1734-VHSC24 with sensors

[jenk00001]

Hello to all and thanks for help in advance.
I am trying to set up sensors to detect blade RPM of a mower using 2 sensors and a VHSC24 card. I have been able to get this to work using "encoder" mode with one sensor counting and the other in B channel making count up or down so I can detect direction with some math but it is just a count. I am wondering how i could make this a live count value so that I can see as the RPMs fluctuate. It appears from the manual that my only option would be PWM? I have not had any luck getting any values in that mode.

Apologies if this is not clear.

 

[Ken Roach]

Welcome to the PLCTalk forum community !

Does this mower mechanism actually run in both directions ?

Typically to detect both speed and direction, the sensors are physically placed so that the signals are 90 degrees out of phase, which is where the term "quadrature" comes from. The order in which the rising edges of the two signals are received is opposite when the direction is opposite.

The POINT I/O VHSC modules are generally meant to count pulses and turn on local outputs very fast. That's how Encoder mode works.

But there are also three modes to measure the frequency of the input. They differ slightly in how they function. From the User Manual:

Period/Rate Mode: Count internal clocks during the On period, return a frequency. Outputs updated only at the end of the period.

Continuous/Rate Mode: Count internal clocks during the On period, return a frequency. Outputs are updated continuously during the period.

Rate Measurement Mode: Read pulses during the sample period, return a frequency.


You definitely don't want PWM mode: that actually uses the high-speed circuitry of the module to generate a pulsed *output*.

 

[jenk00001]

If wired incorrectly during assembly, the mower may turn backwards which is what I was going for with the 2nd sensor. I have some logic that will compare present value to preset value and if the count is higher - correct direction. I was thinking that the continuous rate mode would get me the information I want to see but I can only tie 1 sensor to that card.
I want to display the live RPM on an HMI and then hold a value after a set amount of time. The problem with encoder mode is that it will be an "average" number based on how long the mower runs.

 

[Ken Roach]

Thanks for those details and perspective !

The 1734-VHSCx modules only have one high-speed counter, and the Count and Speed modes really are (mostly) mutually exclusive. You can get very accurate Rate measurement or you can get position and (implicit) direction, but not both.

I think that I would approach your application by using Encoder x1 mode, so that the position values can be compared to determine direction.

It is relatively straightforward to calculate speed when you have regularly updating position values, especially if you grab the controller's free-running clock value every time the VHSC count updates, and use that to calculate the time interval between position values.

Both functions have to account for rollover of the counter (and the clock); I don't have a good example at my fingertips but you're in the right place to get recommendations !