Stepping conveyor control...

Borte · Jun 9, 2005

Hello Guys!

This is not really a plc question; it’s more a question about what solution to go for…

I have a conveyor chain running inside a oven. The conveyor is working in a "stepping" manner, meaning it receives a part on the loading side. Moves one "step" forward then start’s all over again until the parts hits the unloading side of the conveyor. Then it waits for a robot to unload the parts before allowing next step.

The "step" size is controlled by means of a sensor sensing on 5 pins mounted on the motor axle. The plc starts the motor then runs it until the sensor goes on then it stops. This is working fine except that I now have to find a way to control this "step" size from the plc. The reason for this is that we need wider spacing between the parts on the conveyor.

My first though was that I had to get an encoder and mount it to the motor axle and then control the motor based on the value from the encoder, but then I started thinking about the 5 pins I have on the axle. Would it be possible to use this as a simple encoder? I was thinking about creating a counter in the plc based on the clock (10ms resolution) in the plc and then controlling this counter based on the 5 pins. This way I could get a precise measurement between the pins and thereby knowing how much to turn the axle to get to a specified position.

Does anyone have any ideas / comments on this? I have never actually done this it was just an idea I came up with… I’m not even sure if it’s possible.

Cheers
Borte

jstolaruk · Jun 9, 2005

That's what I would do. On the surface, I would tend to think that the encoder would give you more resolution than you need for the application.

brucechase · Jun 9, 2005

Something simple

Depending on what kind of resolution you may need, I use a photoeye to load my oven. The part comes up to the photoeye then I run it for a fixed amount of time after the part clears the photoeye. By adjusting the timer, you can get any spacing you want.

rsdoran · Jun 9, 2005

I have worked with several applications that used a sensor to read "pins" or teeth. Determine the distance traveled per "pin" and/or per revolution. Use that to define the distance to travel as needed. Lets say 1 revolution (5 counts) was 100mm and you wanted to space 200mm then you would need 10 counts.

As was mentioned the input resolution is limited so the accuracy of the "move" may be harder to obtain but can be done.

If you need more resolution you can change the "wheel" to a device with more pins or teeth.

As was mentioned you can use time to provide the gap and not sure you would need another sensor. When the product is loaded just move it using time to obtain the distance you want then load/unload as desired. The use of a sensor at the load/unload point could "verify" product present.

DanielCamacho · Jun 9, 2005

Courious...

I am just a little more courious about the exit of the piece. Does that area where the robot picks up the piece have its own sensor and centering station? Or was it all working only based on revolutions of the shaft? And what PLC is this?

Borte · Jun 10, 2005

I am just a little more courious about the exit of the piece. Does that area where the robot picks up the piece have its own sensor and centering station? Or was it all working only based on revolutions of the shaft? And what PLC is this?

Yes the exit has its own sensors and centering station.
The plc is a s7 series.

I have worked with several applications that used a sensor to read "pins" or teeth. Determine the distance traveled per "pin" and/or per revolution. Use that to define the distance to travel as needed. Lets say 1 revolution (5 counts) was 100mm and you wanted to space 200mm then you would need 10 counts.

As was mentioned the input resolution is limited so the accuracy of the "move" may be harder to obtain but can be done.

If you need more resolution you can change the "wheel" to a device with more pins or teeth.

As was mentioned you can use time to provide the gap and not sure you would need another sensor. When the product is loaded just move it using time to obtain the distance you want then load/unload as desired. The use of a sensor at the load/unload point could "verify" product present.

I don't want to change the hardware or use a timer to run it, because the problem is that I will have 5 sensing positions pr. revolution so using a timer will not help. I have to do a count of some sort. A count of only 5 times pr. revolution will not give the desired resolution for this application. That's why I was thinking about using the internal clock pulses available in the plc to create a better resolution.

Doug-P · Jun 10, 2005

Borte said:
A count of only 5 times pr. revolution will not give the desired resolution for this application.

Just for the record, what is the desired resolution? Is the stopping position fixed or variable? Would ten PPR be enough?

Borte said:
That's why I was thinking about using the internal clock pulses available in the plc to create a better resolution.

This can bring its own troubles. Does the line speed change if the load weight or ambient temperature varies?

Borte · Jun 10, 2005

Just for the record, what is the desired resolution? Is the stopping position fixed or variable? Would ten PPR be enough?

I don't have a requirement for the resolution, just that it has to be better and more flexible than the existing solution.

This can bring its own troubles. Does the line speed change if the load weight or ambient temperature varies?

The load and ambient temperature is not a factor here, since the chain is running inside a oven with 500 degrees celsius and the stepping (with parts) will only happen at this temperature.

But anyhow if I'm using the sensors to retrigg the internal counter (based on the time) the counter should be fairly precise with the actual distance moved.

Cheers
Borte

tommy23t · Jun 10, 2005

How about a combination of above suggestions?- You could use the counter for rough position then start a timer when that count is reached. In other words, you want to stop at position 6.5. Use your counter to count to 6, then start the timer to cruise in to your final .5 position. Not the easiest to adjust, but the easiest to install since it's already there.
Tom

OkiePC · Jun 10, 2005

You can use the existing pins/sensor, and the formula D=RT (distance=rate*time) to get a pretty good feedback position depending on the answers to the following questions.
1. What is the speed of the chain conveyor?
2. Does it vary a lot?
3. What's the control type (analog speed/start stop)?
4. Does the PLC "know" the speed of the conveyor or is it a separate operator function?
However, there may be some positioning error with this method so you need to do the math and figure out what your resolution needs to be. Find the actual speed (range if it varies) of the line, and how much error can it tolerate. With this information you can determine what type of feedback is required.

Stepping conveyor control...

Borte

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jstolaruk

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brucechase

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rsdoran

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DanielCamacho

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Borte

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Doug-P

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Borte

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tommy23t

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OkiePC

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