Robot Wars Home Page Southampton University
Robot Wars 1999/2000

RC BANDIT
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INTRODUCTION
ARM CONTROL
VALVE CONTROL
CONCLUSIONS
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ACTIVE WEAPON
INTRODUCTION
The arm motion and weapon systems are powered using a hydraulic system that enables the arm to open and close and the spike to extend and retract. Mentorn have strict regulations on the use of hydraulics and pneumatics, as both are potentially dangerous. Safety is thus a critical consideration in the design of the hydraulic system and must be considered at all stages. Prior to discussing the design and manufacture of the actual system, a brief description of hydraulics and hydraulic systems will be performed.

Fluid unlike gas is uncompressible, thus when the pump is moved, the cylinder moves immediately in the opposite direction provided the force on the cylinder is sufficient to overcome the load.



ARM CONTROL
The main weapon of the robot consists of two hydraulic rams, which extend and retract. Which is controlled by a pair of solenoid control valves. Start, stop and direction of pressurised fluid (provided by a hydraulic power pack) is achieved by the use of directional control valves. An example circuit representation of a valve with 4 service ports and 3 switching positions is:


There are two main categories of control valve - separated according to their design:

    1. Directional poppet valves
    2. Directional spool valves

Both can be either directly or indirectly operated i.e. manual or servo controlled. Directional spool valves were chosen for the project as they could be moved directly by a signal, without using any auxiliary force for switching. Direct operation can be carried out mechanically, hydraulically, pneumatically or electrically. The later method being the reason for selection, combined with the lower weights involved as opposed to poppet valves. On the negative side, poppet valves can achieve leak-free closing, while spool valves cannot achieve this due to the tolerances required between spool and housing.


VALVE CONTROL
The solenoids for the valves and the pump are controlled using a similar circuit as the hydraulic motor, only without the diode. These are also to be run off the same 12 Volt marine battery. According to the datasheet for the valves, the maximum current drawn by the solenoids is 2.2 Amps. However, during testing it was discovered that when the solenoids were connected to the battery, it allowed a larger peak current to occur when the solenoid were switched on, which then settled down to a constant current of just over 2 Amps.


The control circuit for one of the solenoids can be seen in the figure above. Each solenoid is turned on and off using an IRL2203N mosfet. These can take a drain current of up to 100 Amps and were originally to be used for the hydraulic motor. However, since four were needed in parallel to control the motor, which made wiring awkward, it was decided replace them with the isotops and use the IRL2203Ns for the solenoids.
CONCLUSIONS
Hydraulic systems are inherently complex and the analysis conducted within this report covers only a fraction of that undertaken in the design of the system. Unfortunately, due to unreliable supplier quotes on weight and power, the actual system used is somewhat over specified for the task required. Having said this, the weight saving posed by downgrading the system to the new 'micro-packs' is significantly less than the power decrease, 20% less weight compared to 75% less power.

The control of the system required careful design due to the high currents involved, however, the final system is able to survive even the most extreme loading that it is subjected to by the motor.

ACTIVE WEAPON