I want to create a fairly small 1 axis gimbal, and I am trying to find a balance in finding a motor that can output a fair amount of torque while still being lightweight.
I came across the SimpleFOCMini V1.0, which seems like like it would certainly help to minimize space and weight for my application. I would like to use it in conjunction with the F2004 Fpv Racing Drone Motor 3-6S KV1700/KV3000, as it weighs just 16.4 grams. However, on the listing for the SimpleFOCMini, there is this at the very bottom:
BEWARE : SimpleFOCMini is designed for gimbal motors
SimpleFOCMini has been designed for gimbals motors with internal resistance around 10 Ohms. Using this board with high-power drone motors may burn the DRV8313 chip integrated on the board. The chip can handle up to 3Amps of current for short periods of time, but in for long-term operation (without the heat-sink) the current should stay under 2 Amps.
the motor specs show an internal resistance of 221 mili ohms, a peak current of 21.2 amps, and an idle current of 0.38 amps.
Now, below the warning, there was this line: - It is possible to use this board with low resistance motors but it requires a bit of experience and it is not recommended for beginners.Ask community about it!
So, that’s what I’m doing. Any advice on how to make this work would be appreciated.
The torque generally depends on current and voltage and motor radius (I’m lying here but just go with me) so the motor itself doesn’t really mater. Just find a small gimbal motor. You will not gain anything from going the other route, and actually lose torque because these drone motors are designed to develop maximum torque at high RPM. Please read up on motor physics.
If you really, really want to go that route and your form factor is that important, you need a pancake motor with KV under 50.
Or even better, get the motor, open it, rip the existing wires, and re-wind yourself with low KV thin copper wire.
Or, as many people have found out, the king’s road solution is to get a geared sensored gimbal. Massive torque/power and extreme precision positioning running SimpleFOC with very low curent.
Thanks for your help. I had initially looked into repurposing dji gimbal motors from the dji mini, because I think their form factor would suffice. Unfortunately, I wasn’t able to find any data sheets on them, and I’m fairly certain that their connectors are proprietary. Do you know of any well documented gimbal motors of a similar size for a somewhat resaonable price point? **
It should not be easy to destroy these drivers because they have overcurrent protection. As long as you don’t connect a dead short across the phases, the motor inductance will prevent the current from rising too quickly so the overcurrent protection will have time to react.
In my experience using the DRV8316, even with bad commutation signals and a low resistance motor of 0.3 ohms, the driver just shuts down almost immediately and nothing bad happens.
You should still consider a different motor because drone motors do not come with sensors onboard, which makes them unsuitable for position control applications. I would suggest taking a look at ipower motors, they seem to have a decent range of gimbal motors of various sizes. Make sure you buy one with a sensor, unless you want to go through the hassle of adding your own sensor and aligning it properly.
Drone motors have too high kV, will never work properly as a gimbal motor unless you rewind them. There’s a reason why all drones use one type of motors for the propellers (fast rpm) and another for the gimbals.
Where that DRV BLDC chip can’t take the current one can move to using IFX007/BTN7960 type half-bridges, or otherwise discrete mosfet solutions.
I’m doing the former at present, and while it is not very effiicient, and the motor windings get hot, there are ways to use very low proportion PWM and exploit the motor’s own inductance to do buck-converter type stuff at low motor speeds. I can get 600 gram * cm from a 1000kV motor at standstill, this should improve a bit when I add proper FOC techniques, it takes 2A at 12V from a power supply to do this, but inside the motor I’ve got negligible voltage but currents of about 15A (time averaged, >20A peak reducing to <1A over the course of each cycle, on a 7.8KHz PWM frequency).
Any playing around you do with low resistance motors, always spec the driver chips to provide huge currents, even if they shouldn’t have to during your use case. But make sure the power rail feeding to the driver chips is set up to be current limited, so it will never supply too great a current, hence protecting the driver chips if they don’t have auto-shutoff in over-current scenarios. Also always add multiple methods of cutting off power in over-current accident scenarios. For example, I’ve got a 3A polyfuse in my power supply rail, the 12V is coming not from a high-current-capable battery but a wall-wart which safely turns of if overdrawn, and I’ve got a hardware circuit with some op amps and AND gates enforcing automatic turn off of any PWM signal which reaches too high a percentage of a cycle in length. So even if the control software fails, the hardware stops an over-length pulse, which stops over-current, and if that somehow fails the polyfuse catches it, and if that fails the wall-wart simply refuses to supply big currents.
The reason your motor driver is inefficient and the motor heats up is because your PWM frequency is too low. Increase it to around 50KHz and you will see a noticable increase in efficiency, because the current does not drop to nearly zero in each PWM cycle.
The reason for inefficiency is even more fundamental than that. I wasn’t expecting efficiency, so much as simply seeing if the principle works. But it comes down to the fact that the drone style motors have short pieces of fatter wire, whereas true gimbal motors have longer thinner wires. The torque produced is
constant*current*wire_length
, and the loss of energy as heat is
current^2*resistance
. So to drive a big enough current to make decent torque with shorter fatter wires you’re looking at increased resistive losses than driving a smaller current in longer thinner wires even though the longer ones would have higher resistance.
The IFX007T works pretty well at 7.8KHz, it doesn’t seem to overheat, atleast not with its slew rate pin pulled directly to ground. But I wouldn’t want to try it at higher frequencies, the limit I’ve seen usually suggested is 10KHz.
Try high current and very low or very high PWM duty at low RPM or just hold position, at 8kHz you will smoke it. It works fine at higher frequency only within certain very narrow regions. The good news is since gimbals operate at very low RPM, if you go down to 5kHz, aside from the horrible noise and a big heatsink, you will be fine.
The theoretical efficiency shouldn’t be affected by the thickness of the wire. This is because if you make the wire thinner, you are increasing the resistance and the length, so the total resistance is proportional to the length squared.
This means that you can’t decrease the power losses in the windings by making the wires thinner and longer.
: SimpleFOCMini is designed for gimbal motors
