I need to test my PID valve driver (FACU unit) and it will be a while before the hydraulic components and power pack arrive for our test leg, so we decided to rib up an DC linear actuator to the tibia joint of the test leg. I have taken the 4-20ma output of the FACU unit used to drive the hydraulic servo valves, and converted it into a voltage suitable for a 6/12 amp servo driver I had in my spares box. I will fix the feedback of the servo driver into mid voltage 12ma position so the driver behaves only as an amplifier in open loop mode, then the FACU will close the loop with the absolute encoder on the tibia joint.
Fortunately I had a couple of ultra motion bug actuators left over from an old job. These are powerfull units, with 6" throw each, so to get a bit more range out of them, we join them, back to back and run two together to give 12" throw. We did this once before on one of the Harry Potter films for the Dragon animatronic, usually you would buy an actuator with a long throw, however, we had to use old stock, so this was the best soloution. The two ultra motion actuators back to back are almost the same length and throw as our tibia hydraulic ram, so this should give us a good representation of how FACU behaves!
Josh is working to get this electrical test rig up and running.. as you can see by his work area :)