allen bradley 1769 0f2 b series

john russell

Member
Join Date
May 2022
Location
yorkshire
Posts
8
Hi, i want the above to control a 4-20mA valve using the 20mA signal to open the valve fully and a lower signal to partially open the valve
how do i create the input signals for the full and partial settings

any help would be much appreciated
 
Is this part of an existing system? or new? If new I would scale is then you can set limits on the output for a min and max, are you having a PID control?
 
this is an existing system, no PID is needed, the plc receives signals from an external software package via B3 inputs
for example B3:2/4 is open valve fast and B3:2/5 is open valve slow i want to use these signals to drive the OF2 4 to 20mA outputs
 
If I understand right...


You will want to use those inputs to pass values to a scaled output.


B3:2/4 ][ --> () MOV 100% to Valve Position


B3:2/5 ][ --> () ADD 10% to Valve Position


with a timer restricting the slow open to add 10% every however many minutes.
 
One approach:

  • increases a value (N7:0 PERCENTAGE_OUTPUT) to 100 by slow steps of 5% every 10s or fast steps of 10% every 10s. as suggested by Chris_M;
  • scaling time and the output rate to the OP's process and particular output card is left as an exercise;
  • this is RSLogix500; in Logix it could be written with fewer branches to use more horizontal and less vertical real estate;
  • this has the disadvantage that the valve will not start opening until 10s (or whatever time is chosen) after either of the discrete inputs becomes 1;
    • fixing that is left as another exercise
open_slow_or_fast.png
 
If I understand right...


You will want to use those inputs to pass values to a scaled output.


B3:2/4 ][ --> () MOV 100% to Valve Position


B3:2/5 ][ --> () ADD 10% to Valve Position


with a timer restricting the slow open to add 10% every however many minutes.

sorry Chris i dont think i have explained my problem clearly

the OF2 will be used to control an actuator valve on a cement weigh vessel
the valve is controlled by software that has a target weight so it will open the valve fully B3:2/4 then when the weight is within 50kg of target it will switch to slow fill position B3:2/5 until target is reached.
once these positions have been established they will be fixed so B3:2/4 will be 100% open and B3:2/5 could be 30% it will depend on the flow rate and how accurate it is
thanks for your input on this problem
 
thanks for the ladder, this might give you a better idea of what my problem is
the OF2 will be used to control an actuator valve on a cement weigh vessel
the valve is controlled by software that has a target weight so it will open the valve fully B3:2/4 then when the weight is within 50kg of target it will switch to slow fill position B3:2/5 until target is reached.
once these positions have been established they will be fixed so B3:2/4 will be 100% open and B3:2/5 could be 30% it will depend on the flow rate and how accurate it is
thanks for your input on this problem
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Thanks for clarifying
--
B3:2/4 ] [ MOV 100% to Valve Position
B3:2/5 ] [ MOV 30% to Valve Position


Valve Position will be a REAL tag. Since you are not adjusting your valve in real time, just opening and closing presets you could skip scaling the output and just send values. I think the default of those cards would be engineering units and sending a value of 16384 would be equivalent to 20 mA VERIFY that that is correct. If so your 4 mA value would be 3276 and you could calculate what value you want to send for your slow valve.
Hope that helps.
 
what should happen when B3:2/4 is 0 AND B3:2/5 is also 0?

When B3:2/4 is 1, will the "fast fill" valve position target always be 100%?

When B3:2/5 is 1, with the "slow fill" valve position target always be 30%?

Or will the ""slow fill" and "fast fill" positions be varied by the operator over time, perhaps from an HMI?
 
DR hit the nil on the head, if both bits are 0 then the valve will stay in it's last position so either move 0 (or the 4ma value) into the analogue before the logic that moves the required values or use NOT B3:2/4 AND NOT B3:2/5 MOVE 0 Analogue (or 4ma value).

i.e.
Always True bit
----| |----------[MOV 0 Analogue_Var]

B3:2/4
----| |----------[MOV 100% Analogue_Var]

B3:2/5
----| |----------[MOV 30% Analogue_Var]


instead of the first rung then after the last rung

B3:2/4 B3:2/5
----|/|-------|/|---[MOV 0% Analogue_Var]
 
current status

OF2 using engineering units
MOV O:1/0 target set at 16000
if both B3:2/4 or 5 are off then a 0 target is sent to MOV O:1/0
if B3:2/4 is on then a target of 16000 is sent
if B3:2/5 is on a target of 6000 is sent

when i activate B3:2/4 the MOV O:1/0 dest goes to 16000 and stays there

when i check the mA it flashes a number then shows 0 every time i take the meter leads off then back on it does the same

there is no actual 4 to 20 mA device wired to it yet

Valve Position will be a REAL tag... what is this and how do i do it

thanks for your help so far
 
when i check the mA it flashes a number then shows 0 every time i take the meter leads off then back on it does the same


The current output probably needs a load or else it shuts down; a VOM measuring current is not a load, or at least is is a very small load. There are meters that do this, or put a small resistor in series with the VOM, or put a resistor across the output and measure the voltage across the resistor.
 

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