Encoder application

Hi there,
Need help to use encoder and PLC to make a rejection timing.
We have a machine which detects reject at one point and timed to be rejected off the belt at the next point.

Anyone can describe a way to get same result but using an incremental encoder. Just cannot figure out how it will register when a reject is detected and start counting pulses til the next point.
Thanks in advance.

Measure the distance between the detect point and action point in encoder pulses.

As long as there are no rejects, keep the encoder counter zeroed.

If there is a reject, let the counter count until you reach the count at the action point and do something.

If you can have more than one reject in line at a time, stuff the counts into a FIFO stack.

franchois13 said:

Hi there,
Need help to use encoder and PLC to make a rejection timing.
We have a machine which detects reject at one point and timed to be rejected off the belt at the next point.

Anyone can describe a way to get same result but using an incremental encoder. Just cannot figure out how it will register when a reject is detected and start counting pulses til the next point.
Thanks in advance.

Click to expand...

Instead of the FIFO (which is a good idea) if the length is long, and the parts are small (meaning that a great number of 'tracking' positions might be active, you could make a virtual racetrack out of the encoder, resetting it at the "maximum counts between detect to reject" count. Then, when a defect is detected, you would create a flag in a echo array, the index of which would be offset by the distance described in the OR. In this way, you could have a limitless amount of rejects, all at random timing, and, if the flag were 'per count', the length of the reject could be variable, as well.
Yes, a lot more work, but, very flexible solution.

A bit-shift register would be another method that will work well with your encoder. The reject detector inserts a "1" into the shifting register, which then will shift to the memory point that represents your reject point (at the same time that the part arrives there).

Thank you guys for different ideas. For accuracy, do I need a special module for counting or the standard input module would be enough? The belt I would say is not running at high speed.

What PLC are you using?
What is the frequency of your encoder?

Lancie1 said:

A bit-shift register would be another method that will work well with your encoder. The reject detector inserts a "1" into the shifting register, which then will shift to the memory point that represents your reject point (at the same time that the part arrives there).

Click to expand...

The distance between detection and reject point is not that long, can I still use encoder for bit-shift method?

boneless said:

What PLC are you using?
What is the frequency of your encoder?

Click to expand...

PLC is Siemens S7-300 (315 2DP), and encoder we have on shelf says 10,000 PPR

The distance between detection and reject point is not that long, can I still use encoder for bit-shift method?

Click to expand...

Yes, if there ever will be more than 1 part between detection and reject points. If only 1 part in gap, you don't need anything special. Just set a bit at the detection station for a bad part, then at reject station, reject if that bit = 1.

rdrast said:

Measure the distance between the detect point and action point in encoder pulses.

As long as there are no rejects, keep the encoder counter zeroed.

If there is a reject, let the counter count until you reach the count at the action point and do something.

If you can have more than one reject in line at a time, stuff the counts into a FIFO stack.

Click to expand...

Is FIFO similar to bit shift method?

A First In, First Out instruction creates a word shift, instead of a bit shift. FIFOs are more useful where there is no available encoder. As generally used, a FIFO maintains the order of events, but not necessarily the position of an object. A bit-shift register can maintain the order of events and also track the position of objects (if used with an encoder or other tracking device).

Lancie1 said:

A First In, First Out instruction creates a word shift, instead of a bit shift. FIFOs are more useful where there is no available encoder. As generally used, a FIFO maintains the order of events, but not necessarily the position of an object. A bit-shift register can maintain the order of events and also track the position of objects (if used with an encoder or other tracking device).

Click to expand...

On our application there will only be one reject at a time so the bit shift method sounds a bit simplier to implement. Could you please describe a typical application where FIFO is best use.

One reject at a time will make your life much easier!

FIFOs can be used to track and reject products down a conveyor where there's multiple products between the inspection and reject point - loading and unloading encoder values to initialize events.

I think you should probably reconsider using the encoder ... I have never seen an encoder with 10000 PPR but that certainly doesn't mean they don't exist.....If you wanted a resolution of 1" and the encoder were driven by a disc that turned by friction with it's edge on the belt, the disc would have to be 3000" in diameter

More realistically, if the encoder were attached to the end of a shaft that drove a 4" pulley that drove the belt, you would get a resolution of .0012" per pulse... if the belt traveled at 1 foot per second, you would have an encoder frequency of 120,000 Hz

I think what I'm getting at is that since the distance is small, and it's a REJECT operation, meaning all you need to do is knock it off the belt, you might be better off with just the timer, or at any rate an encoder with less PPR. Using an encoder and having only one reject at a time, you don't need any kind of register... just count the pulses and knock the reject off when the count matches.

Using a timer means NO extra mechanical hardware or inputs or nuthin

Do you have a part sensor at the reject station? If so, you don't need a FIFO or a bit-shift register.

If you don't have a sensor at the reject station, but the distance is short AND only 1 part EVER will be between detector and reject, then you still don't need a FIFO or bit-shift.