PLC Speed Sensing

Hello!

I am looking for some help to know if I have the correct setup to monitor the speed of a motor running. The motor RRM is 3490 and the Frequency is 60 HZ. I have never tried this before so I am not sure how to completely do the calculations to see if I have a PLC fast enough to keep up. What I want to do is, use a dc prox sensor to sense a pulser disk connected to the motor. Right now I am just using a 1769-L18ER-BB1B with a 1734-IB8 input card. The pulser disk has 16 alternating poles will be either 2.5 inches or 4 inches in diameter. I do not think I am able to keep up with the current setup, but I cannot figure out how to calculate if its the prox sensor or the input card that cannot keep up. I just tried a normal allen bradley prox sensor 872c-dn5np18-d4.

Thanks for any help!!

Install a VFD and read the RPM parameter directly out of the Drive.

Yes that would be very easy but would just add another unnecessary cost. I already have everything I need and thats not what the customer wants. Thanks though!

I think that’s his point, you’re going to need different or additional hardware.

Most standard input cards have at best a .5 ms minimum on/off transition. At the rpm you listed you have 64 revolutions per second. No bueno.

3490rpm is ~58rps*, times 16 pulses/rev is over 900Hz, so of order 1ms/pulse so 0.5ms per half-state; or if I am misunderstanding "pulser disk" then maybe 450Hz or 2ms/pulse or 1ms per half.


I don't know about the prox, but unless the PLC was doing nothing else, the PLC would almost certainly miss some transitions on a typical discrete input (as _Dock_ says).

• Can you use a high-speed counter card (1734-VHSC...)?
• Can you block 15 of the pulses**?

* I remember seeing [RPS = Revoutions Per Second!] in a Flash DC Comic many moons ago


** See here, but the guts of it is Qd in the image below; I imagine there exists a single 4-bit counting chip, and a little soldering should get you there.

Here you go, 7493, see here:

If you plan to stick with 16 pulses/rev then the suggestion drbitboy made of adding a 1734-VHSC is probably the easiest thing to do.

If you don't want to/can't add another card then I would change your pulser disc from 16 pulse to 4 pulse (make sure they are 50% duty cycle pulses), restructure your I/O to get the pulser prox in the embedded I/O set and set up an event task based on the input. The four pulses makes it less likely you will keep the PLC busy doing nothing but counting pulses. The 50% duty cycle just optimizes the on/off duration so you are less likely to miss a transition.

Keith

Thanks for the response! I can definitely get a different input card if needed, if there is one that would work for this. But covering up 15 of the 16 pulses is not a bad idea. I can look into doing that if that option would work!

Cody Anderson said:

Thanks for the response! I can definitely get a different input card if needed, if there is one that would work for this. But covering up 15 of the 16 pulses is not a bad idea. I can look into doing that if that option would work!

Click to expand...

If these are holes in a disk, then ...

Keep in mind that covering 15 of the 16 transitions doesn't eliminate the base problem; that being the ability to detect an off-on-off transition. If leaving one transition pattern open works then leaving all 16 open will work as well.

Keith

kamenges said:

Keep in mind that covering 15 of the 16 transitions doesn't eliminate the base problem; that being the ability to detect an off-on-off transition. If leaving one transition pattern open works then leaving all 16 open will work as well.

Keith

Click to expand...

d'Oh! Keith is right of course; you may need the counting chip. (or use a Dremel to extend one of the holes).


Although a simple 555 timer - aka the duct tape of chips - and a few components could make a pulse stretcher.

A flip-flop output feeding the -IB8 driven by two opposing pulses' rising edges might also work, or just one for that matter.

One more idea: an Frequency-to-Voltage circuit, which has the added advantage of some filtering but still noisy and non-linear, typically.


My Dad did this with some anemometers he was building (he used to live and windsurf on the Outer Banks of North Carolina, USA).


It's a pretty simple circuit: the rising edges are stretched, by a 555, to pulses of fixed duration (less than the smallest pulse period), and the pulses charge ("fill") a capacitor while a resistor discharges ("drains") the cap. For any given speed the cap wobbles around an equilibrium voltage directly related to the rotation speed. The noise/wobble comes from the alternating charging/discharging, and is worse (lower S/N) at lower speeds.


That might be a fun lab in an electronics course, but the -VHSC card seems like the best, least expensive, and most professional, bet here: how to convert the pulses to frequency is done in the card and the PLC simply reads the number and scales it as necessary; and no duct tape.

Is this a homework assignment
the only way you can do it in a plc is with a high speed encoder card
standard inputs don't react fast enough to count
and jus using the output frequency is no good if the motor name plate says 3490 rpm
that tell me that it's a 2 pole motor and the base speed at 60 hz is 3600 rpm the 3490 is the slip at rated full load. the true rpm's will be someplace between.
you are not going to b able to count the motor rpm's without some type of a pick up
as was said before your best way would be to install a vfd and read it from the vfd panel.
the high speed counters have a rate output that can be read in the plc

It depends on the PLC, some of them have built in High Speed inputs that are connected to hardware counters, these are not enabled by default and often used for normal inputs, however, when enabled they act as high speed inputs to hardware counters and the value is able to be accessed within the program.
Mitsubishi is one example the FX range inputs X0-X7 can be used as High speed inputs even encoders up to 200khz I believe.