Thursday, October 27, 2011

Strobe Attack

The bad thing about having tinyKart around is that we will just keep driving it until something breaks.


Besides drifting the tires off, the only reliability problem I've seen so far with tinyKart is the motors. Electromagnetically, the Turnigy SK-6374-170's are very good. But mechanically, they suck. We already had to replace the left motor at Maker Faire because the entire rotor was loose and would shift axially until the motor locked up. Last week, the right motor suffered the same fate.

Additionally, the outside rotors are out of balance and the sound like they are going to detonate at full speed (7,500rpm). The vibrations are so bad that the entire kart needs to be clamped to a table to do a no-load test, lest it resonate like the giant aluminum flexure it is. And I'm sure all that vibration would eventually lead to more problems.

tinyKart was originally designed with the larger "melon-class" 80mm motors in mind, but the 63mm "grapefruit-class" with KBS36101 controllers is a better match for its size, weight, and performance. When we built it over the summer, the SK-6374-170 was the only 63mm or 80mm motor in stock, so we didn't have much choice. (Well, okay, we could have dropped $360 on Hacker A60's...) But now there is a new line of inexpensive Hobby King motors called SK3, and there are quite a few 63mm ones:

SK-6374-170 (left) and SK3-6364-190 (right).
They look very nice, and the construction quality is definitely improved. The SK3 has a large radial bearing on the shaft side of the rotor, making it far less prone to shaking itself apart violently at high speeds. It should also help better-constrain the rotor axially. My only real problem with the SK3 63mm motors is that they are not actually 63mm:


The outer diameter is 59mm. The stator is proportionally smaller. And because of the can bearing, the active length is reduced as well. The shaft is also smaller: 8mm instead of 10mm. They don't look much different in the pictures but if you held one in each hand you'd clearly be able to tell that the SK3 is a shrunk-down model, probably to cut materials costs while still passing it off as a 63mm motor.

All of this I would be fine with if it was as good, electromagnetically, as the old SK. But the first indication that it is not is the line-to-line resistance: 37mΩ for the SK3-6364-190 as opposed to 23mΩ for the old SK-6374-170. Normally, it wouldn't be fair to compare a 190rpm/V motor to a 170rpm/V motor. But in my testing, the old SK actually was closer to 190rpm/V anyway, so they have virtually the same back EMF constant. So, for a given amount of current, you get the same torque from each, but the new SK3 generates 60% more heat.

Maybe it's not a big deal. After all, the comparable Hacker A60 is 59mm and 32mΩ. Even at 37mΩ, the power lost to winding resistance at 80A is only 237W per motor, or about 10% of the peak power input. If, with more robust motors, the speed can be increased past the 75% please-don't-blow-up limit its been at so far, the peak power might even go up. And tinyKart has never had motor overheating problems. So, it's worth a try. Swapping motors will mean changing the pulleys to 8mm bore, but that's easy. The bigger problem is the Hall effect sensors...


Because the can diameter is 4mm smaller, the sensors are now 2mm farther from the rotor can. They're already relying on the small amount of field leaking out of the side of the can, so every millimeter counts. It turns out they have no trouble picking up the field from the larger distance and I could still time the motor in the normal way. 

But, reverse sucks.

When timed correctly for forward commutation, the reverse is awful. I didn't recall this being the case on the old SK's, but a further test with them confirmed that forward and reverse have always been asymmetric. The extra distance between the sensors and the rotor can, and possibly a thicker rotor can on the SK3, seem to exaggerate the problem. I wasn't 100% sure it was a sensor problem, though. So, I went to borrow a strobotach from the Edgerton Center:

Borrowing a strobotach from the Edgerton Center is kind-of like borrowing an airplane from the Wright Brothers Museum...
This strobotach is an old-school box of vacuum tubes with a giant dial that can adjust the strobe rate. One way to use it is to point the strobe at something spinning at a constant rate and dial the knob until it appears to stop moving. Then, you can read the rpm off the dial. But, I was interested in using the external trigger, which flashes the strobe in response to an electronic signal. I tied this in to one of the Hall effect sensors. The signal bias is 54V for some reason that I'm sure has to do with the vacuum tubes, so I had to make a quick transistor trigger circuit so it would run off the 5V Hall effect signal.


To make a position indicator on the rotor, I passed a fixed current into all six permutations of phase wires and put a tick mark on a piece of yellow tape at whatever fixed position the most was held at by the current. I used three different color tick marks, for A, B, and C phase. There are seven ticks of each color around the motor, since there are seven pole pairs.

The strobe makes it very easy to see where the Hall effect sensor is firing, in forward and in reverse, as well as the physical effect of moving the sensor mount:


The strobe aliases with the video camera's frame rate a bit, but you can definitely see the tick marks. Here are still pictures of the forward and reverse case as well:

Forward.
Reverse.
I had already timed the motor to run properly in the forward direction. So, in forward, the Hall effect sensor (and the strobe) fire almost exactly on a fixed-current motor position (green). A long time ago I convinced myself that this should be the case. But, if it's firing on green in forward, it should be firing 180º electrical out of phase in reverse, or, somewhere about halfway between red and black. Instead, it's firing on the wrong side of black, about 70º away from where it should. The reversing problem is definitely a timing issue, then.

I tried a similar experiment with the old SK, for which the gap between the sensors and the rotor is much smaller, and the reverse mark was still offset from where it should be. But, it was better. Probably enough so that the SK would actually run in reverse, if not happily. I also tried making a flux-concentrating extension for the sensors:

i.e. I glued cap screw heads to the sensors.
This also brought the reverse mark closer to the correct position, but it still was offset by about 60º. I think the problem is due to the Hall effect sensor hysteresis (about 8mT for these sensors). If the field outside the can is weak enough that the total swing is only, say, +/-20mT, then the hysteresis will have a huge effect on the forward/reverse timing.

I could try getting the sensors themselves closer to the rotor, which I think will be better than gluing cap screw heads to them existing ones. I could also try more sensitive Hall effect IC's with a small hysteresis band. Of course, I would not want to have to make bunch of acrylic ones to test all this. Luckily...

4 comments:

  1. Borrow a gauss meter and map the magnetic field on the outside of the motor. You should find a sweet spot where it peaks... and it may not be where you expect. Mount the halls there.

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  2. Do you mean there might be a better place to mount them axially? Or radially? Or both? Or neither?

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  3. Any chance of getting a copy of the gerber files for the pcb? I'd like to use this design on a project I am working on.

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  4. Sure:

    web.mit.edu/scolton/www/63mm_sensor_EAG.zip
    web.mit.edu/scolton/www/63mm_sensor_CAM.zip

    Now that I know how to do it, I might make a 59mm version as well for the SK3 and Hacker A60, and an 80mm one for larger motors.

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