Prolblem with control of Brooks MFC with FP-AO-200

Question

When setpoint is 0, the while loop executes only one cycle then stop. When setpoint is not zero, error reportspen current loop detected. The module has detected an open current loop. check wire connections for possible breakage or disconnection. Thanks a lot.
Attachments:
cable_wiring_for_MFC.ppt ‏34 KB

Answer 1

Bo,
If you are receiving an Open Current Loop error, it is because the module is attempting to drive a load (specifically, it was told to output a non-zero current) but that there is not a complete circuit for current flow. There are several reasons why you may see this error. First of all, there may be an incomplete circuit. Double check your wiring to make sure that there are no loose wires. Secondly, the impedance of the device being driven may be too great. The drivable impedance from a FP-AO-200 varies with the voltage of the external power supply wired to the V & C terminals of the FP-AO-200's terminal base. This leads into the third option; no power supply wired to the V & C terminals of the FP-AO-200. The FP-AO-200 uses an external power supply, wired
to the V & C terminals to source power. Internally, there is a 3 Volt drop for the regulation circuitry. Thus if you use a 5 volt supply, and 3 volts are dropped internally in the module, than only 2 volts are available to drive the load. Remember V=IR, than at the full scale of 21 mA, with 2 Volts, we can drive a load of 95 Ohms. With a 24 volt supply, we can drive a 1000 Ohm impedance.
Looking at your diagram, it looks like you are using a 24 volt supply, so you should be able to drive the full range of 1000 ohms. Since I do not know what the connector looks like on a Brooks MFC, I can not verify whether your wiring is correct or not. However, please not that by connect the V & C terminals of the network module to the V & C terminals of the FP-AI & FP-AO modules, you have bypassed the normal isolation barrier in those 3 modules, although the isolation barrier on the FP-RLY module is still intact.
Regards,
Aaron