Rotary Encoder

Hello Guys,
In order to measure the length of a product using a photo-eye and a Timer .. It's not a reliable operation even if you got it right for some time, So how to Program a Rotary encoder to measure the length of a product you want to accurately cut, and is there any other solutions to this application ??

Many PLC's have High speed inputs or dedicated cards for reading either encoders or high speed pulses off a sensor.
A single ended encoder will give x pulses per revolution, this is connected to one of the high speed inputs (many PLC's i.e. brick type like RW, Siemens, Mitsubishi etc.) have the first few inputs that can be used as high speed anywhere from 50-200khz, depending on the speed of the product is travelling the accuracy you expect a simple high speed proximity with a toothed wheel can give good results. The high speed inputs on these PLC's will almost certainly have high speed counter registers that are built into the hardware, so independent of the scan time of the PLC, when you configure the HSC using the standard functions it takes care of the counting. you need to tell us what make of PLC you are intending to use, the expected speed of the component you want to measure i.e. mm per second or even metres per minute, from this we may be able to give you some ideas.

In order to measure the length of a product

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It depends on what you mean. Are the products of varying length and you are trying to measure them? Are you trying to get a repeat length of the same size, say a cutting machine? Do you need to vary the length from one machine cycle to the next, or do you only need to change the length on a day? Are these long lengths or short lengths? We need ore information to say what might be the best measuring solution.

Do you need to sense length or sense movement ?

For sensing the movement of the material an encoder is a really good solution.
The trick is to have something transfer the motion from the material to the encoder.
We have had good results with a dedicated encoder wheel that has no other function than to pick up the movement of the material. The encoder wheel can move up/down freely and has a good grip on the material either simply by its own weight or by spring tension. No problem getting +/- 0.1 mm even over long distances which is enough for our application.

BryanG said:

It depends on what you mean. Are the products of varying length and you are trying to measure them? Are you trying to get a repeat length of the same size, say a cutting machine? Do you need to vary the length from one machine cycle to the next, or do you only need to change the length on a day? Are these long lengths or short lengths? We need ore information to say what might be the best measuring solution.

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Yes, it's A cutting Machine, but they have different shapes and sizes they need to customize the length to be cut from HMI .... So the operator will enter the length to be cut in Centimeters then start the automatic operation on that length until he decides to change it later ...

parky said:

Many PLC's have High speed inputs or dedicated cards for reading either encoders or high speed pulses off a sensor.
A single ended encoder will give x pulses per revolution, this is connected to one of the high speed inputs (many PLC's i.e. brick type like RW, Siemens, Mitsubishi etc.) have the first few inputs that can be used as high speed anywhere from 50-200khz, depending on the speed of the product is travelling the accuracy you expect a simple high speed proximity with a toothed wheel can give good results. The high speed inputs on these PLC's will almost certainly have high speed counter registers that are built into the hardware, so independent of the scan time of the PLC, when you configure the HSC using the standard functions it takes care of the counting. you need to tell us what make of PLC you are intending to use, the expected speed of the component you want to measure i.e. mm per second or even metres per minute, from this we may be able to give you some ideas.

Click to expand...

I will use Delta PLC , It has HSC built in .. and I can Read the input pulses from the encoder, I need to convert that Pulses into length unit, so the operator can enter the length of the product to be cut on HMI

how many encoder pulses will there be per cm, or per metre, of material?

will the material be moving continuously? or will it be stopped before and after each cut, and before the material starts moving for the next cut?

can the PLC control the motion of the material? I.e. can it make the material move faster or slower? Can it stop and stop the material motion?

how does the PLC initiate the cutting process?

ielkhatib said:

... I need to convert that Pulses into length unit, ...

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Is this the main question that you are asking? Because if it is, I think you actually want to convert the other way around, i.e. you want to convert the length, specified by the operator, into some number of pulses, call it Ptarget, and then have the PLC count the pulses as the material moves, and when that pulse count reaches Ptarget you want the PLC to cut the material.

drbitboy said:

how many encoder pulses will there be per cm, or per metre, of material?

will the material be moving continuously? or will it be stopped before and after each cut, and before the material starts moving for the next cut?

can the PLC control the motion of the material? I.e. can it make the material move faster or slower? Can it stop and stop the material motion?

how does the PLC initiate the cutting process?

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- The encoder resolution is 1024 pulses per revolution.
- preferably continuously moving, but I can make a quick stop before and after each cut..
- i don't understand the question.
- I can install a sensor to detect the beginning of the product.

drbitboy, DMD.

DMD = dentist.

Peter Nachtwey said:

drbitboy, DMD.

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Not the first time ... LOL

Dear fellow, The image posted by @Peter Nachtwey illustrates the problem perfectly. If I asked you "Please advise me how to fix my automobile," but gave you no more information (make and model and year of automobile, gas or electric, etc.), do you think you could provide a useful answer? Well, this PLC and this material and this cutting device are your automobile, and the people reading this thread know almost nothing about it. How do you expect anyone to provide a useful answer?

ielkhatib said:

drbitboy said:

how many encoder pulses will there be per cm, or per metre, of material?

Click to expand...

- The encoder resolution is 1024 pulses per revolution.

Click to expand...

Oh dear. That does not answer the question. I asked for pulses per length of material motion, not pulses per revolution. What is revolving? Is the material revolving, or is something else revolving? If the material is not revolving, how much does the material move in one revolution, of whatever is revolving

ielkhatib said:

drbitboy said:

will the material be moving continuously? or will it be stopped before and after each cut, and before the material starts moving for the next cut?

Click to expand...

- preferably continuously moving, but I can make a quick stop before and after each cut..

Click to expand...

This partially answers the next question.

ielkhatib said:

drbitboy said:

can the PLC control the motion of the material? I.e. can it make the material move faster or slower? Can it stop and stop the material motion?

Click to expand...

- i don't understand the question.

Click to expand...

Can the PLC start and stop the material from moving (apparently yes, from the previous answer)?

Can the PLC control the speed of the material motion, beyond just stopping and starting the material motion?

ielkhatib said:

drbitboy said:

how does the PLC initiate the cutting process?

Click to expand...

- I can install a sensor to detect the beginning of the product.

Click to expand...

Oh dear. I meant, as the material is moving and the PLC is counting pulses, and the PLC decides the time has come to make the cut, how does it activate the cutting device (knife, saw, laser, light sabre, etc.)?

ielkhatib said:

... I need to convert that Pulses into length unit, ...

Click to expand...

Is this the main question that you are asking? Because if it is, I think you actually want to convert the other way around, i.e. you want to convert the length, specified by the operator, into some number of pulses, call it Ptarget, and then have the PLC count the pulses as the material moves, and when that pulse count reaches Ptarget you want the PLC to cut the material.

These are all links to Siemens examples but the principles are the same. Download the PDF for each and read the relevant section.


Measuring using time based I/O
Measuring using HSC
This is very Siemens specific hardware features that may or may not be available in Delta. There's also and application example, somewhere.

drbitboy said:

Not the first time ... LOL

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It won't be the last.
You are now drbitboy DMD.


I wish people would state their application first so we know the context of the questions.

Peter Nachtwey said:

It won't be the last.
You are now drbitboy DMD.


I wish people would state their application first so we know the context of the questions.

Click to expand...

LOL

If wishes were horses, beggars would ride.

I am pretty sure this OP only needs to know how to multiply by unity.

drbitboy said:

Dear fellow, The image posted by @Peter Nachtwey illustrates the problem perfectly. If I asked you "Please advise me how to fix my automobile," but gave you no more information (make and model and year of automobile, gas or electric, etc.), do you think you could provide a useful answer? Well, this PLC and this material and this cutting device are your automobile, and the people reading this thread know almost nothing about it. How do you expect anyone to provide a useful answer?





Oh dear. That does not answer the question. I asked for pulses per length of material motion, not pulses per revolution. What is revolving? Is the material revolving, or is something else revolving? If the material is not revolving, how much does the material move in one revolution, of whatever is revolving




This partially answers the next question.





Can the PLC start and stop the material from moving (apparently yes, from the previous answer)?

Can the PLC control the speed of the material motion, beyond just stopping and starting the material motion?



Oh dear. I meant, as the material is moving and the PLC is counting pulses, and the PLC decides the time has come to make the cut, how does it activate the cutting device (knife, saw, laser, light sabre, etc.)?




Is this the main question that you are asking? Because if it is, I think you actually want to convert the other way around, i.e. you want to convert the length, specified by the operator, into some number of pulses, call it Ptarget, and then have the PLC count the pulses as the material moves, and when that pulse count reaches Ptarget you want the PLC to cut the material.

Click to expand...

Okay, I will provide you with more details about the application I need to do ..
1. The product enters a conveyor..
2. The encoder will start counting and When the product reaches the length specified from HMI, a knife that is installed on an upper head of a carriage will get down, as the Head is coming down the carriage itself will be moving with the same speed of the conveyor ... The knife cuts the product and get back up and the carriage goes back to its original place with maximum speed ..
There are limit switches to determine the upper and lower place for the knife, and the same way the carriage has two limit switches to determine the start and the end of the stroke ..

That's the whole process ..
Now the system is working using some mechanical way to count and activate a coil that starts the knife when it reaches the required length ...
Again what I needed is to find a way to replace the mechanical system with an electrical system to determine the length of the material to be cut .. I thought an encoder might be a good solution .. so I need to know how to use the encoder count to calculate the travelled length ...