Pneumatics - Cylinders

The cylinder (or actuator) is the business end of the whole system. The one pictured here is the primary weapon on More Panda Monium and shows a number of interesting points. A cylinder is basically a bicycle pump - a plunger within a metal tube. As you push air in behind the plunger, you force the plunger forward - also exhausting any air which is infront of the plunger. The simplest cylinders have a spring at one end to return the plunger to its original position. For the purposes of robot wars though we will assume a double acting cylinder where you push air in each end alternately to move the plunger back and forth.

At each end of the cylinder is a 'hole' or port. This hole is used to both allow air in and out. If you take the picture as an example, as shown here the air is being pushed in through the front port (the spike end) to hold the plunger in the retracted position. Release this air and push air in the hole at the tail end of the cylinder and the plunger is moved forward out of the cylinder, or extended. The control of the air flow is done by a control valve.

The cylinder is made of aluminium and contains a number of rubber and plastic seals which seal the air inside. Beware that these seals wear and eventually break. The are particularly susceptible to liquid carbon dioxide which has a habit of expanding quickly and bursting the seals. It's worth having a set of spare seals before competing.

Connection to the cylinder is through fittings connected to plastic tubing. The size of the hole (and subsequently the fittings) will determine the flowrate of air into and out of the cylinder and therefore the speed at which the plunger operates. As basic physics say force = mass * acceleration, the mass of the plunger is fixed, so to get more force you need more acceleration. Go for the biggest ports you can and keep the fittings and tubing that size to. We have 1/4" fittings on this cylinder but a larger exhaust valve on the front so that air can be escaped from the front faster than it can be supplied to the rear. This means that you create a vaccuum in the front of plunger which pulls it while it is also being pushed by the gas from the rear. This gives more speed on the outward thrust.

The other way to increase force is to use a larger bore of cylinder. There are two dimensions which determine the thrust that can be produced by the cylinder: bore and stroke. The bore is the diameter of the plunger itself and the stroke is the length that the plunger can move along. The weapon above has a bore of 40mm and a stroke of 200mm. This gives a thrust of about 1200 Newtons at an air pressure of 10bar. We have increased that with tricks such as the vaccuum exhaust and some internal modifications. These were done by our sponsor, Siebe, who custom built the cylinder for us - we do not recommend messing around with the internals of the cylinder. If you increase the bore of the cylinder, the plunger has a larger surface area and as such more air pressure is required to move it. Greater force to move it means greater thrust when it does get moving! John Reid uses a huge 80mm bore cylinder for the axe of Killerhurtz. The downside to using a big cylinder is that it uses up more air. Every time you fire the cylinder you use up gas moving it forward and gas to return it to home. This gas is exhausted and cannot be reused. The bigger the cylinder, the more is used each time and you will only ever have a finite amount of gas on board the robot. You need to do some calculations to determine the amount of times you can fire the cylinders based on the amount of gas in the system. For example:

According to Boyles Law:

(P1 * V1) / T1 = (P2 * V2) / T2

        P1 = 1000psi / 14.5 +1 = 70 bar
        V1 = 5.6 litres, the volume of our fire extinguisher
        P2 = 5 + 1 = 6bar
        :   V2 = 65.3 litres of air (assuming constant temperature
            and assuming a nominal working pressure of 5 bar.

The 65.3l is the amount of air you will have in the system. If the you cylinder used 1 litre for a extend/retract cycle, you would get 65 shots out of the system. In practice, it would be less than this because of losses due to not total compression of air and tiny leaks in the system.

Cylinders come in all sorts of shapes and sizes and you need to look carefully at getting the right one for your job. We have a large cylinder for the weapon, three smaller (25x120mm) cylinders for the lifting shovel as we need force over a larger area but not using lots of air, and a tiny (12x150mm) cylinder to do the lifting of the weapon as we only needed control and no real force is required. Most commercial cylinders run at 10bar but a few can run at 12bar or more. You can usually order them in lots of different sizes too. If you look in a sales catalogue, you will find thrust tables and dimensions of the cylinders which will allow you to work out what you need for your task.