:45 pm
content: Quote from: langwadt on March 20, 2022, 11:54:23 amMaybe, maybe not. You'd think a high resolution encoder would be great for commutation, but quadrature encoders have no information at startup to tell the controller what to do to get the motor turning. The BLDC servos that I've seen have all used Hall sensors for commutation. The high res quad encoders were just for position/speed control.

author: langwadt
author_rank: Super Contributor
timestamp: March 20, 2022, 12:50:02 pm
content: Quote from: eugene on March 20, 2022, 12:22:45 pmsure that hall sensors aren't just used for synchronization and "coarse" commutation? There is no point in doingan estimated angle from speed and hall sensors when you have a high resolution encoder with the actual angle for FOC

author: eugene
author_rank: Frequent Contributor
timestamp: March 20, 2022, 04:56:24 pm
content: Quote from: langwadt on March 20, 2022, 12:50:02 pmI did say maybe.In any case, I was thinking of permanent magnet BLDC motors. The OP (and you) said AC servo. My knowledge of AC servos is not deep. Do they even have phases of rotation? Somehow, I was under the impression that AC servos do not have permanent magnets. The AC current in the stator coils (plural) generates AC magnetic fields in the rotor. This results in complimentary AC currents in the rotor which, in turn, generate a magnetic field that causes the rotor to turn. In other words, the phase of the AC signal and the angle of the rotor don't need to be synchronized in any way.I'll repeat: my knowledge of AC servos is weak. I'm just keeping the conversation going in the hopes that others will correct me where I'm wrong.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2022, 05:44:35 pm
content: There are several types of AC motors. They can by synchrous like a BLDC or asynchrous like a conventional induction motor. Even if synchonous it is not clear if the motor itself uses an encoder and especially the main encoder for the servo loop. It is a good idea to check if the encoder ppr really have to match or whether this is just for convenience for the scale factors.

author: Kjelt
author_rank: Super Contributor
timestamp: March 20, 2022, 07:22:05 pm
content: It is a three phase motor. Only U,V,W output of the driver.The encoder is the feedback channel for the servo driver.Only A and B are needed and those are the only signals.The servo driver has multiple possible inputs, modbus or step/dir for instance.Those inputs define the setpoint how “far” the motor should turn and the encoder feedback is used by the servo driver to keep track of the travel progress (actual position)If the delta between expected position and actual position exceeds a preset max allowed error the servo stops in an error mode. So yes the 2000 vs 2500ppr do really matter esp for fast movements.So no hall sensors or other things.

author: langwadt
author_rank: Super Contributor
timestamp: March 20, 2022, 09:20:30 pm
content: Quote from: Kleinstein on March 20, 2022, 05:44:35 pman induction motor wouldn't need hallsensors, for a synchronous motor they would makes sense for startup and coarse commutation.In both cases it would make sense to use the high resolution encoder for FOC