hi!!! i'm quite confused about the required No. of I/Os in a motorized valve for dcs control..

 

For open and close operation need 1 D/O and 2 D/I, using relay for output for two position and u can make 4 possible inputs with two open/close inputs, i.e. open, close, fault, travel. For analog type u need 1 analog input (position feedback) and 1 analog output required, and 1 process value input if controlling PID loop. MOV's have many optional outputs like, thermostat trip, mid travel, end position, local/rem select, torque trip, ESD command, auxiliary air valve operation etc, depend how much u use.

 

IO requirement for a Motorised Control Valve

you have not mentioned for what purpose this valve is used. simple flow isolation or flow control itself. The IO assignment depends on nature of the application and also on the MOV control arrangement. however you will need at least this

For a simple flow isolation/open MOV
a. MOV open command
b. MOV close command
c. MOV cotrol in Local/remote feedback
d. MOV open feedback
e. MOV close feedback

if it is inching type valve used for rudimentary flow control then you need

f. MOV position feedback

the above is the most common IO arrangement for MOV's. having said that i have seen a few motorised valves ued for precision flow control. they used the following IO arrangement

a. MOV run/stop command (1 for run 0 for stop)

b. MOV forward/reverse command (1 for clockwise direction , 0 for anticlockwise direction)

c. MOV full open feedback

d. MOV full close feedback

e. MOV position feedback

the bottom line is the IO assignment depends on both the application and the type of the MOV you are working with. Hope this helps

 

I, too, am confused because "it depends".

A motorized valve can use a variety of control signals
- an analog input like 0-10Vdc or 4-20mA

- a digital Fieldbus protocol like Foundation Fieldbus or Profibus

- a motor drive circuit, sometimes called position proportional (among others)

So the I/O depends on what the specific brand, model actuator and drive signal is for a specific field unit.

Position proportional uses two (binary) digital outputs (I/O) from the controller, each of which supplies a drive voltage to the motor for movement in one or the two possible directions, either open or closed or clockwise or counterclockwise. The other output drives in the opposite direction of the former.

Analog feedback to the controller is sometimes used to know when the motor/actuator/valve has reached its position, other times the control is open loop, where the control algorithm times the duration of the drive signal and assumes the motor drives the valve to the required position.

For feedback control, one needs 2 DO's and one AI at the controller.

For open loop control, one needs only 2 DO's.

For Fieldbus, one needs Fieldbus capability.

For analog control, one needs 1 AO to drive the actuator and one more if valve position feedback is required.

Some Process Automation Controllers (PACs) or hybrid controllers have function blocks for either position proportional or open loop (Honeywell calls it '3 position step control') which calculate the appropriate outputs for a input setpoint in "percent output". Since large damper drive (motor) actuators are part of the DCS world, a DCS might have similar functionality.

 

In my personal opinion, with motor operated valves (MOV) you will inevitably end up with a large number of signal wires, I/O cards, and system tags if you go with hardwiring. You may end up running one multi-core cable to each MOV, and you still don't get the remote setup and diagnostics. Electric actuators are one of the many reasons we need to get away from hardwired signals. For most projects you find several of the MOV control and feedback signals cannot afford to be used due to the high cost of hardwired I/O. This means the full functionality of the MOV is not utilized, the automation, and therefore the plant is not at its full potential. Incorporating some of these signals later in the project would be very costly and cause serious delays.

MOVs lend themselves extremely well to digital communication. Connect your MOVs using fieldbus instead and you end up having one pair of signal wires supporting 12 MOVs instead of 12 pairs of signal wires supporting one MOV. The wiring savings are huge.

However, beware of proprietary communication protocols. Proprietary communications need dedicated network cables, gateways, drivers, data mapping, planning/engineering and configuration for your DCS. The network cannot be shared with other devices around the MOV. With a proprietary protocol there is no interoperability. You will be locked in with a single vendor, you cannot cannot replace just one MOV with a third-party MOV. You would need to replace the whole subsystem.

Instead using FOUNDATION fieldbus drastically reduces wiring, yet still provides freedom to chose MOV from many suppliers. There is no register mapping, just point and click to link blocks. The MOV can share the same bus as other devices around the plant such as transmitter, valves, tank gauging systems, gas chromatographs, and many other things. Worst case, go with PROFIBUS or Modbus.

Because FOUNDATION fieldbus uses "soft marshaling" it is vary flexible. Incorporating additional signals into the control strategy at a late stage of the project, or even after the plant is running, is easy as no additional cables have to be laid, no cards installed, no marshaling, no drawings changed, and no spares used up.

. Open/stop/close control
. Desired valve position control
. Valve position status (end point limit switches)
. Percentage open
. Available for control
. Local/remote switch
. Motor running open direction
. Motor running closed direction
. Torque switch tripped
. Percentage torque
. Motor thermostat tripped
. Battery condition low


MOV configuration/setup and diagnostics is possible from the same intelligent device management software as other devices around the plant. Thanks to EDDL (www.eddl.org) this is possible without having to create custom screens.

Cheers,
Jonas