Looking to use ODrive to rig up a force trim release system for a cyclic control in a helicopter simulator. The goal is to use the DC motors to simulator spring return to center. So if the pilot moves the cyclic lever forward and releases it, the DC motors will move the cyclic back to the center. The reason I don’t just mechanically rig this with actual springs is that the “center” can be changed by the pilot for various regimes of flight.
Stage 2 of this project will see an autopilot implemented which will move the cyclic around based on parameters received from the flight simulation software.
From what I can tell so far, I’ll likely need to use a planetary gear system combined with a pulley system to get a reasonable amount of torque off the motors.
The question are as follows: What am I looking for as far as power supply specs for the motors (planning on using the D5065 motor) seeing as they will be operated at such slow speeds. I gather than I need to make sure that I’m current limiting as the motor will be back driven in a stalled condition somewhat frequently.
Start by calculating the current you need based on the torque requirements:
T = 8.269 Im/Kv => Im = T Kv / 8.26
Find out the power of your motor at stall:
P = Im^2 Rm
Where Rm is the motor phase resistance, you can measure that with a multimeter. Keep in mind that this will be drawn as heat from the motor, so if it is a lot you might need active cooling.
Decide on a PSU voltage and get your PSU current requirements based on power:
Ip = P/Vp
Pay attention that Im != Ip , ie the current in the motor windings is not the same as the current delivered by the PSU to the ODrive. Also the above is for a single motor obviously, and the motor at standstill (i.e. no back-EMF).
On the mechanical side, I’d just use a timing belt/pulley.
Ah, interesting. Let’s see, spring resistance is linear so if you set the motors in position control with no integrator (vel_integrator_gain = 0) then this will behave like a force trim that you can move around when you hit the trim button.
Thanks for the advice. I confess that I don’t understand the formulas that yconst provided (yes, I’m that dumb when it comes to this stuff and have a tendency to bite off way more than I can chew).
I suppose I can always buy the odrive and a couple motors to see how it works and slowly work my way up in torque and current right?
Not DCS Gazelle wetmelon, although that would be a lot of fun! X-Plane, hopefully with a decent set of VR goggles!
So I just purchased an ODrive and 2x D5065 motors. I’m just trying to sort out the power supply and trying to figure out the stakes of getting this wrong.
Let’s just say I buy a 24V 20A power supply and rig up active cooling on the MOSFETs and motors. If I stall the motors, what happens? I’m no where near that 56A stall rating. Am I damaging anything? If I current limit the ODrive to say 10A, does that mean the power supply will only ever be asked for 10A of power?
There are different ways to stall a motor, there’s the motor’s physical stall torque which assumes you can pull as much amperage as you want, this is the physical limits of current going through the motor wires and heating, avoid that.
There’s also stalling a motor when it has a software current limit installed, let’s say you put a 10A software current limit on your motor, then the stall torque is much smaller (torque and current are directly related, see yconst’s equation) since you’re throttling how much current can go to the motor, which means you’re throttling the torque that the motor can provide, as well as throttling how hot the motor will get, You will not damage anything this way as long as the current limit is set reasonably and the motor and Odrive are not overheating.
There are some really great studies on this forum that others have done that look at how much current you can safely pull using different cooling methods and for how long.
Finally, it’s important to note that even though there might be 10A in the motor, that doesn’t mean that 10A is being pulled from your power supply. This is because the speed of a motor is directly proportional to the voltage, so if you’re stalling a motor, the voltage is very small and the current is very large. But the battery/power supply is giving you a constant voltage so the motor controller can convert 24V/5Amps to roughly 5V/24Amps (neglecting losses). Do some digging around this community, there’s a lot of posts that have more in depth explanations if you’re interested.