Industry statistics reveal that a fair percentage of control loops are controlled manually, and are not automated. This fact was brought home last week, when a caller told us he needed “something to adjust the valve position so that the valve stays where it’s been set. And it’s real important that it can’t accidentally go off its own”.
What he was describing is what we call a manual station. It’s a controller where the 4-20mA current output stays fixed where it is until someone pushes buttons on on the front of the controller: up to manually raise the output value, and down to manually lower it. Typically, the output value is displayed for the operator as a digital number from 0 to 100 percent.
So what’s available to fill the bill?
I pulled a Honeywell UDC 2500 (a model # UDC 2500 C0-0A00-200-0000E0-00) off the shelf to see if I could lock it into manual mode so it could not be switched into auto mode where it would “go off on its own” and attempt to control without even an input. Success on the first shot! A couple minutes messing with its configuration showed that it can indeed be locked in manual mode, as needed.
When powered up, the controller starts up at a configurable output value. It displays the output value as 0.0 to 100.0% in its digital display, with the letter M, to show it’s in manual mode. The lower display shows OT for “output”. The up arrow raises the output and down arrow lowers the output. Pressing the M-A (auto/manual) key just shows Key ERR because the M-A key has been disabled in the configuration to prevent the controller from being put into automatic mode.
A few configuration settings are critical.
- The power up setting (PWR UP) must be “MAN for manual mode.
- AUTOMA = DIS which disables automatic mode.
- Input 1 type must be 4-20 (not the default) and must be linear (LIN), not the default setting.
- The failsafe setting (FAILSF) in the control group is the value at which the controller powers up.
- In the Display group, the default TUNITs (temperature units) is F (deg F) which lights up the letter F in the display, but the letter F can be turned off by setting TUNIT to NONE.
I suggest wiring the 4-20mA output in series with the controller’s Analog Input #1, then continue the loop wiring in series to the field device. Connect terminal 19 [output (+)] to terminal 26 [Input (+)], terminal 27 [input (-)] to the field device’s (+), field device (-) to terminal 21 [output (-)]. Wire the 250 ohm resistor across the terminal 26 (+) and terminal 27 (-).
For those who want to tackle configuration of a manual station, here’s the settings:
GAIN/PB = don’t care
RATE T = don’t care
I MIN = don’t care
SECUR = default 0 or your 4 digit security code (don’t call me if you forget it)
LOCK = NONE to get to next settings
AUTOMA = DIS (disables automatic mode, must be done while the controller is in manual mode)
RN HLD = DIS
SP SEL = DIS (I recommend LOCK = CAL or higher level security, not NONE)
SPRAMP = DIS
SPRATE = DIS
FUZZY = DIS
TUNE = DIS
CTRALG = PIDA (don’t care)
TIMER = DIS
OUTALG = CUR
CRANGE = 4-20
INP 1 TYP = 4-20
XMITR1 = LIN
IN 1 HI = 100.0 (can be fudged up to 105.0 or lower, if needed)
IN1 LO = 0.00 (can be fudged down to -5.0, if needed)
RATIO1 = 1.00
BIAS 1 = 0.0 Fudge factor, adds or subtracts from displayed value
FILTR 1 = 0
BRNOUT = NONE, UP, DOWN, or NOFS,
PIDSET = ONE
LSP’S = ONE
SP TRK = NONE
PWR UP = MAN
SP Hi = 100.0
SP Lo = 0.00
ACTION = REV
OUT Hi = 100.0
OUT Lo = 0.0
FAILSF = 5.0 (when it powers up what will its output value be?)
(if controller goes into fault mode, what will its output value be?)
FSmode = NO L
PB or GN = PB (don’t care)
MINRPM = MIN (don’t care)
ComADDR = 3 (only matters if you’re using PIE software)
ComSTA = DIS
IRENAB = ENAB (only matters if you’re using PIE software)
A1S1TY = NONE
A1S2TY = NONE
A2S1TY = NONE
A2S2TY = NONE
HYST = 0.1
BLOCK = DIS
DIA AL = DIS
DECMAL = ONE
TUNITS = NONE
FREQ = 60
LWRDSP = ENAB
LNGURG = ENGL
CAL (if you see this, I recommend going back to TUNING group and changing the LOCK setting to CAL (it’s now NONE), otherwise someone will get into calibration and screw things up.
The Honeywell UDC nicely fills the bill where there’s the need for a manual station is needed.