Robot joint Control

As I mentioned a moment ago, actuators are the devices that actually move the robot joints.

And there are a number of different types of actuators in common use for robotics. A good definition of an actuator is it is a device that causes motion and it can cause linear motion or rotary motion. On the top here, we have a linear actuator and the shaft of that linear actuator moves backwards and forwards along a line. The more common form of electric motor has a rotary output. The output of an actuator could be considered as either a speed or a force. That is, for a particular input, the motor might adopt a particular speed or it might exert a particular force or torque in the rotational case.

There are many different types of actuators and they vary in the types of underlying physics that’s used to cause the motion. The most common types of actuators are hydraulic which use compressed oil to cause the motion, pneumatic which use compressed air to cause the motion, and electric which use electric current and magnets to cause the motion.

Hydraulic actuators are very common in heavy machineries such as used in construction and in the mining industry. They are very compact devices that are capable of generating very high forces and that’s important when I want to pull a bucket of an excavator through the ground or I want to lift up many, many tons of earth and stones.

The hydraulic actuators are the cylindrical devices you see in this image. Up close, you can see a large cylinder with some hoses going to it and a shiny rod sticking out one end that moves backwards and forwards. A hydraulic actuator is really a very simple device. The cylindrical body has got two ports where compressed oil enters and leaves and then, there is the moving part, the rod which moves backwards and forwards and exerts a force on the machine on the environment. 

A simple schematic of a hydraulic actuator looks something like this. Down the bottom, we have the cylinder itself. Inside that, there is a piston which is attached to the rod which is the piece that does the work.

There is a valve assembly and high-pressure oil from the pump enters the valve and oil leaves the valve and returns to the pump. The hydraulic actuator converts oil pressure to a linear force. If I apply a positive signal to the valve and oil flows as shown by the black arrows, and the piston moves to the right. Oil enters the left port of the cylinder and it exits from the right hand port of the cylinder.

If the area of the piston is A and if the pressure of the oil is P, then the force of the piston is P x A. And because the oil pressure is so high, the force that can be exerted by a hydraulic actuator is very, very large. If I apply negative signal to the valve, then the oil flows in the opposite direction and the piston moves to the left. If I apply a zero signal to the valve, then all the ports are blocked and this means that no oil can enter or leave the hydraulic cylinder. Because the oil is virtually incompressible, this means that the piston is effectively locked and it can’t move left or right.

Although the linear hydraulic actuator is by far the most common, there are also rotary hydraulic motors such are shown here in the motor which is driving this caterpillar track. You can think of it essentially as an oil-driven turbine or paddle wheel.

The hydraulic actuators are very compact and can exert a lot of force. But in order to use a hydraulic actuator, you need a lot of other kit. First of all, you need valves to control the direction of oil flow. You need a compressor to supply the high-pressure oil and for large systems, you need a chiller to cool the oil after it’s been compressed. Often you will have an accumulator that holds high-pressure oil and there is always an awful lot of hoses. Way back in 1984, I was involved in developing this flexible manufacturing cell and it contained two robots in addition to a lot of other equipment. There is the electric Puma 560 robot that we’ve met a number of times before in this course and in the foreground is this very large hydraulically-powered Cincinnati Milacron T3 robot.

Back in the day, hydraulics was the only technology that could be used to create a robot that was able to lift very large payloads. Since then, there has been massive improvements in electric motor technology and there are virtually no hydraulically-powered manufacturing robots in existence today.

Pneumatic actuators have a lot of similarity to hydraulic actuators. And here we see a linear pneumatic actuator. Instead of compressed oil, the pneumatic system uses compressed air. Just like the hydraulic system, you need a lot of other kit. You need to have valves, you need to have compressors, you need to have accumulators and of course, you need to have an awful lot of hoses. As an alternative to the linear pneumatic actuator, it’s possible to create an artificial muscle. These devices directly convert air pressure to a linear force. There’s no piston involved.

And here we can see a large number of artificial muscles connected by tendons to the fingers on this robotic hand.

The artificial muscle contains a flexible chamber or bag. As air is pumped into the chamber, the chamber gets wider and it also gets shorter. And we can see that the overall length of the muscle has reduced as it is inflated. One characteristic of these artificial muscles just like our own muscles is that they can only pull, they can’t push. In order to make an effective actuation system, we use them in what we called antagonistic pairs. You can see that illustrated here. As I pressurize one muscle, it gets shorter and the wheel at the top rotates to the right. As I depressurize that muscle and pressurized the other muscle, it gets shorter and the wheel at the top rotates towards the left. These artificial muscles have to be used in pairs.