Rslogix 5000: calculate motor speed RPM

[drled]

Hello,

I'm trying to understand a servo motion system. I want to know the motor speed in RPM at a given speed setpoint entered in a MAJ instruction.


In the servo/axis settings the following is set:
Drive resolution: 200000 Drive counts / Motor Rev

Feedback device:
Type: Resolver
2 cycles per rev
4096 feedback counts/rev

Conversion constant:
Positioning mode: Rotary
conversion constant = 211437 drive counts/ 1cm

How can i calculate the actual motor speed in RPM based on this info?
ie. speed entered in MAJ instruction is 5cm/sec what is the motor RPM with this setpoint?

 

[widelto]

My guess is that your servo module or drive will be the one who provides rpm info to the plc and to you.
You did not mention how your servo drive is connected to the plc.

 

[drbitboy]

Conversion constants are unity i.e. 1 i.e. 1.0.


Multiplying by unity is the "Multiplicative Identity Operation;" that means that you can multiply a quantity by unity (1) and the product will be the same quantity.


For example, the OP states that [conversion constant = 211437 drive counts / 1cm], which says that [211437 drive counts] of drive motion is the same as 1cm in linear motion, or

211437 drive counts = 1cm


Dividing both sides by 1cm yields:


211437 drive counts     1cm
-------------------  =  ---
      1cm               1cm

Anything divided by itself is unity (1), so (1cm/1cm) is unity, and since (211437[drive counts]/cm) is equal to (1cm/1cm), it too is equal to unity (if A = B and B = C, then A = C).

211437 drive counts
-------------------  =  unity = 1
      1cm

Starting with the OP query of 5cm/s, and multiplying by unity = (211437[drive counts]/cm), which changes the units but does not change the quantity:

  cm           [drive counts]
5 --  * 211437 --------------
  s                   cm

 

             cm [drive counts]
= 5 * 211437 -----------------
             s  cm


Since the unit of cm is in the numerator and denominator, [cm] can be removed:

 
          [drive counts]
= 1057185 --------------
                s

Other conversion constants from common knowledge and from the OP:

                           s
60s = 1min ==> unity = 60 ---
                          min


                                              1        Rev
200000[drive counts] = 1 [Rev] ==> unity = ------ -------------
                                           200000 [drive counts]

So, multiplying by unity several times, which does not change the quantity, and keeping track of units:

        [drive counts]        s       1          Rev
1057185 --------------  * 60 --- * ------  --------------
              s              min   200000  [drive counts]
 

                          [drive counts] s Rev
= (1057185 * 60 / 200000) --------------------
                           s min [drive counts]

Removing any units that are in both numerator an denominator (s, [drive counts]), yield the answer:

        Rev
= 317.2 --- = 317.2 RPM
        min

90% of engineering is multiplying by unity; the other 10% is understanding the value of unity. If this is a class assignment, then send me 50% of your tuition, because if you understand this fully then the rest of your classes will be trivial.

 

[drled]

drbitboy, thank you very much for the clear explanation.

It isn't a class assignment, we are looking to replace some motors that are end of life and want to check the expected max. RPM.

 

[drbitboy]

drbitboy, thank you very much for the clear explanation.

It isn't a class assignment, we are looking to replace some motors that are end of life and want to check the expected max. RPM.

You're welcome. Still, if I were in your place, rather than only accept an answer you got from some ego-inflated, bozo Yank on the Internet, I would also put a strobe light or some other device* on the running motor to validate that calculation is within a few percent.


* You could make a circular disk with five radial slots around the circumference, plus a finger hole offset from the center, and screw the disk center into the end of a broom handle, and if the motor appears stopped when viewed through the slots, while using the finger hole to spin the disk near once per second, then an answer of order 300RPM (300 [Rev/min] * 1/60 [min/s] = 5 [Rev/s]) is correct.