QPPS units vs Encoder Speed vs Events vs Cycles

Questions about using encoders with the Roboclaw product line
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Joined: Sun Feb 03, 2019 1:51 pm

QPPS units vs Encoder Speed vs Events vs Cycles

Post by wireline » Sun Feb 03, 2019 2:13 pm


I hope someone can help as I am rather confused over terminology.

I have 2 servocity motors whose encoder specs are as follows:

Encoder Cycles Per Revolution (Encoder Shaft) 7 (Rises of Ch A)
Encoder Cycles Per Revolution (Output Shaft) 374.325 (Rises of Ch A)
Encoder Countable Events Per Revolution (Encoder Shaft) 28 (Rises & Falls of Ch A & B)
Encoder Countable Events Per Revolution (Output Shaft) 1,497.325 (Rises & Falls of Ch A & B)

https://www.gobilda.com/5201-series-53- ... w-encoder/

I am also using roboclaw_ros, which wants the following information:
max_speed Max speed allowed for motors in meters per second
ticks_per_meter The number of encoder ticks per meter of movement
Countable Events per Revolution is 4 times Encoder cycles per revolution, which is fine. However I am not able to find information explaining if the encoder speed in QPPS is the countable events, or the cycles per revolution. I need this to find the maximum speed in meters per second, and assumed I need to use the 374.325 figure (cycles), then use that to find degrees per pulse, then use v = omega * r to find tangential speed having found the pulses per second. Of course this will be wrong by a factor of 4 if I am using the wrong item - countable events or cycles per revolution. Other internet sources tell me that cycles are the same as pulses.

Should the QPPS figure from the motion control program be divided by 4 in the above calculation? I note that when rotating a motor by hand, I get about +1497 on the position, so I assume that uses "countable events". Is this true for the QPPS / speed too or does it use Cycles Per Revolution?

Secondly, what does the author of roboclaw_ros mean when s/he says "ticks"?

The use of three sets of terms terms between servocity, roboclaw and the ros node author has me quite stumped. Please can someone explain?

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Basicmicro Support
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Re: QPPS units vs Encoder Speed vs Events vs Cycles

Post by Basicmicro Support » Mon Feb 04, 2019 10:10 am

Coutable events are what you want to look at. Each edge of A and B signals is one count(eg 28 per rotation of the motor shaft. The output shaft is just 28 * the gear ratio which is apparently 1497.325. QPPS is maximum counts per second, eg RPM at 100% duty * 1497.325 / 60. Or you can just measure it by running the motor at full power(100% duty) using Motion Studio and read out the maximum speed(eg maximum counts per second).

I would suspect ticks is just another term for counts in this case. Yes the terminology on quadrature encoders in general is all over the place. Many encoder manufacturers cal Cycles per rotation "lines" to make it even more confusin. In our case our "counts" are just encoder pulse edges.

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