Linear encoders

Would 24 bit binary scales work ? Same data protocol as a lot of digital calipers , a lot of us are using these on our Shumatech DRO's . My 24" X axis scale was less than a hundred bucks , the 8" on my Y axis was around $35 .

Well, you'll be looking for absolute position encoders unless you can work a homing routine into the configuration. You indicate you don't need fast speeds, but I think your constant cycling dictates a non-contact type of encoder (i.e. not mechanical contacts.

What is the travel needed, resolution and cycle rate? Can you drive a rotary encoder with a tensioned cable and pulley arrangement?

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How about string pots? Repeatability is good on the better quality units, and linearity and monotonicity are generally guaranteed.

The way I use them is to do a low-force "home" operation at the beginning of a machine session or cycle (if possible without wasting process time), and calibrate whatever is "zero" for that position in software.

LLoyd

I have used some UsDigital products in a similar situation. They are resonably priced and quite reliable.

It's called a string pot if you do that with a potentiometer, and you can buy them off the shelf.

I'd be somewhat concerned with the left expectancy of a mechanical wiper potentiometer used in that application, and linear slide pots are available anyway. My thought was the inexpensive rotary optical encoders intended for front panel control applications, or perhaps some low end "real" rotary encoders from US Dig or the like. The real solution is the totally contact, friction and wear free LVDT sensors, but I think that looses on the cost end.

I don't know how LVDTs really work over long strokes, either -- I've always used them for 1/2" or less. More, and I've seen linear encoders used.

And LVDTs cost a bundle. They're one of those theoretically easy things that have all sorts of practical difficulties tied up in their manufacture. Worse, my experience with LVDT companies (and resolver companies) leads me to believe that the technology was conceived and refined long enough ago that all the folks that really know how they work have passed away.

Maybe that's not true outside of the US, but it sure seems to be the case here. I've had vendors eager to sell me parts who can't for the life of them come up with engineers that can do more than repackage the same old same old into a different frame while quoting electrical specs from a 10th-generation Xerox copy of a mimeograph whose original is long gone.

(Remember mimeograph machines? My dad actually owned one for a while -- I don't know if he was planning on using it to produce catalogs, or if it was one of his charity buys from a friend hard on his luck. I just remember that it was fun, and messy, and stinky, and that we only had it around for six months or so).

A quick look on the Digi-Key site shows LVDTs up to 100mm and change.

Yep.

Quite possibly. They sure are simple in theory though. My only experience with them was owning some 3/4" U-Matic VCRs that used LVDTs connected to tension arms for supply and takeup tension control.

Nice.

My teacher in elementary school called them "purple poopers", and I usually got tasked with running the machine.

The US Digital MA3 series miniature magnetic absolute rotary encoders looks kind of promising. Absolute position sensing for the 360 degrees of shaft rotation, non-contact magnetic sensing and ball bearings for long service life, and a 10bit resolution analog output that a PLC can probably directly accept, for about $35 in modest quantity. With the appropriate mechanical linkage to change the end-to-end stroke to one rev, it should work pretty reliably for a long time.

Bingo!

For positioning with hydraulics they often use LDT, some hydraulic cylinders are made for them. I did a servo hydraulic press design for GM and we used an Allen Bradley 1771-QB linear positioning card, MTS Temposonics LDT's, and Atchley jet pipe servo valves. This is expensive stuff but we were pressing control arm bushings with a few tons force and .001" positioning capability. I saw a similar setup used to make a programmable bender. Most of this stuff can be bought for about 1/10 the cost of new on eBay, also, a proportional valve would probably be fine instead of using servo valves.

The US digital linear encoder strips may be better for your application but I just wanted to let you know about the hydraulic servo options. Many PLC's have high speed counter inputs that will count encoder positions.

RogerN

I won't have to control cylinder speed and the machines "home" all cylinders at the beginning of each cycle.

Many molding machines use big industrial-strength sealed linear pots to report the position of hydraulic cylinders. Probably a few hundred dollars each, but they're absolute position measurement devices.

Best regards, Spehro Pefhany

I looked at those, but I don't think they are the best choice for your application. The cost is somewhat high since the LIN is just the clock strip and you still need to add an encoder head to it, as well as do extensive fabrication to mount the Mylar LIN strip in a protected environment with the encoder head. I also suspect your PLC isn't up to counting encoder pulses to track position (don't know what PLC of course). I think the US Digital MA3 analog output absolute encoders would be a better choice.

OK but I don't need absolute position, all cylinders "home" before each cycle.

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Then, with the fairly low precision you're looking for, you can use the much less expensive string pots to accomplish the same function.

Any PLC with A/D inputs will read them.

I use them on pressing equipment that must measure the height of the media load. I get resolution down to about 0.002" with string pots that retail under $60. They do tend to wear and wander in total resistance a little over their useful lives, but that wear is accommodated by a homing action to establish the "zero" position, and an occasional calibration run to record what a full-scale excursion amounts to. Those figures are stored in the PLC's flash ram, and are used as the base figures for all positioning until the next calibration run. The calibration could be done on every new power-up of the machine, but we haven't found it necessary to do more often than about every 90 days.

LLoyd

LLoyd

Those MA3 analog output ball bearing magnetic absolute encoders run about $35 in tens. Obviously you have to add a pulley or similar setup to convert linear to rotary, but that should be simpler than building an entire housing for a Mylar LIN strip and encoder.

A calibration routine in the machine cycle is a good idea of course, but the MA3 encoders, being magnetic with no contacts or wipers to wear should be pretty consistent.

Well, how about some details so we can make better suggestions?

- Stroke length

- Cycle rate

- Resolution needed

- PLC capabilities for precision pulse counting from an encoder

- PLC analog input capabilities

If you are always at a mechanical stop at the start of a cycle, that simplifies a pulse type encoder since all you need is a single channel at that point (no need to detect direction or index). My concern with a pulse type encoder is the PLC may not be designed to handle accurately counting rapid pulse trains, but I've not played with any newer PLCs so they may well be up to the task. I'm pretty sure the PLC should have an analog input which is why the analog output MA3 encoders look promising.

I expect that the environment is fairly dirty, so in assembling any sort of optical linear encoder, environmental sealing would be an issue.

Possibly this is all overkill, and a normal limit switch mounted on the moving part of the machine and operating against an easy adjustment point like a fine thread setscrew with backup nylock would be a simpler solution. Possibly it is already like this and you want to adjust in software vs. hardware, which is nice, but the logistics of looking at the output of the machine and making adjustments at a keyboard may be a lot less practical than twisting an allen wrench while directly watching the machine.