As is the case for all pneumatic actuators, the pressure in the PPAMs needs to be controlled by pneumatic valves. This can be done by off-the-shelf pressure regulating servo-valves, either continuously or on-off controlled. The former type was found to be too heavy and too slow for our application. Therefore fast switching on-off valves have been used to make fast and lightweight proportional pressure servo-valves. By making them ourselves, full control over the servo-valve control system was gained, which is usually concealed in commercial valves.
The requirements for the ON/OFF valves are:
Pneumatic Solenoid valve Matrix 821 2/2 NC
- Low weight
- Sufficient flow rate
- Fast switching
- Weight : 25 g
- Flow rate : 180 Nl/min
- Opening time : 1 ms
Because fast switching is required the opening time was improved by increasing the supply voltage, but during a smaller time period.
In the graphs below, one can see the influence of the internal spring and increased supply voltage. Note that the differential pressure over the inlet valve is always between 3 to 6 bar, for the outlet valve this interval is 0 to 3 bar.
valve (0-3 bar over the valve) with internal spring
Valve (3-6 bar over the valve) without spring
Experiments showed a pressure control with 6 Matrix valves (2 inlet valves and 4 outlet valve) can be used to control the pressure in the muscles fast and accurate. A special lightweight collector has been build to make a compact pressure control.
CAD Drawing of collector with 2 inlet and 4 outlet valves
An adapted bang-bang controller with a death zone is used to control the pressure. The actions of the controller are visualized in the next graph.
a) -60 mbar Open all outlet valves
b) -20 mbar Open only one outlet valve
c) -15 mbar Close all outlet valves
d) 15 mbar Close all inlet valves
e) 20 mbar Open only one inlet valve
f) 60 mbar Open both inlet valves
The collector with 6 valves is connected with the electronics for fast switching.