The new motor controller and one of the motors I was hoping to use on the project arrived the other day.
After the previous issues with the motor controllers there was no doubt that a better design was needed. Revision 2 looked like this
You cant really see it on the photos but each of the motor controllers now has a header socket similar to that on the Arduino board soldered on to the connections at the bottom. This means that should one of them die, all you have to do is slide it off the white socket. You’ll still have to unsolder the wires at the top but this isn’t much of a big deal.
Having all of the white sockets at 90 degrees solves the problem of not being able to adjust the current limiting pots as you can now fit a screwdriver in there. This board is a little bigger than the previous one but still fits alright in the project box.
The motors that I hoped to use are from the same crowd that make the controllers, once again from Robokits. The biggest motor that Robokits stocks has a holding torque of 1400 g-cm but not having done any calculations I had no idea if this would be enough or not. The whole design is going to have to be as balanced as possible but still if the mass of the head is too great then the motors wont be able to move it. I decided to order one, run some tests and if it didn’t work then I’d have to find something else but at least I’d know that before buying all of them.
Mr Test Motor arrived with bare leads so I connected a cable from another project that would slot onto the header pin / DB25 botched dongle I’d mocked up so as to be able to connect the motors to the DB25 port.
I then combined this with another stepper motor kindly donated by Peter and some clear perspex and mounting hubs and mounted the whole thing on the top of a tripod to run some tests.
The tests didn’t quite go as hoped. In 1/4, 1/8 and 1/16 wave step mode the motors turn fine, in 1/2 and 1/1 wave step mode the motors jumped around a bit. Turns out that even this test setup has too great a mass for the motors to step properly at the high step speed built into the manual control section of the program. In the finer resolution stepping modes the smaller stepping angle means that the motor doesn’t need to accelerate and decelerate the mass of the head as much per step, rather the head rotates at a semi constant speed. In the coarser resolutions the motors have to move the mass a greater distance in the same time period and end up throwing the head around. If you grab the whole lot and try to turn it by hand not much force is required before you overcome the holding torque and are able to spin the whole thing freely. Holding Torque Failure!!
I then added the GoPro camera to the rig and tried to get the had to move the camera. This was an abysmal failure with not much really happening, just a lot of unhappy stepper twitching.
All in all it doesn’t look like these motors are going to work like this. By the time the rest of the head is built it looks like there is going to be too much mass to move and even if the whole thing is meticulously balanced I doubt the motors will be able to overcome the moment of inertia and rotate the head.
From here there are several options. At the moment I’m trying to get a larger stepper for Robokits but have yet to receive a reply from them. These larger steppers are the biggest ones that Pololu make and have 3.17 kg-cm of torqueas apposed to 1400 g-cm. If Robokits are able to get these in then Ill give one a go and put it through similar tests to those above. Another option would be to try and find some larger steppers from somewhere else but the problem is that all the motors I’ve managed to find from suppliers in NZ thus far have been a lot more expensive, I would have to also find one that doesn’t draw more than 2A per coil. My final option would be to combine either of the two options above with some form of gearing. This would give me the required torque and would have the bonus of a finer stepping angle but unless I can find some sort of pre-made gearbox for this sized stepper then it would add unwanted complexity to the head design.