I am currently working on a project to build a Ball balancing robot. I am using 4 onmi wheels connected to servos as shown in the picture.
My idea is to align the four servos so that i can use the roll and pitch data from my fusion sensors to drive each pair of servos. However, I ran into some issues when I was tuning with the PID algorithm. The servo was too slow, it is only 60RPM. Technically, the robot can never experience an "overshooting" from the servo motors.
Thus, I am now looking at brushless motors that has high rpm, but I am unsure of the torque. I am afraid that the robot might be too heavy for the brushless motors. How do I choose a suitable brushless motor? I checked hobbyking website and I found Inrunner and Outrunner motor. This Inrunner looks good to me Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking
Alright, I know that I need around 500 to 700 rpm and around 6.7 kg cm to balance the robot.
The following are the specs of the motor:
RPM/v: 820kv
Dimensions: 51 x 23mm
Shaft: 6.0mm
Voltage: 11.1~14.8V
Weight: 194g
No-load current: 2.8a @ 11.1v
Max Current: 60A
Max Watts: 680W
Since I will be using a 6400mah 3S 40~80C Lipo Pack, the voltage supply would be 11.1v.
So assuming that I will be running at 11.1v, the rpm would be 820 x 11.1 = 9102 rpm.
and the formula for torque is (correct me if I am wrong):
Torque (pound foot) = 5252 x horsepower / RPM
and Max Watt = 680W = 0.911895021 horsepower
Thus, torque = 5252 x 0.911895021 / 9102 = 0.52618 pound foot = 7.27 kg cm
So the torque is ok (what I need is 6.7kg cm) when I used max watt and max rpm to calculate the torque.
But I am still confused, does that mean that at lower rpm, such as 500 to 700 rpm, the torque will also be 7.27 kg cm?
The max RPM isn't especially useful since you will only achieve that when the motor is unloaded i.e. not doing anything useful.
You need to know the torque and speed characteristics that you want delivered i.e. how much torque you want available and the speed range over which you want it available. Since the available torque provided by a motor drops off with speed, the worst case is usually high speed and this will determine the power requirements of your motor.
You might find a motor which can provide the required speed and torque combination directly, but more usually you'd want to take a motor with higher speed and lower torque and gear it down (to reduce the speed and increase the torque).
A good way to approach this is to calculate the power requirements (maximum torque times maximum speed) and pick a motor which can provide that much power with a suitable margin. Then decide what gearing is needed to produce the speed and torque combination you need. If you have a choice of several motors all with suitable power then you can start considering which ones would be easiest to package - if you have one which could be used directly and another which would need gearing down, the one that can be used directly might be more convenient.
