Spirit flow-x Function Reference page 267


Check if the current setpoint needs to be gradually ramped up or down to the required
setpoint:
If (SP - SP
CUR PRV
SP = SP
CUR
Else if (SP - SP
PRV
SP = SP
CUR

Calculate the current error:
If Control direction = Forward then
e = SP
CUR CUR
Else
e = PV
CUR CUR

Calculate the Proportional part:
P = K * e
P CUR

Calculate the Integral part:
I = I + Ki * Δt * ( e
CUR PRV CUR
Note: the latter part is required to avoid anti-windup.

Calculate the Derivative part:
D = Kd / Δt * (e
CUR

Calculate the required control output:
v = [ P + I + D - Low scale value ] / [ High scale vale – Low scale value ]
CUR CUR

Check if change in control output is within the slew rate
If v - u > Upwards slew rate * Δt then
CUR PRV
u = u + ( Upwards slew rate * Δt )
CUR PRV
Else if Δu < - ( Downwards slew rate * Δt ) then
u = u - ( Downwards slew rate * Δt )
CUR PRV
Else
u = v
CUR CUR

Check if new control output is outside its limits
If u > u then
CUR MAX
u = u
CUR MAX
) > ( Upwards SP clamp rate * Δt ) then
+ ( Upwards SP clamp rate * Δt )
PRV
) > ( Downwards SP clamp rate * Δt) then
CUR
- ( Downwards SP clamp rate * Δt )
PRV
– PV
CUR
– SP
CUR
+ (u - v )*(High scale value - Low scale value)/100 )
PRV PRV
- e )
PRV
Flow-X─ Function Reference
Flow-X General functions - fxPID
4-267