Control of a stepper motor? Need to index position

Either encoder type would need environmental protection. Dust/fluff could be a nuisance with the optical type; metal filings/particles with the Hall type.
How many index positions are there per rotation?

There will likely be 15 positions.

Most motor mounted encoders already have enough protection. This device is typically for indoor use and not subject to iron filings or harsh conditions. That being said.....ya know who things go...

There will likely be 15 positions

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Then it would be fairly easy to just put 15 notches on the edge of a disc and have the roller-arm of a microswitch running on the disc edge. 15 x 4 holes in the disc would make a 4-bit optical encoder to encode those positions and would be easy to engineer.

With a correctly chosen (large enough) stepper motor, and enough gearing, it will be practically impossible to "skip steps" by bumping it.

Lots of rotary tables for metal machining use about 96:1 gearing in a worm drive (that resists back-driving) with a decent size stepper motor and will resist thousands of pounds back driving force.

With stepper motors the general rule is; don't use underpowered motors for the task, or a bad design, and then you never have to worry about missing a step.

I will admit that I probably have not explained enough on the device but I DO NOT want radical torque or someone will get hurt. I actually WANT it to error out at the slightest irregularity. There should be next to no torque required if everything runs right.

I have been rethinking the stepper and tend to agree that a stepper can probably work fine and I am just over thinking all the failure modes. If I just initiate a homing routine at start up and ""maybe(??)" make good use of a current monitor to look at both max and min current, it could ensure the motor is working if a fault is presented, just turn it off/on which would rehome and go from there.

Regarding the stepper rotary table though, I am DEEP in CNC equipment and have rotaries. I don't know one shop owner that would touch a stepper system for anything unless it had feedback. You just don't put a $2000 block of Ti on it and "hope" it does not skip steps. We have a rotary in here now that is a 96:1 made 20 yrs ago with a DC servo and 1000 line encoder. Certainly not picking a fight but I guess I would like to learn more about who, in the machining business, uses stepper rotaries??

This thread is a little hard for me to understand, I'll add my $.02 worth.

Think of two limit switches. These "catch" over travel and the HIGH limit would normally never be reached. Motor basically shuts down fast.

The LOW limit is used in almost the same way. Now, let's create a precision position sensor called ORG or ORIGIN. This would be located a little bit away from the LOW limit sensor. That ORG sensor has the ability to be precisely adjusted.

So, when the system powers up, it has no idea where it is, but it knows the direction toward HOME (The ORG sensor). It goes in that direction until it sees a spike on the ORG sensor and then slows down. If it happens to find the LOW limit, the motor will stop. It then moves very slowly toward the ORG sensor until it finds it.

I operation, the stepper can know to slow down when it's approaching ORG.

I believe the "stepper" use is primarily for hobbyists without feedback. Nothing really beats an optical encoder.

I really thought I could move ahead with the stepper BUT I was recently told by someone that a stepper tends to draw the same current regardless of load so using a current sensor to create an overload fault codition may not work. I must havea way to detect overloads and fault out. This is a low torque application and want it to alarm out if something impedes rotations. just keep hitting road blocks here....lol

If you're going with a stepper plus some sort of position encoder a micro can be used to command so many steps then check position. If target position is not reached there must have been an obstruction/fault. No need then to detect any overload current?

alec_t said:

If you're going with a stepper plus some sort of position encoder a micro can be used to command so many steps then check position. If target position is not reached there must have been an obstruction/fault. No need then to detect any overload current?

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That is pretty harsh on the mechanics, not to mention the block of titanium or the milling bit.

This project is not to machine anything.

Can anyone confirm that steppers pull constant current?

At rest, yes. Now, I'm not sure what happens when you try to resist the stepper. I would assume that the current would go up. Energizing the windings will cause a jump in current. When subjected to a mechanical force, I would expect that under constant voltage excitation, the stepper current will increase and/or it will jump a step or two when it's unable to hold it.

Micro-stepping does use a constant current driver. A 1/2 step is current half way between the full step. So the motor is held between positions or between two magnetic fields. Microstepping changes the ratio between the nearby windings, thus positioning it between a step.

A micro stepping driver is effectively constant current. A regular driver usually isn't.

fastline said:

... who, in the machining business, uses stepper rotaries??

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I certainly won't argue with you about what you use, but there are lots of people on the CNCzone forum using rotary tables with stepper motors. Modern steppers have excellent magnetics and even a "budget" size23 motor will do 400oz-in torque. At 96:1 those rotary tables with smash a tool or stall the spindle before they skip steps.

fastline said:

Can anyone confirm that steppers pull constant current?

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Yes, I can. The stepper driver electronics are a closed-loop current control, designed to maintain a constant pair of currents in the two motor windings. If you want to detect loss of position it's almost impossible to do electrically at the motor or driver.

You could try a force sensor on the turntable, which would detect bump issues and could simply re-home.

Other than that it might only need a low resolution rotary sensor on the table to detect a fault. Like a simple opto disk with one notch for each position the table must stop at. You would easily detect there had been a skipped steps fault as a stepper motor that has skipped will be out by 1/50th of a revolution, (or more, based on 50ths as the skip distance).