Cheap Unipolar Chopper

[jessbussert]

Hi Folks,

I just bought a heavily modded Gerber Scientific PBC photo plotter. This thing originally drove a photo-resist covered PCB around on an x/y vacuum table and exposed the resist using a fine point light source. The thing weights a ton and uses cast parts for the base and the gantry. The previous user hacked off the light source and replaced it with a z axis. They never got the thing running before they past on to that great machine shop in the sky and I bought it at their estate sale.

The original drivers are long since gone. I was able to salvage an untested power supply, four honkin' Superior Electric unipolar steppers, and the main body of the unit. I'm now trying to get it up and running myself.

The x and y steppers are 225 oz/inch, 2.5vdc, 5A, .5ohm, 5 wire unipolar devices. The z and an extra stepper that came with the device are both 200 oz/inch, 3vdc, 4A, .8ohm, 6 wire unipolar devices. I can find no info on the power supply except on an old eBay auction. It is a Gerber model MPS-8B that supposedly supplies 48VDC @ 20A.

My questions are:
1) What 5A+ unipolar choppers will handle these steppers?
2) Would 48V @ 20A be enough to power the x, y, and z? What about if I added the 4th stepper as a rotary table? If not, what would be a good option for a PS?

Thanks for your help!
Jessica

[kreutz]

If you are looking for a DIY approach visit Pminmo's site, if you decide for the Mardus-Kreutz boards, you can reach the design thread here.

Your power supply will be OK for 4 steppers too.

[jessbussert]

Kreutz,

Thanks for the info. I've been bouncing around your "stepperworld.com" link from the other thread. That is a bunch of great information that I'd highly recommend to the beginner!

I'm thinking of using PMinMo's chopper design and running the steppers at 30vdc. Given the specs I've listed above, do you think I'll get acceptable performance out of those Superior Electrics?

Also, is there any good rule of thumb to determine if a 6-wire unipolar will perform well as a bi-polar? What are the dangers of opening a 5-wire unipolar in order to try and seperate the center taps?

Thanks!
Jessica


Thanks,
Jessica

[jeffs555]

A bipolar stepper will get about 1.4 times the static torque of the same motor driven unipolar. If wired bipolar parallel it will also use 1.4 times the unipolar current.
The ease of opening and reconnecting a 5-wire stepper depends on the motor. Some just have a cap over the wiring and removing the end cap allows access to the wires. On some, the bearing is part of the end cap and you have to be very careful when opening. Removing the rotor from the motor is a very bad thing to do, so you have to be careful if the bearing is part of the cap. You can easily demagnetize the rotor by removing it.

[kreutz]

Originally Posted by jeffs555 A bipolar stepper will get about 1.4 times the static torque of the same motor driven unipolar....

That is true unless your motor drive uses automatic torque compensation, as the Mardus-Kreutz does. Unipolar motors driven by M-K micro-stepper drives will (theoretically) produce the same torque as a bipolar drive, independent of the micro-step mode.

Jessica;

30 Volts will probably be too low voltage to get a decent amount of speed before midband resonances strike. Why don't you use the 48v/20A power supply?

I would recommend about 45-60 volts, depending on the motor electric time constant. Higher voltages than required, for the speed you will use, will increment the temperature rise of the motor body unnecessarily.

Kreutz.

[jessbussert]

Hi Kreutz,

I'm a little confused here. Is the Mardus-Kreutz a microstepping chopper? I had been emailing PMinMO and they told me they didn't have anything that could handle my motors, but then I googled your design and found a link on PMinMO...

I'd be happy to do a build-up of your design if it will handle the current requirements on these steppers. Will my max current be 2 coils for each of four steppers? So (5A*2 coils at any given time*2 steppers) + (4A*2 coils at any given time*2 steppers) = 36A maximum?

What is the max voltage I could push through your design? Danaher Motion told me that these steppers could handle as much as 175V. How much is overkill?

Is there a source for already etched PCBs for your design? I -could- etch them myself, but I hate doing so. The machine I'm building is going to be used for PCB routing among other tasks.

Is there a kit available with the BOM already included? (yes, I'm feeling lazy... *grins*)

Thanks for all your help.
Jessica

[kreutz]

Originally Posted by jessbussert Hi Kreutz, I'm a little confused here. Is the Mardus-Kreutz a microstepping chopper? I had been emailing PMinMO and they told me they didn't have anything that could handle my motors, but then I googled your design and found a link on PMinMO... I'd be happy to do a build-up of your design if it will handle the current requirements on these steppers. Will my max current be 2 coils for each of four steppers? So (5A*2 coils at any given time*2 steppers) + (4A*2 coils at any given time*2 steppers) = 36A maximum? What is the max voltage I could push through your design? Danaher Motion told me that these steppers could handle as much as 175V. How much is overkill? Is there a source for already etched PCBs for your design? I -could- etch them myself, but I hate doing so. The machine I'm building is going to be used for PCB routing among other tasks. Is there a kit available with the BOM already included? (yes, I'm feeling lazy... *grins*) Thanks for all your help. Jessica

Hello;

Yes, M-K is a DIY unipolar micro-stepper drive, will work up to 80v (using IRL640N Mosfets), 8amps per phase. micro-step modes are 1/16, 1/8, 1/4, 1/2, Full, 1/10, 1/5. It also has automatic idle current reduction to 50 % of peak current, and automatic torque compensation.

During micro-stepping it will never have maximum current on two coils at the same time so actually the maximum motor current is 1.42 times the peak current adjusted. 4 motors will require a minimum of 4* 0.65 * 1.42 * Icoil_max, well within your power supply ratings.

The higher the power supply voltage the higher the torque you will get, up to a certain point, also the motor heating is proportional to the square of the power supply voltage, so the higher the voltage the hotter the motors will get. Maximum motor temperature will be at idle or very low speeds.

There is a source for the bare PCBs, Ozan, try to PM him, maybe he also sells kits. Here you will see some of the PCBs he sells.

Regards,

Kreutz.

[jessbussert]

I've written your friend about the boards. Do you have a BOM you could post for me? One modified as needed for my current/voltage requirements?

Thanks!
Jessica

[kreutz]

Originally Posted by jessbussert I've written your friend about the boards. Do you have a BOM you could post for me? One modified as needed for my current/voltage requirements? Thanks! Jessica

Please, PM me your e-mail address. The B.O.M. does not need to get modified for up to 8A / 80V, the best option is to use the IRL640N for all power supply voltages. Check the notes on the schematics. I will send you the complete documentation including firmware and gerbers.

Kreutz.