Tuesday, 20 October 2009

Valve PID Control Unit

This box currently called the FACU will close the loop on the four actuators of each leg. The FACU takes four encoder inputs one for each leg, and sends 4-20ma signals down to the VAPS unit for distribution to the appropriate valve (See my earlier post for further info on the VAPS).

As mentioned previously, this design is probably going to change, but this configuration and hardware should surface for our initial test leg.

The FACU has a dsPIC5011 micro controller, and has been programmed in C using the Hitech compiler. The 4-20ma signals are generated using a four channel 10bit DAC, with a 4-20ma loop drivers on each of the outputs. Although the DAC is only 10bits, this only effects the flow rate resolution of the valve, the actual loop resolution is 16bit, effectively reduced to 15bit with out 15bit absolute encoders.

If higher resolution flow control is required, there is a 12bit version of the DAC.