Blog post title by Albert Einstein
After a long winter in Cleveland, it’s been difficult to get up and moving again. You could say that I have a large inertia – I am resistant to change my state of motion.
Motor systems have rotational inertia, which is the resistance of the system to rotational acceleration around the axis. To have good control, the controller must know the system inertia. The greater the inertia, the greater the torque needed to accelerate or decelerate the motor.
There is a common misunderstanding that inertia is equivalent to load. Inertia is the mass that will spin simultaneously with the motor’s rotor, while load is an external torque applied on the motor rotor shaft. An easy way to differentiate the inertia from load is to consider whether the mass will spin with the rotor shaft if the rotor shaft changes direction. Direct couplers, gears and belt pulleys are examples of inertia because they spin directly with the motor shaft.
Quiz: In a washing machine, which of the following is part of the system inertia?A. The water in the machineB. The drum that spins with the motorC. The clothes that are agitated during the wash cycle
The answer is B – the drum that spins with the motor. Notice that C is worded very carefully. As the washing machine spins faster, the centrifugal force presses the clothes against the drum. At that point, the clothes become part of the inertia, because they are spinning directly with the drum, which is spinning with the rotor shaft.
The inertia of many systems changes over time. Imagine the shoulder joint of a robotic arm. The inertia of that system changes not only if the arm is lifting an object, but also with the arm’s orientation in space. The inertia is much greater when the arm is fully extended than it is when the arm is fully retracted.
We recently encountered a customer who was building a universal camera mount motion system that would support many different types of cameras. This means that the motor must be controlled over a wide range of inertia. We simulated the varying inertia of different cameras by attaching weighted bars to the mounting platform. The weight bars allow us to adjust the amount of inertia in the system (see Figure 1). Short bars represent a small inertia and longer bars represent a larger inertia.
We tested the range of inertia that could be controlled and found that with LineStream's controller we were able to tolerate a 4x change in inertia without having to retune! To do this with a traditional controller would be at least months of tuning effort, and it might not even be achievable.
As we shed the last vestiges of winter, and our inertia changes, it’s a good time to investigate a new approach- stay tuned for the LineStream powered ARM platform for motor control, available soon!
Great post Adam and made all the better by a compelling headline. BTW, I did the quiz coincidentally after dropping a load of wash into the machine.