stall detection

[mechanicer]

Hi,

I want to make a stall detection system. How can it be?
I suppose,
It will sense the step commands coming from computer and signals from step motor (with optical sensor or etc.) that whether it is turning or not.
While steps are coming, it will check if step motor is turning or not. If motor is turning no problem but if motor is not turning, it will shut the system.(means stall).

Anybody can help me in this subject.

Best regards.

[NC Cams]

You are essentially describing exactly what the feedback system of a servo does.

So, all you have to do is add a feedback encoder and wire it in and create the logic to respond to it.

I'd look into merely installing a servo if it were my choice.

[jl15]

You could use an optical encoder on the motor shaft and input the shaft position and step commands to a Picmicrocontroller. The PIC can then be used to shut down the system when a stall is detected.

[Syp]

Another idea would be to use an accelerometer on the axis you want to detect stall conditions. If the accelerometer is reading zero when you are moving, probably you are stalled. :-)

A 2 axis accelerometer is about $29 nowadays.

Syp

[ger21]

You can get a system that will detect if the machine is out of position that works with Mach3 from www.rogersmachine.net

Works with both steppers and servos, and will pause the machine if it loses position.

[NC Cams]

Accelerometer = not so good idea

Here's why:

At rest, you have ZERO acceleration

At ANY constant velocity you have ZERO acceleration.

When you have any change in velocity, you have acceleration (positive=acceleraion whereas negative=deceleration). Its gonna be real hard to keep an error signal from occurring when you have to find a awy to come up with a control logic that will "think" about what's going on and when.

With some AND or OR logic, you could get an accelerometer to work but a tachometer feedback would probably your easiest thing to integrate. If you were stepping, you better have an RPM of some king. Trouble is, if you step only enough to get a few thou movement, you won't generate a perceptible RPM.

Still say, use a servo and drive - it is ultimately what you're trying to emulate and it is a bolt on if you are willing to spend the $$$.

[lucas]

I have been thinking on this also, when I saw a small tool heavily bending because the Z stalled.
The easiest way is indeed to detect motor, screw or table movement optically and compare that with the step pulses, there's no need for accuracy, it's only a safety issue.
There could even be 10 step pulses for only one feedback pulse, a few thou shouldn't matter, it's not a servo emulation. Benefit of a servo is that this feature is incorporated.

I have a basic concept in mind but no details yet.

@ Gerry: The board you suggested looks like overkill for only this purpose. Do you intend to use it for your dual motor gantry? In your case a stall detection (for whatever reason) would be very usefull.

BTW: It's also possible for the stepper driver to detect a stall, some have it and there's also a application note on the subject. But it seems quite complicated to implement it.

[max_imum2000]

according to my knowledge

There are very easy solution that can be done and cost almost nothing. you can sense the current going to the motor , as dc motors takes too much current when in stall
so you wither

1. make a circuit to detect the current and stop the system or do whatever u want

2. add a fuse with rating close to stall so it blows up

3. add a fused switch (like the one found on homes) that when certain amperage is reached the switch is turend off

[ger21]

Originally Posted by lucas @ Gerry: The board you suggested looks like overkill for only this purpose. Do you intend to use it for your dual motor gantry? In your case a stall detection (for whatever reason) would be very usefull.

That board was designed to prevent damaging parts if you do happen to lose steps, not just let you know that you have.

I don't plan on needing that for mine. If it's losing steps, I'll get bigger motors/ better drives.

[lucas]

Originally Posted by ger21 That board was designed to prevent damaging parts if you do happen to lose steps, not just let you know that you have. I don't plan on needing that for mine. If it's losing steps, I'll get bigger motors/ better drives.

Gerry, Mechanicer (and me) would like to automatically stop the machine in time, not just signalling it to the operator to hit the E-stop and then it will be too late: ruined parts, tools or even wrecked machine.

Aren't you worried on this? Your motors, drivers could be well sized and function properly but then one day the totally unexpected thing happens: failing drive, damaged cables (motors or limit switches), the cleaning lady does something wrong...
Even the "unthinkable" like a mice feeding from cables or peeing on drives and when you start the machine the gantry get's wrecked.

I've seen several stepper driven systems with optical feedback only for safety and I personally would like to have it on a big machine where a lot of time and money has been invested.

[Geof]

Originally Posted by lucas ....The easiest way is indeed to detect motor, screw or table movement optically and compare that with the step pulses....I have a basic concept in mind but no details yet....

Here is a cheap optical system: Mount an aluminum disc with a brushed or fine sand blasted finish somewhere on the drive system. Get a cheap optical mouse and mount it so it is looking at the disc and plug it into an available usb port. Write a program that looks for "mouse" signals at that port when step pulses are being sent; no signals no motion, stop the system.

[lucas]

The real simple setup wich I would like to try is a 16 slot disc mounted on the screw with an optical sensor.
There would be 12.5 steppulses for each sensor pulse using a 200 step/rev motor.
Feed the steppulses in a simple counter and reset the counter with the sensor pulse.
The counter would count up to 12 and then be reset to 0 by the sensor.
When it reaches 16: something's wrong: create E-stop. 2 or 3 IC's should do the job.
One problem: on a direction change when the counter has reached 9 and the motor reverses it will count up to 18 before the next sensor pulse arrives, a change in direction should also reset the counter.

The number of slots and counter values are just an example, other combinations should work also.
This is just an idea wich needs some more thinking but it would be a simple solution.