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Simple way(s) to run a bipolar stepper motor?

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sorveltaja:

--- Quote from: Noitoen on November 29, 2023, 07:41:46 AM ---Old post but to run a simple stepper motor as a regular small speed motor, an AC supply of 24V and 2 none polarized 4,7 uF capacitors in parallel will drive the motor like a single phase motor at mains frequency. Capacitors may vary depending on the motor

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I've seen some Youtube videos of how to do that, and from what I understand, it's rather limited way. Also, I doubt if there is any useable torque available.

I haven't watched all videos of the subject, but generally they just seem to show, that it's possible to make a stepper motor rotate using that method. But how does it handle even modest load?

As Joe mentioned, there just has to be certain signal for both coils(when talking about bipolar stepper) in certain order.  I guess, that's what makes it possible to produce torque.

--
I know it's been a while since I last posted. I really would like to get back to this project, but meh, there are other things that keep me occupied. It just sucks when that happens.

Muzzerboy:
You have to realise that stepper motors need to be current driven, rather than voltage driven. Most drivers regulate or limit the current once it has reached the level needed to index the motor to its next position. If they didn't do that, there is nothing to limit the winding current in the motor. If you had a 30V power supply and a 3 ohm motor, you would end up with a steady state current of 10A (for a fairly short time!). Instead, they typically use a full bridge with PWM modulation to limit the current and hold it at a sensible (programmable) level.

When you control the position with multiple microsteps, you come closer to driving the windings with 2 sinusoidal currents that are 90 degrees out of phase. However as noted, these need to be currents, not voltages and they need to be a sinusoidal function of the rotor position, not uncontrolled.

As the motor speed rises, you need more voltage to overcome the inductance. Or if you like, the motor develops a back emf (voltage) that is proportional to speed - and when that back emf reaches the power supply voltage, you can't drive any current into the windings. As torque is proportional to current, that means your torque has fallen to zero. This is something that often gets overlooked when people focus on the headline (stall) torque, not appreciating that the torque will fall to zero with speed.

That opamp circuit with the MOSFETs is a (linear) voltage driver, so not really suitable. Ideally it would be a current source, either PWM or linear.

Take the time to study a modern microstepping driver IC, like the I've linked to here. You may not want to take the easy route out but if you digest the datasheet and application notes, you will gain a better understanding of what is needed to successfully drive a stepper motor.
https://www.allegromicro.com/-/media/files/datasheets/a4988-datasheet.pdf

Noitoen:
Many years ago, on a plant that I used to do some maintenance, there was a need to automate a 6 meter high recycled water tank. We needed to know where the water level was with some accuracy. At the time, analog ultrasonic range sensors were very expensive so, I designed an "electromechanical" level sensor. It consisted of a 200 mm diameter nylon flat drum/pulley with a rounded thread with the pitch of the diameter of a thin 2 conductor + shield cable (around 4 mm) and 3 concentric grooves were cut on the side of the drum. These accommodated 3 brass rings to make up 3 simple slip-rings to connect the cable to a regular level relay. The drum was driven by a small stepper motor with a gearbox. A 5 K/ohm, 10 turn potentiometer was connected to the shaft to give feedback of the drum position. On the end of the cable, a small weight was hung and 3 probes (ref., Lower and Upper) were soldered to the wires.

These probes would indicate if the tip was touching the water or fully submerged. The stepper motor was driven by 24 VAC with a capacitor rolling or unrolling the cable. If the upper probe was touching the water, the cable would be pulled until it wasn't and if the lower wasn't touching, it would lower the probes. The level was determined by the feedback potentiometer's value. This type of AC drive was chosen because there were no brushes to wear out on the motor since it was always moving up and down "hunting" for the water surface.

The system worked for many years without any issues.

Muzzerboy:
FWIW, many of the commercial tank level meters use a submerged pipe (to the bottom of a tank) and a small pump to supply a continuous (but low) flow of air through the pipe. If you know the density of the liquid, you can directly relate the back pressure to the depth of the fluid. No moving parts, apart from the pump itself.

Noitoen:

--- Quote from: Muzzerboy on December 16, 2023, 09:33:49 AM ---FWIW, many of the commercial tank level meters use a submerged pipe (to the bottom of a tank) and a small pump to supply a continuous (but low) flow of air through the pipe. If you know the density of the liquid, you can directly relate the back pressure to the depth of the fluid. No moving parts, apart from the pump itself.

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Been there, done that  :D I've built the controls of a sinkable floating dock where the "sinking part was controlled by the filling of 20 ballast tanks. The control systems are built in the 4 corner towers where the pumps and fill/empty valves were located. The level inside the tanks were monitoring by a constant air flow regulator and a manifold of multiplexing valves which took turns of measuring the blowoff pressure of 5 tanks on each corner to determine the water levels at each ballast tank.

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