PID Mode (Auto/Manual) being Controlled from two locations

plcnovel

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Feb 2006
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Hi All
I am using Compact Logix L35E with Beijer Touch Screen T100 as HMI. I have a following problem:

There is a field (multiple choice object) on HMI screen, that depending upon its value, turns a PID from manual to auto or auto to manual (If Multiple choice value = 1, then PID mode = 1 (AUto) ). This field is also event driven, means user must put a value inorder to make it change the mode. Inside my ladder logic, i am waiting for that event o take place. The thing is i would also like do the same thing from remote end. Actually, i want to operate this field from a DCS. For example if DCS will send a Physical signal to my plc panel, and based on that physical signal i am changing the choice of this field, which in turn change the mode of PID.
Lets talk abt a scenario to justify my explanation: lets say from the HMI screen, operator turns the PID to Manual Mode, now, dcs Sends a command to one of my physical channels, based on the state, i have to change the PID mode to Auto. So the multiple choice field is being written from two locations, how can i make it like that and safe as well.

Thanks in advance
 
One way or another, you have to determine which of those two stations has priority!

Imagine this... at one station, the operator wants the process in Manual... at the other, the operator there wants the process in Auto!

Do you see the problem? Who has priority? Who has ultimate control of the process???

Just as a slave can't work for two masters... so too a device can not do so... unless there is an arrangement of some kind between the masters which is recognized by that device!

I have similar situations for motor control. In my case, I figure that the guy that is trying to control the motor AT THE MOTOR has priority! If he chooses Manual Control then, by God, he has Manual Control... the rest of the process simply has to accept that!

When he surrenders Manual Control... that is, when he places the motor in Auto Control... then, at least in terms of that particular location, the control of the motor then falls under the control of the other location. Then, if that "other location" is set to Manual Control... then the other location is controlling the motor manually... otherwise... automatically.

What this means is... eventually, this is a matter of preordained co-ordination at the human (master) level.

The method of control doesn't matter... switch, button, DCS... whatever... it is the locale of the particular control that matter!

In general, I have a "Primary Control Station" (in sight of the motor) and a "Secondary Control Station" (out of sight of the motor). If the Primary Station calls for Manual Control... it gets it, period! The rest of the system has to accommodate! The station that is closest to the physical process has primary control... whenever it wants it! Otherwise, it "surrenders" control to the more remote station.

Under "normal operating conditions" the Secondary Control Station (remote from the motor) has control of the motor. In this case, the Primary Control Station (at the motor) is in Auto and is, in effect, "surrendering control" to the Secondary Station.

Actually, with respect to Primary/Secondary vs. Local/Remote... it could work either way... Primary = Local, or Primary = Remote... you just have to have all of your ducks in a row!

I simply prefer having the on-sight controller as the primary controller.

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