Design and Use of Low Pressure Pneumatics on Fighting Combat Robots

We hope you will find this guidance of some assistance.

Section A:        Safety with Pneumatics

Section B:         Introduction

Section C:        Rams

Section D:        Gas Flow and Valves

Section E:         Design Considerations

Section F:         Regulators

Section G:        Buffer Tanks

Section H:        Final Design

Please read this safety section before anything else

Section A - Safety with Pneumatics

Compressed gas is a source of energy and should be treated with respect.  Never direct compressed air onto the body or block exhausting ports with the hand.  Before pressurising a system, make sure all connections are tight and any actuators are mechanically secure.  Always disconnect / isolate and vent pneumatic systems before any maintenance work / adjustments are undertaken. 

Actuators are moving parts and capable of achieving high velocities with tremendous forces in very short periods of time so please exercise extreme care when working with pneumatic systems.  Keep well clear of a pressurised system as radio controlled operation cannot be guaranteed to be free of false triggering.

Remember, robots have the potential to kill so exercise care at all times.

Section B - Introduction

Pneumatics is becoming an increasingly popular energy source for fighting robots (hereafter simply referred to as robots or bots).  There are 2 main types of pneumatic systems which are commonly referred to as full bottle and low pressure.  This guide will only deal with the low-pressure application which although more complex does use commonly available components that are used in their intended application.  Full bottle pressure designs do carry additional risks and is discouraged unless you are competent in this technology.

Whilst it is possible to use pneumatics on a number of weight category bots, this guide will assume it is a heavy weight class but the principles can be applied to lower weight classes.

This guide will not consider the mechanical aspect of the weapon or even what type of weapon it is but focus on delivering the gas to a pneumatic cylinder (hereafter called a ram) at the desired pressure in the desired time.  The guide will refer to a practical example and that is of the robot called M2 which can be seen at http://www.m2robot.com

Section C - Rams

In order for the pneumatics system to do work, a ram is most commonly used.  The ram is basically a piston in a tube (cylinder, similar to that in a car engine).  When gas under pressure enters the ram it acts on the surface of the piston.  Once sufficient pressure is built up in the tube to overcome the resistance of the piston, the piston will move (stroke).  The higher the resistance, the greater the force required and thus the greater the pressure required to stroke.

The force required is dependent on the mechanical design (efficiency and mechanical advantage) and intended load.  As mentioned above, this guide will not delve into the mechanical aspect but will quote M2 as an example for reference.

The force is a product of pressure and the effective piston area (bore), the greater the area for a given pressure, the greater the force and visa-versa.  M2 uses a 100mm diameter ram with an operating pressure of 16 bar thus:

                        Force = pi x Diameter² x pressure 

                                                40

where the force is measured in Newton’s, bore diameter in mm and pressure in bar

Substituting in the values for M2 gives:

                        Force = 3.1415 x 100mm² x 16 bar      = 12566N

                                                40

A force of 12566N is equivalent to a force of 12566/9.81 = 1280kgf

M2 has a mechanical advantage of 3:1 giving a potential force at the tip of the flipper of 1280/3 = 427kgf assuming 100% efficiency.

For a double acting ram where pressure can be applied to both sides of the piston, the force available from the down stroke is not as high as the up stroke because the effective piston area is reduced by the diameter of the output shaft.

To size the ram, you can work backwards from the force you are trying to generate.  Beware, we have only identified Norgren as having a range of pneumatic rams and valves rated at 16 bar.  The more common maximum working pressure is 8 or 10 bar.

Rams are available as double acting as above or single acting with spring return.  There are others available including rotary but we will focus on double acting as being the most commonly used type for weapons. 

Section D - Gas Flow and Valves

In order to operate or stroke the ram, gas needs to be switched into and out of the ram.  This is the function of the solenoid valve where the solenoid is the electrical actuator for the valve.  They’re are many types of valve and are commonly referred to as 2/2, 3/2, 5/2, 5/3 and others.  The numbers relate to the number of ports (first number) and the number of ways of the valve (second number).  For example, a 2/2 valve has 2 ports (in & out) and has 2 positions (open and closed).  A common way of controlling a ram is to use a 5/2 valve as follows:

This is a spring return 5-port valve.  The air supply enters the middle port on the bottom, the top two ports connect to each end of the ram, and the remaining two ports are vents to atmosphere.  The symbol shows the valve in the two possible states, the left hand side of the symbol shows the valve internal connections with the solenoid de-energised (spring position).  The right hand side shows the energised internal connections.  So, de-energised has the air supply going to the retract or down stroke side of the ram and the upstroke side vented to atmosphere via the valve.  When energised, the reverse is true, i.e.: air supply to the upstroke and the down stroke vented.  Thus, as the valve is energised and de-energised, the ram strokes out and in respectively.

These valves often have a rather small internal bore which tends to restrict the gas flow rate which for typical application is not a major problem.  For high speed weapons on robots, it is not only the force that can be developed but also how quickly.  This is true for both flippers and axes, it is very well being able to lift 100kg but to throw 100kg requires both force and gas velocity.  The gas velocity is dependant not only on the restriction posed by the valve but also by tubing, fittings, regulator and even how fast you can exhaust the gas out of the venting side of the ram.

This section is believed to be correct but would welcome any suggestion to the contrary.  Gas velocity is the speed at which the gas flows and is typically measured in metres / second (m/s).  Valves are quoted normally in flow rate which is a volumetric measure rather than speed.  The ram in M2 has an internal volume when fully stroked of 1 litre, which is 1 litre of free air at atmospheric pressure (approximately 1 bar absolute where absolute relates to a vacuum).  If it is charged to 17 bar absolute or as normally quoted 16 barg (where the g indicates gauge that is with respect to atmospheric pressure), this equates to 17 litres of free air.  If a valve states for example 900l/m, this means 900 litres of free air per minute.  Assuming no other losses, it would take

17/900 x 60  = 1.13 seconds to charge the ram on M2 to 16 bar.  Assuming that the unloaded ram will extend with say 1 barg of pressure, it would take 2/900 x 60 = 0.133 seconds to extend fully.  This is why a flipper often disappoints in action, as the velocity is not sufficient when lifting an opponent.  To complicate matters further, valves can indicate their flow rate in a number of ways such as: ‘C’, ‘b’, ‘Cv’, ‘Kv’ and others.  To accurately calculate the maximum flow rate into the ram, you would need to include the ‘Cv’ factors for the tubing, fittings etc and even the critical or sonic flow characteristics of the valve.  This is getting rather complicated and perhaps unnecessary as the general rule is the bigger the better if you desire performance.

Airflow through nylon tubing:

As you will be able to see, at 10 barg pressure, 12mm tubing has over twice the flow rate as 8mm tubing at the same pressure and velocity.

Most pneumatics are designed for industrial systems where the plant air supply is limited to about 8 barg which is one of the main reasons that most pneumatic components are limited to 8 or 10 barg working pressure.  In the case of M2 where it desired to increase the pressure to 16 barg, this greatly limited the availability of suitable components that has the desired characteristics of high pressure and high flow rate.  You will see later how this was overcome.

Section E - Designs Considerations

The arrangement shown in the previous section of a 5/2 valve controlling a ram is quite reasonable for axes but does have limitations for flippers:

Ø      All the gas into and out of the ram has to pass through the valve which typically has rather small orifices although larger ported models are available.

Ø      It uses full pressure to retract the ram which uses valuable gas.

Ø      When up-stroking the ram, it is necessary to also exhaust the compressed gas behind the piston.

To address these limitations, the standard arrangement can be improved:

Quick Exhaust Valve - Rather than only exhaust the gas through the valve, a device called a quick exhaust valve (QEV) can be used.  This is a 3-port device that connects between the ram down-stroke port and its pressure line; the third connection is a vent to atmosphere.  The way it works is that the QEV allows gas flow in certain directions depending on port pressures.  If the solenoid valve is de-energised thus putting pressure towards the down-stroke side of the ram, the QEV closes its vent port and allows the gas into the ram.  On the up-stroke, the solenoid valve vents the gas contained in the down-stroke side of the ram as normal but, the QEV senses the pressure change and opens its vent port which allows the gas to be vented direct to atmosphere at a higher flow rate.  This allows the pressure to decay quicker thus allowing the ram to up-stroke quicker.

Using full pressure to retract the ram is most often only required on 180 degree axes or as in the case of M2 where the flipper is also doubles up as a low powered crusher.  Quite often a reduced pressure is sufficient to retract as gravity often help here.  The pressure can be reduced by using a pressure regulator with 2 off 3/2 valves or as in the case of M2, the down-stroke pressure is zero.  This contradicts the first sentence of this paragraph but the solution used was to use a 5/3 solenoid valve.  The 3 positions are up-stroke, vent both sides of the ram and down-stroke.  The normal position is both sides of the ram vented, on the up-stroke the piston pushes out air at atmospheric pressure behind the piston through the solenoid vent and QEV.  When the solenoid valve returns to its vent position, the weight of the flipper retracts the ram.  Alternatively, using the third position, it is possible to pressurise the down-stroke side to bring the flipper down quickly and hopefully onto an opponent.  

Section F - Regulators

To enable plenty of operations of your pneumatics system, you will need to store the gas on-board compressed to very high pressures.  The standard rules at present state the maximum pressure to be 1000psi or 69 bar.  These pressures are only achievable using non-liquefying gases such as air, Nitrogen and Argon.  CO2 however liquefies at about 750 psi or 50 bar at room ambient temperatures which is why CO2 is sold by weight rather than by pressure.  When your on-board Co2 bottle is recharged, you recharge it to a maximum weight as stamped on its outside.  Why use CO2?  Well with M2 both air and CO2 have been tried but the main reason for choosing CO2 is capacity.  You have considerable more gas available for your pneumatics by using liquefied CO2 than simply a compressed inert gas. 

By example: A 2kg CO2 extinguisher charged to 69 bar gives a capacity of 69 x 3 (a 2kg CO2 extinguisher is about 3 litres capacity) which is 207 litres of gas at atmospheric pressure.  Remembering that it takes 16 atmospheres of gas per up-stroke on M2, this gives only 207/16 = 12.94 strokes.  Now each 1kg of CO2 gives about 500 atmospheres of CO2 gas so a fully charged 2kg CO2 extinguisher gives 1000/16 = 62 strokes. 

Now there is a problem with CO2 and that is it takes heat from its surroundings when it turns from liquid into gas.  If you use a non-liquefying gas, as you use the gas up, the storage pressure drops proportionally.  This also means that the flow rate through the regulator will drop as the differential pressure across it falls.  Foe CO2 this does not happen, as you use the gas, the liquid CO2 turns into gas to maintain the liquefying pressure for that temperature.  As the liquid CO2 turns into gas, the bottle will chill and if you use the gas fast enough you will even see frost on the outside.  Cooling also takes place as the pressure is reduced across the regulator but is even more severe if liquid CO2 reaches the regulator.  This can be so severe as to freeze up the regulator orifices restricting or even stopping the gas flow.  The gas can be easily 20 degrees below zero and this can also cause problems to seals, clearance tolerances on rams etc.

There are a number of things that can be done to reduce this effect.

      Ensure liquid CO2 does not reach the regulator, this is achieved in a number of ways including:

•  Orientation of the CO2 storage bottle, a CO2 extinguisher as standard contains a dip tube which is often removed for upright bottle installations or bent upwards for horizontal bottle installations.
• Install an expansion chamber between the CO2 storage bottle and the regulator, this device has baffles inside that tend to hold back liquid and also create an opportunity to turn into gas especially if heated.
• Install a long length of copper pipe between the CO2 bottle and regulator, this allows a resonance time for heat transfer between the liquid CO2 and surface temperature of the copper pipe.

      If you cannot prevent the liquid CO2 reaching the regulator, use a heated regulator or a regulator with a large surface area or a finned body.  Also choosing a regulator with the capacity of high flow rates means its internal orifices are larger and less likely to ‘ice’ up.

      Reduce the flow rate out of the regulator.

If you are looking for a high gas flow rate for your high-speed flipper or axe, it is unlikely that a regulator alone will be able to provide the required performance.  The technique commonly used is to use low-pressure buffer tanks.

Section G - Buffer Tanks

In order to provide plenty of gas at pressure for the ram to stroke, large pneumatic systems find that a regulator is not capable of delivering the gas flow rate.  The arrangement used on many bots including M2 is to use a low-pressure buffer tank.  The idea is that the buffer tank is charged over a relatively short period of time from the regulator but is discharged through a large ported solenoid valve into the ram in a fraction of the time.  This offers a number of advantages including requiring a smaller regulator (noting the comments re: freezing made above) and warming of the gas before entering the ram.

The size of the buffer tank is important.  For the purposes of this example, we will assume that the regulator flow rate is so low as not to significantly re-charge the buffer tank whilst the buffer tank is discharging into the ram.  Example: M2 initially had a 1 litre buffer tank and as stated above, the ram has a 1 litre internal volume.  A fully charged buffer tank to 16 bar when discharged into the ram initially puts a pressure of 16 bar onto the piston (ignoring pipe volumes) thus providing full force.  However, as the ram strokes the pressure from the buffer tank lowers as the available gas begins to fill an increasing volume of buffer tank + ram.  When the ram has completed its stroke, the gas has now filled both the 1 litre buffer tank and the 1 litre ram.  Using P1V1=P2V2 we get a final pressure of:

16 bar x 1 litre = P2 x 2 litre which gives P2=(16 x 1)/2 = 8 bar.

Thus, as the ram strokes, the available force lowers.  Currently M2 has 2 litres of buffer which gives a final pressure of (16 x 2)/3 = 10.67 bar.

Section H - Final Design

We have now discussed the major considerations for a pneumatic system which are in summary:

Selection of operating pressure

Selection of gas type

Selection of size of ram (the stroke is dependant on the mechanical design)

Selection of tubing and fittings size

Selection of solenoid valve

Measures to prevent freezing

Selection of regulator

Buffer tank sizing

Methods to increase flow rate in and out of the ram

There is one further method of increasing the gas flow into the ram and it is this method that has provided M2 with a flipper performance comparable to many full bottle pressure designs.  Recognising that 5 port valves are inherently of small bore design unless specialised ones are purchased and that you only require the high flow rate during the up-stroke, M2 uses a large bore 2/2 valve in parallel with the 5 port valve.

The current design of the M2 low-pressure pneumatics system:

The CO2 pressure is regulated down from the nominal 50 bar to 16 barg by regulator 1.  Regulator 1 charges the 2 buffer tanks in about 2 seconds to a pressure of 16 barg.  A safety valve set to 17.5 bar protects all the low-pressure pneumatics in case of a fault with regulator 1.  Regulator 2 reduces the pressure from the buffer tanks to 8 bar for the pilot supply for the 5/3 valve.  The pilot supply is needed as the 16 bar rated 5/3 valve requires an external pilot supply of less than 10 bar to operate.  This pilot supply is not normally needed for pneumatic components with an operating pressure of less than 10 bar.  On the up-stroke, both the 2/2 solenoid and the 5/3 up solenoid are fired together, this allows gas to flow into the ram both through the 5/3 valve and the 2/2 valve giving a flow rate of about 5500l/m maximum.  When these solenoids are de-energised, the ram is vented through the 5/3 valve as normal and the ram retracts under gravity.  Alternatively the 5/3 valve down solenoid can be energised causing the ram to retract at a lower flow rate but ultimately still to the full 16 bar pressure.

Paul Cooper

16/1/02