Need a Precise Electrical Indicator.Switch

[len]

Hi,

Is there an easy way to get a precise, repeatable electrical contact for a moving machine member. For example, an absolute postion indicator for a rotary table.

The switch should have a resolution and repeatable accuracy of less than 0.001", Hysteresis should be about the same.

I have lots of micro-switches but I don't think they will meet my requirements. Is optical or magnetic the way to go? Thanks.

Oh, yes, almost forgot, it must be cheap...

len

[AndrewMawson]

Len, I would imagine the most repeatable will be an optical switch but you may have some issues with keeping it clear of swarf & other debris. At the crudist / cheapest, most floppy drives / printers / etc abound with them so find a scrap unit and start experimenting for free !

The type that consist of a pair of square pillars facing each other (sensor one side, emitter on the other) just need a vane interrupter between then and are quite cheap to buy new but remember to screen them from stray light.

[Doug_C]

Hi Len,

You don't say if this is under motor control or hand.

Even if you were to switch control a moving member, there will be some overshoot that may need contained by a solid stop for repeatability below .001. In a few applications I have seen this work with current limit control to a solid stop within a creep mode after a switch is made. This will give some adjustability to both stop position and force/velocity of the members natural hysteresis once something solid is sensed above normal loads.

Don't know if it would help much to have a fulcrum point with say a 10:1 moment on an arm to increase the sensitivity of a sensor for position. This could be used in a continuity circuit with an LED or flashlight for just indication of contact?



DC

[len]

Doug,

My need is for an electrical indication only, so I can provide a reference point for a rotary incremental encoder that does not have a built-in zero reference point (or Z output). The hysteresis I mentioned is in reference to direction change of the moving member. Control design factors, such as velocity, do not matter.

[Doug_C]

I got ya Len,

The cheapest way might be to find another encoder with a Z pulse.

Trying to simulate a homing pulse, huh? Interesting dilema!

If this is a canned cycle process then you will be stuck with the switch idea. That depends also on where you locate it. Best would be off the motor shaft or common with the encoder, prior to the gearing ratio for least amount of trip deviation.

Maybe a prox, hall effect, micro off of the drive coupling? This might help to reduce the precision requirement. Even the idea Andrew had with the optical pickup assembly with a seperate disk for this feature might work?

I have seen in the software end of the control this can be triggered with a reference count between 2 micro-switches. The first is made which starts a count as the control switches to low velocity, continuing in the same direction seeking the second micro. Once the second micro is found the motor is reversed to find the first micro again. This count is averaged and a reference is established with inherent error, but fairly reliable between the 2 switches.

At any rate, cheating the main stream engineering is mostly the fun part of the challenge, right? [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

DC

[Vampyr]

The center stations on our old PnP,s use an opticle switch and flag to find the base position and self calibrate off it. There are also two dual switches in every mechanical mouse... www.digikey.com has several available for very little money. Look for opto interrupters

BTW. This doesn't need to be right at "zero", you just need to trip the switch, back it off until it clears, then count X- number of steps to get to "zero" (What am I thinking! it's the same procedure you'd use for any axis where "zero" is not at the full extent of the travel).

[John Garner]

Len --

I've worked with some fair-sized rotary tables (1 meter, 42 inch, 48 inch) that use a plunger-actuated Microswitch to generate a "home" signal, and they've done it very repeatably (well under 0.005 degree).

The basic setup uses a small bi-directional cam attached to the table, which is rotated at fairly high speed until the cam activates the switch through a full cycle [(open, close) or (close, open) . . . I don't remember which]. Then the table is reversed and run at slow speed until the switch is triggered going up the cam, at which point the table speed is reduced even more. Once the switch is triggered coming back down the cam ramp, HOME!

The same approach should work on a linear system.

John

[Guest]

I found a $10 Radio Shack Ohmmeter works pretty good. Actualluy an old Simpson analog ohmmeter worlks even better.

Jim H