grubb

Controlling Cylinder Stroke Length

16 posts in this topic

Hello - I've recently started my adventure into PLC programming and I've found this site to be very helpful. I took on a project that requires control of a cylinder - the short and skinny is that the cylinder will have to provide position feedback and stroke to various set lengths determined by position of a selector switch. So - If position 1 is selected, the cylinder will stroke 20"inches. If Position 2 is selected, cylinder will stroke to 25"inches - so on and so on.

 

My questions is how to best go about this process. I may be having a brain-block and not viewing this clearly, but something is not clicking for me and wanted some advice. Planned on using Allen-Bradley's Micro850 PLC (had one on the shelf, and CCW software is free so why not).

Thanks

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1. How is the cylinder actuated: pneumatic, hydraulic, electrically driven?

2. How is position being determined?

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Welcome,

You will find it almost impossible to control a driven rod cylinder, you would require a rodless cylinder which can be controlled, with feedback given by simple reed switches mounted at the controlled stop positions.

Rodless cylinder

Steve

Edited by nehpets
poor english

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@pop29684 1. I am open to options, would prefer to stay away from hydraulic though? It is being used in a food application and I feel like hydraulic leaks in the area would be bad..

This is my first project of this nature so really just trying to bounce ideas out there and get input from others with more knowledge than me, thanks

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I do not think the rodless cylinder would provide enough force at the end for this application? I could be wrong though, would have to investigate further.

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Unless you can explain how the cylinder is being used, the forces and speeds required, hard to help you.

Might I suggest you look at something like an IAI Robocylinder.  I've used them as a standalone application or controlled through either digital IO or EtherNet/IP from a PLC.  I've used them in clean room environments, as well.

https://www.intelligentactuator.com/robo-cylinders/

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@pcmccartney1 thanks for the link, very interesting and going to dive deeper into researching on those cylinders.

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There are (2) tasks at hand...move a cylinder (pneumatic or electric or hydraulic?), move the cylinder to a position.  The 2nd task will require position feedback and closed-loop control.  Both tasks are highly mechanical for specification (the controls guys take care of the programming and wiring).  There is no way to concept the controls with a questionable mechanical concept.

What is the specification for position accuracy?  How precise must the cylinder final destination be held to (+/- fractional thousandths, +/- an inch).

What is the specification for cylinder speed?  How much time is allowed for the cylinder position to be changed.

What is the specification for total load to be moved?

Reed switches are often used in cylinders for general cylinder actuation feedback.  However, these buggers are a pain to adjust with high accuracy.  Reed switches are binary...they only tell you if you are at Position A or B.  If you budget is low and the specification is Low Accuracy, a cylinder with multiple reed switches per the customer location is an option.

If your budget is higher and the specification is for High Accuracy, a servo cylinder is an option (servo-pneumatic or servo-hydraulic).  Such as a Festo servopneumatic system.   https://www.festo.com/cms/nl-be_be/16425.htm

Parker, Festo, IAI...good places to start

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One way to do this would be to position Prox. switches along your travel, and start and stop the cylinder.

This would also require that you  use a center off, or center hold valve, and while not perfect, it would work with out having to use a more expensive linear slide.

But a linear slide would give you a much more positive positioning opinion.

Just my maintenance man opinion.

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@kaiser_will thanks for the information; I’d prefer to go with pneumatic or electric controlled cylinder. Must press within a equal to or less than 0.5mm threshold, and will have various settings for stroke length (lets just say 3). So a selector switch to determine three various stroke length set points. Time allowance would not be hateful, whenever the operator is clear (at the proper operating station) to hit the cycle button the cylinder extends to x length and retracts to ‘home position’/retracts. The servo cylinders seem ideal to allow better control from the information you provided, thanks.

 

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I’ve done some minor programming projects (mostly conveyor systems). And wanted to tackle something new, so all the information/help everyone is providing is greatly appreciated!

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what am I missing???

how do you stop a  pneumatic cylinder mid-stroke without a mechanical means???

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If you have two solenoid valves with center close porting, then yes you might be able to move to a mid position.  It might not be very pretty and accuracy would be poor.  Hence, the suggestions of a servo or slide.  Might have to have some additional means of exhaust.

I have used a cylinder that was capable of retracted and then two extended positions, but technically is was two cylinders in one and required two of single acting/spring solenoids.  With both solenoids off it went to the retracted position, one solenoid on it went to first extended position, then with both solenoids on it went to the second extended position.  It's been a few years, but I think it was by Festo. 

Edited by pcmccartney1

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Run the cylinder against a physical stop. Add cushioning if necessary. Make the stops moveable - there are lots of ways to do this, multiple actuated stops, lead screw, depending on how many and how finely tuned you need it. If you need a sensor add a sensor to the moveable part(s).

Simple, cheap and easy to understand.

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how many positions we are talking about? you can have cascaded cylinders and control each individually. then you would have sum/difference of all strokes.

or just don't use pneumatics, there are servo actuators for same job. still clean...

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We have a depositing machine which uses a linear displacement transducer to measure piston stroke, this allows control of how much product is deposited,

with limit switches for min and max as reference points

Edited by matt_C_25
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