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incremental rotary encoder 1

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powerjunx

Electrical
Sep 13, 2002
448
hello..
any one could give me the actual application and difference of incremental and absolute rotary encoder?
i had encounter a steel rolling machine which uses as what they say an incremental encoder.., the machine now was inoperational due to it was on the alarm display:check machine rotary encoder.
anyone could give me some basic procedures, perhaps a standard procedures to test or diagnose the rotary encoder from defects and abnormalities?
my colleague test the output pulses of the unit, phase a, phase b, phase z and phase a', b' as well..but i doubt that while stall(not in motion)we got pulses on phases..does it mean that the encoder is not defective?
by the way, the unit is rated on 5 voltdc, 500 pulse where the ouput is conected to a control cards..not in PLC.

thanks..
 
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The A and B are put of phase with each other as you mentioned and are used by the card to tell direction if that is needed. Usually the encoders are mounted so the case can be rotated if a couple of screws that hold the body are untightened. Mark the position of the case before you do in case the Z output is used to set a zero position. Rotating the case will allow you to see the A & B output pulses. The Z is only once per revolution. Some encoders have differential outputs so there are two inverted outputs for each phase. Usual problems are damaged bearings that cause the optics to be scraped or the light source fails. Sometimes the coupler that links it to the machine snaps.
 
Basically, Operahouse did a good job of describing an incremental encoder. Here is a little additional info. The pulse output between phase A and phase B are 90 degrees apart. That means one phase should always be high. Even at standstill. A system can tell which direction the encoder is turning, by which phase is high first. The problem with incremental encoders is the system must remember the position of the device tied to the encoder. It can track position, once it is referenced, as long as it can read the pulses. However if the system is off, and the device the encoder is tied to is moved, there is no way for the system to know this so it has lost sync. Usually the cure for this is to re-sync the system,which is usually called homing the system, so it is not that much of a problem.
An absolute encoder has multiple connections to it, power, common and enough lines to give you a set of outputs. Each output is unique, it is really a part of a data set. 4 lines could give you a hex out of 16 cominations, 0-15. The more lines out the more resolution. The advantage of this is the absolute encoder, always gives a unique set of data. So if the system is off when the encoder is moved, once power is reapplied, the system can still read the unique value of where the encoder is, by the data word output.

Now, it sounds as if your system only needs speed feedback, so an incremental encoder is used (i.e. you don't need all the additional position info). The phasing of A-B tells direction and the rate of the pulses being output sets the speed.

You can get a rough test with a good digital meter which reads dc volts and hertz. At standstill read A to A Not, B to B not. One should read high, one should read low. Keep the comm lead of the meter on the respective Not phase. You can also read from tach Common to A, Anot, B, Bnot. Look at the good tach to get the values you should see. Now put the meter in Hertz mode and read A-Anot, B-Bnot. The hertz should be the same on both phases. Then try phase A to comm, Anot to comm, B to comm and B not to comm. The Frequency should be the same on all.

I assume you have a drive on this, so if it is a modern DC drive, put the feedback in Arm Volts mode so you can turn the motor slowly while you do the hertz test. If it is an AC drive, hopefully it will allow you to switch from closed loop vector to either open loop vector or volt/hz.

Running the drive in one of these other modes also lets you check out the encoder with a scope, which is a much better way of finding what the encoder is doing.

Good luck.
 
thank you all for your suggestion i admire it much!!

i got your point RADARRY!! ;-)i never undertake freq/hertz measurement..il try with that!

god bless!!
 
I am coming across a strange problem.

I am using a 1000 ppr incremental encoder mounted on to 1440 RPM 4 pole vertical mounting induction motor. The shafts are coupled using a flexible coupling. The encoder signals are connected an A-B PowerFlex 700 AC Drive. The encoder takes a 4.75 V to 30 V DC input. The A, B and Z signals of the encoder are connected to corresponding terminals on the drive. At low RPMs, say 150 RPM, the encoder speed parameter of the drive frequently drops to 2 - 3 RPM and then returns back to 150 RPM. This happens intermittently and the duration of this hardly for a second. On checking the counts parameter of the drive, it too suddenly goes to as low as 1. However the frequency of this problem reduces drastically as the speed of the motor is increased.

Can anyone suggest the possible cause and solution for this problem.

Thanks and regards,
Mathew
 
Have you checked the coupler lately? I have seen a number of couplers crack because of small alignment problems on machine tools.
 
A=____----____----
B=--____----____--
So both may be high or low or only one of them -- all 4
combination happens to have the same probability.

Coupler intenmittent problems usually are caused by
incorrect levels ( overload, etc.) or lack of bebouncing.



<nbucska@pcperipherals.com>
 
Suggestion: Encoder malfunctions should not be ruled out.
 
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