Robot joint Control
The fundamental physical principle underlying all electric motors is magnetism. So, let’s quickly review what it is we know about magnets.
Every magnet has a south pole and a north pole, so called because the north pole is attracted to the north pole of our planet. If I put two magnets together like this, we know that opposite poles attract and like poles repel each other. Now, let’s imagine we take two magnets and lay one on top of the other and pin them so that the top magnet is free to rotate. Then, what happens is that the similar poles repel and the top magnet tends to rotate so that the north pole in the top magnet is lying over the south pole of the bottom magnet.
This is the underlying principle of rotary electric motors. Let’s re-run the previous example and the top magnet, once again, rotates. And, when it’s nearly at rest, what we do is we pull a trick and we reverse the bottom magnet.
So, now it looks like this. And now, the top magnet is going to keep rotating so that its north pole is attracted to the south pole and then we pull the same trick again.
So, in this way, we can keep the top magnet rotating endlessly and this trick is referred to as commutation. It’s a fundamental principle of all electric rotary motors. So, it’s a lot like a dog chasing its tail. It’s never ever going to get to the destination. When the top magnet gets close to its destination, we change the destination and we keep doing this over and over again.
Here is a picture of the first ever electric motor. It was invented in 1827 in Hungary. That’s nearly 200 years ago. Now, the principal components of this electric motor are, firstly, the Stator. That’s the stationary electromagnet that’s highlighted here and it has got the magnetic field vector indicated in red.
Now, there is another electromagnet and this one rotates. We call it the rotor and it has a magnetic field indicated by the blue vector. The laws of magnetic attraction and repulsion cause the rotor to rotate so that the magnetic fields are aligned. When the magnetic fields are almost aligned, the Commutator reverses the direction of current flowing through the rotor electromagnet that reverses the direction of the magnetic field so the rotor keeps rotating to, again, try and align its magnetic field with that of the Stator and this process is repeated indefinitely. So, the Commutator is the key innovation that allows a rotary electric motor to operate.
If we look at a modern electric motor, we see that it has exactly the same components. It has the stationary electromagnet which we call the Stator. It has the rotary electromagnet which we call the rotor. And, at this end of the motor, we have the Commutator. Modern electric motors are low cost, compact and reliable. We can consider them as a device that converts a current flowing in to the motor in to a torque applied to the output shaft of the motor and this relationship is linear. An alternative way to consider an electric motor is that it is a device that converts an input voltage to a rotational velocity omega that is the speed of rotation of the output shaft of the motor.
The motors that we have discussed so far are rotary motors. That is the output is rotary motion. But, frequently, in real applications, we require linear motion, not rotary motion. There is a couple of ways we can achieve this. One is to convert rotary motion from a rotary electric motor to linear motion using a rack and pinion mechanism. An alternative is a linear electric motor or linear actuator. And, in this type of device, the shaft moves in and out of the body of the actuator, somewhat like the hydraulic and pneumatic actuators that we looked at earlier.
Here is an example of a modern electric motor. These devices are very effective, very reliable and very, very cheap. This is a rotary motor. So, as I pass current in to these wires here, the output shaft rotates in either the clockwise or the anti-clockwise direction.
Here is an example of an integrated motor and encoder assembly. So, there is a DC motor in the front part and this is the output shaft here. And, on the back is the incremental encoder which gives the signal to the controller about which direction and how fast the motor is turning. Here we have an even more integrated assembly. This has got an electric motor here. There is a gearbox and here is the incremental encoder on this end of the assembly. So, this is quite a common device in many robots in mechatronics systems, in integrated motor gearbox and encoder assembly.
Finally, here, we have an example of a hobby class servo motor and this is one of the wheel drives from the small robot that I have been building. This is an even more integrated assembly than the last one we looked at. It contains an electric motor, a gearbox, an encoder and a small computer which actually controls the motor and the motor controls signals coming through this, three wire cable which provides power and also the velocity command signal.
The most common type of actuator is a rotary electric motor so let’s look at the basic principles.
High school mathematics
This content assumes an understanding of high school-level mathematics, e.g. trigonometry, algebra, calculus, physics (optics) and some knowledge/experience of programming (any language).