I recently salvaged this monstrous metal carving robot:
It has x,y and z stepper motors, lead screws, lots of homing switches (4 for x, 3 for y, and 2 for z) and a mess of cut wires and old serial ports, but no other control electronics. I have spent a few days reading to try and give myself a crash course in electronic control system design but I think I have hit a wall. I think the most important question i need to ask is what stepper motor driver should i buy?
This is what I have worked out so far. All three motors are made by MAE, they are hybrid, 200 steps per revolution, 4 phase and 8 wire. the y has no information on it at all, though i assume it is identical to the x, which says it is 3,5A/phase and 1.1 Ohms. The Z motor (which plunges the cutter, which is quite a different job) is 4A/phase and, (i think, the writing is super hard to read right here) 0.85 Ohms.
Something else which has really thrown me is that I tried to reverse engineer the previous system. Each motor has 6 wires coming out of it, which run to a connector block right by the motor that consolidates 2 pairs of wires into one, then those four wires run back to a serial port with 12 wires in total, a red yellow blue and green for each motor. so I was expecting to find an extra pair of wires in the motor, which I did, but they were not connected to each other or anything else, they were just isolated in there. surely this must mean that they abandoned one or two of the phases. It used to be an engraving machine so maybe they did this to sacrifice torq for speed, but it just seems crazy. Anyway, what i was thinking is that I could bring those wires back out and run all 8 wires to a board. that way i can use silicon and relay switches to decide how i want the coils to be wired up (series or parallel) depending on the requirements of the job. Is that a logical plan?
I'm learning as I go so please do correct my miss use of terminology or any fundamental errors, and thanks for reading this far.
First I'd try to figure out how the coils are connect to the cables. That's not so easy, when the coil resistance is about 1 Ohm, and two (center tapped) coils may share a common line. A scope and an AC supply may be helpful. Next comes the orientation of the coils, i.e. which pairs of coils work in same/opposite direction.
its quite logical: red, yellow, orange and black are all different coils, and there are four white cables with a red, yellow, orange or black stripe that are the other end of each coil of the corresponding colour, and there are no center taps. I dont understand what you mean about the direction of the coil. I think it is the sequence and polarity by which they are energized that determines the direction of rotation. Does it affect which aduino and what stepper drivers i should buy?
You don't say how much background knowldege you have and may get some useful info in Stepper Motor Basics.
You need to get stepper motor drivers that can comfortably supply the current for your motors. Unfortunately the cheap hobby drivers cannot supply more than about 2 amps so you will need small industial drivers - Leadshine and Gecko are two brands. AFAIK if you buy a single driver that can manage 3 or 4 motors it will work out cheaper - but you need to size it for the motor that takes the most amps.
Your description is a bit confusing. At one point you say there are 8 wires per motor and elsewhere you say 6.
Use your multimeter to measure the resistance between each pair of wires and write down the results. Some wires should show infinite resistance (i.e. they are separate coils) and others should show similar readings. If there is a centre tap on the coils some readings will be twice the others.
While tedious it is worth doing this carefully for every motor - it helps keep the smoke inside.
Wikipedia suggests that an 8-lead stepper motor (typically?) is a two phase unipolar motor with separated coils. In this case you have to find out which pairs of coils work in the same direction (same phase), and can be driven separately (unipolar) or together (bipolar, in series or parallel). The wire colors may follow an standard, telling the sequence of activation.
In unipolar operation you apply power to the 4 coils in sequence, with only one coil powered at a time. Instead of reversing the polarity, you power the opposite direction coil. In this model you can use any DC driver board or discrete transistors, which support the required current. The driver circuit should allow for a constant current, best with programmable current for the various operation phases (step slow, step fast, hold/sleep). Dedicated stepper motor drivers should include such circuitry already. When an unipolar driver chip or board does not support the required current, a more powerful transistor can be added to the outputs easily. Some experiments may be required, to find out the right sequence of activation. Since high torque is not required for that test, you can use a low voltage and limit the current to a reasonable minimum.
When you think of sequence and polarity, you need a bipolar driver module. Then you power two pairs of coils in sequence, with reversed polarity in the next sequence. You can start with only two coils of orthogonal orientation. When the motor won't turn, you picked two coils of the same orientation, and you replace one by another coil. This way you can find out which coils belong to the same phase (orientation). Then you can wire the two coils of the same orientation in series or in parallel. When you reversed one coil, the motor won't turn because the coils compensate each other (the resulting magnetic field vanishes). In this case reverse one coil, and you're done. Finally check the movement direction, and (optionally) reverse one of the pair of coils to reverse the turning direction.
thanks guys, that's really helpful. I think I was stuck on wanting bipolar, but I think I'll start by getting them going in unipolar then upgrade later if i need more out of the motors. I have ordered all the components I need to build 3 of these circuits for a total of £15, including breadboard, jumper leads and a power supply
They are definitely 4 phase and 8 wire. I had to take the motors apart to find 2 of the wires, which is why I said they only had 6 wires coming out of them. I will also definitely test the resistance to confirm my assumptions about what those wires do once i find my multi meter. It is going to be about a month before all my parts arrive from china but i expect i'll get stuck and come back for more advice soon enough. thanks again, you really got me out of a rut