Encoder and PLC Program Question

Hi guys,

I just have a simple question regarding encoder uses:

I have 2 encoders coupled to 2 different wheels. One wheel is driven by a motor, the other is an 'idle' wheel. The motor is driving a wheel which pushes a length of product. To prevent the driven wheel slipping on the product, the idle wheel is used to detect the motion of the product; i.e. if the drive wheel encoder is detecting movement and the idle wheel encoder is not, the drive wheel is slipping on the product.

Basically, I need the PLC to compare the signals from each encoder to verify that when the drive wheel is in motion, the product is too. Can I use the encoder signals for this sort of function? If so, how is this written into the ladder logic (RSLogix 500)? (I'm not looking for someone to do it for me)

Also, the drive wheel encoder is used to measure (calculate) RPM and the product can move in either forward or backward.


Thanks for any help you can give.

Joel

How are the encoders connected to the PLC system? How closely do their counts match each other if there is no slipping?

bernie_carlton said:

How are the encoders connected to the PLC system? How closely do their counts match each other if there is no slipping?

Click to expand...

One encoder is connected to a high speed counter (for determining RPM), however the other I am not sure; thats really half my question.

I wasn't looking at counting the pulses from the idle wheels to detect motion. Is it possible to connect the encoders to normal digital inputs (as well as a HSC input for the drive wheel encoder) which energise XIC's that stay on whilst the encoder shaft is rotating?

Joel

Since one is using the HSC it probably is too fast for a standard input to register.

To get any information from an encoder signal the input must be able to react to the ON/OFF changes.

If the signal is being processed by ordinary ladder logic then the ON and OFF times must at least be as long or longer than the scan time.

Exactly which PLC is being used and how is the current encoder connected - to an onboard high speed input or to an add-on HSC card.

The encoder connected to hsc is attached straight to motor? The other encoder is attached to wheel that is running from movement of the product? How fast is product moving?

I'm using a Micrologix 1400 L32BWA, it has an onboard HSC. The drive wheel encoder is connected to a HSC; i've yet to connect the idle wheel encoder.

@TurpoUrpo
Yes thats correct, HSC is connected to drive wheel encoder. The product moves as slow as 0.1 m/s, up to 1.5 m/s; however provision is made for a speed up to 3.6 m/s.

Joel

How many pulses per second at 3.6 meters/second?

I'm using a 500 PPR encoder, 3.6 m/s = 460 rpm, so about 3833 pulses/second.

So, far faster than a standard input can handle.

You need hsc for that one too.

So really, I should be connecting both encoders to HSC inputs to detect if the either wheel is in motion, then evaluate if either encoder is sending pulses to the PLC over a period of time. Since the slowest speed (17 rpm) amounts to 142 pulses/second, the period of time would have to sufficient to capture pulses at this speed while also accounting for the pulses not being in sync. I can then use the PLC to check if both HSC inputs are receiving pulses over 'x' period of time and then control the output based on this.

Does that sound right/feasible?