What is the least expensive distance measurement device?
What is the least expensive distance measurement device?
(OP)
Hi all,
I am designing a machine that has a pneumatic cylinder and ram system. I want to measure the cylinder stroke with a system that is relatively inexpensive ($200~$300 would be great).
The stroke is 30 inches, and it cycles only about once a day. It moves very slowly in extend, and retracts in about 60 seconds. I would like accuracy to be within ±1/8 inch, although worst could be considered if cost saving warrants.
I know there are linear encoders and glass scales but all of these offer much better accuracy than I need, and cost more than this machine model has budgeted for this option. LVDTs are of interest, but I thought there might be better ideas than this.
Packaging is also a consideration, a typical LVDT arrangement makes sense, although I would likely have to add spherical joints to both ends to avoid binding.
Servo pneumatics really don't serve my application. I also don't think direct cylinder piston sensing will work for me.
Any ideas would be appreciated,
ERT
I am designing a machine that has a pneumatic cylinder and ram system. I want to measure the cylinder stroke with a system that is relatively inexpensive ($200~$300 would be great).
The stroke is 30 inches, and it cycles only about once a day. It moves very slowly in extend, and retracts in about 60 seconds. I would like accuracy to be within ±1/8 inch, although worst could be considered if cost saving warrants.
I know there are linear encoders and glass scales but all of these offer much better accuracy than I need, and cost more than this machine model has budgeted for this option. LVDTs are of interest, but I thought there might be better ideas than this.
Packaging is also a consideration, a typical LVDT arrangement makes sense, although I would likely have to add spherical joints to both ends to avoid binding.
Servo pneumatics really don't serve my application. I also don't think direct cylinder piston sensing will work for me.
Any ideas would be appreciated,
ERT





RE: What is the least expensive distance measurement device?
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You will of course also need a small power supply to excite it, and an ADC to read the position.
Mike Halloran
Pembroke Pines, FL, USA
RE: What is the least expensive distance measurement device?
Regards
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RE: What is the least expensive distance measurement device?
Pat - thanks as well, but I do need a measurement system along the entire stroke. Within the discrete realm, I had thought of using several proximity sensors or photoeyes on flags staggered at the ±1/8 resolution, but this would be messy and require homing.
Any other thoughts are appreciated.
ERT
RE: What is the least expensive distance measurement device?
Do you need the same accuracy in both directions?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What is the least expensive distance measurement device?
Yes good questions, say 5 year life. Which would mean a life criteria of only 1000 strokes.
And yes, I only need the accuracy on the slow stroke. I have had second thoughts of how this device could be arranged - I think the slow stroke when the measuring occurs is better suited to the retract direction (regardless of cable pot, or piston type device).
I should also mention this is a heavy industrial atmosphere. Temperature is ok at 15°C to 40°C, but there is a fair bit of dust in the air.
Thanks again for any suggestions.
ERT
RE: What is the least expensive distance measurement device?
Take a power hacksaw blade (cost 30 dollars), a hardened steel pin ($1) and a cheap microphone from RadioShack ($10)
Arrange the blade so that it clicks over the pin
Use the mike to count the clicks.
You'll need some signal conditioning, say $5
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What is the least expensive distance measurement device?
Given the low speed, I might use a switch to turn it into an incremental encoder. You need a second switch to sense that the ram is at one position, say one end. Both switches have to be debounced, but you could do it in software with a PIC or something similar.
The only disadvantage of an incremental encoder is that it loses track of where you are when there's a power glitch, so you'd want to run the electronics from a battery. A 9V battery can run a PIC for a few days at least.
Mike Halloran
Pembroke Pines, FL, USA
RE: What is the least expensive distance measurement device?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What is the least expensive distance measurement device?
Is that RS Electronics, or someone else?
ERT
RE: What is the least expensive distance measurement device?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What is the least expensive distance measurement device?
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RE: What is the least expensive distance measurement device?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What is the least expensive distance measurement device?
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I like the bandsaw blade incremental encoder idea. Use two Hall Effect proximity sensors set at 1/2 the tooth pitch to get A quadrature B signals.
RE: What is the least expensive distance measurement device?
RE: What is the least expensive distance measurement device?
Yes, we have tried the floor money approach, not really practical for us as we have over 100 of these systems.
We are currently trialing the string pot idea. We actually went with a low resolution encoder on a wire drawn unit.
Regards,
ERT
RE: What is the least expensive distance measurement device?
RE: What is the least expensive distance measurement device?
Dan - Owner

http://www.Hi-TecDesigns.com