Real World Examples of Analog Output Card Applications.


Thread Starter


During my last class teaching NOAA PLC basics, I was asked for examples using PLC analog output cards, in which motion control was my obvious answer. but then I froze coming up with any other example. so thought I would pose that question to this group.

Thanks to all who remain professional and reply.
A PLC AO could drive a

- I/P to a pneumatic actuator for a control valve or damper

- positioner to a pneumatic actuator for a control valve or damper

- Electric actuator for control valve or damper

- VFD that drives the motor that drives the pump/conveyor/whatever

- SCR/thyristor that modulates power to heating elements
ie. the stuff considered a 'final control element'
Are you asking about a 4 - 20 mA card?

4 - 20 mA is the standard current signal to final control elements in the process industry. 4-20mA can represent the speed setpoint to a VFD, or the command signal to a control valve/louver/guide vane etc..
My first reply would have been valve control (4-20 mA output) or hydraulic actuator control (+/- 20 mA output) using a servo valve.

Setpoints to external controllers (VFD, Temperature, air flow, etc.) would be additional examples.
As a secondary function, an AO card could output process parameters (Temperature/Pressure/Level/Any such) to Panel Mount Indicators or Recorders installed in remote locations like a Control Room/Field Local Station.

I have also seen installation examples whereby an AO Card of one DCS/PLC being connected to AI Card of another DCS/PLC enabling Analog signals communication, though it would be rare to have this in these times of Digital Communication protocols.
Here are a few things I've seen personally:

1) Temperature control: PLC receives thermocouple inputs of mold and punch temperatures and uses them to control continuously variable SSRs with 4-20mA signals.

2) Another temperature control: Process controller receives thermocouple input of molten metal temperature in a furnace, uses it to control the melt furnace power level with a 0-10V signal.

3) Process parameter display: PLC receives a 0-10V signal slaved from a sensor indicating furnace tilt angle (0-100°). This signal is then passed on to a large marquee display.

4) Process parameter logging: PLC uses 4-20mA loops to pass parameters to a datalogger (electronic chart recorder).

5) Master/Slave VFD system: master drive receives speed command from a potentiometer; uses 0-10V to pass its speed feedback signal to the slave drive to be its speed command.

Curt Wuollet

Probably the largest application of analog outputs these days is to control VFDs for moving air, either in new installations or to replace damper controls and the like in the interest of saving energy. They are also much used for VFDs for other motor speed control as well as various DC drives.