WEG CFW100 - Academic PID Controller; Parameters

14
APPLICATIONS
Putting into Operation Check the state of the PID controller application in parameter P900. Value equal to 4
indicates the application is already in operation. Value equal to 3 indicates the application is stopped; therefore, it
is necessary to change the command value for the SoftPLC in parameter P901 to 1 (execute application). Value
different from 3 or 4 indicates the application cannot go into operation. For further details, refer to the inverter
SoftPLC manual.
1. Manual Operation (DI2 open): keeping DI2 open (Manual), check the indication of the process variable on the
HMI (P916) based on an external measurement of the sensor signal (transducer) in analog input AI1.
Then, changes the manual setpoint value of the PID controller (P918) until reaching the desired process
variable value. Check if the control setpoint value (P911) is set to this value, and then put the PID controller
into the automatic mode.
✓
NOTE!
The PID controller only begins the speed regulation when the motor reaches the minimum speed
programmed in P133, as it was configured to operate from 0.0 to 100.0 %, where 0.0 %
corresponds to the minimum speed programmed in P133, and 100.0 % corresponds to the
maximum speed programmed in P134.
2. Automatic Operation (DI2 closed): close DI2 and make the dynamic adjustment of the PID controller, that
is, of the proportional (P931), integral (P932) and derivative (P933) gains, checking if the regulation is being
done correctly. In order to do so, just compare the control setpoint and the process variable and check if the
values are close. Also check how fast the motor responds to the oscillations of the process variable.
It is important to point out that the setting of the PID controller gains is a step that requires some attempt
and error procedure to reach the desired response time. If the system responds quickly and oscillates close
to the control setpoint, then the proportional gain is too high. If the system responds slowly and it takes a
long time to reach the control setpoint, the proportional gain is too low and must be increased. In case the
process variable does not reach the required value (control setpoint), the integral gain must be set then.
14.1.2 Academic PID Controller
The PID controller implemented on the inverter is academic. See below the equations that characterize the
Academic PID controller, which is the base of this function algorithm.
The transfer function in the frequency domain of the Academic PID controller is:
y(s) = K
p
x e(s) x

1 +
1
sT
i
+ sT
d

Replacing the integrator by a sum and the derivative by the incremental quotient, we obtain the approximation for
the discrete (recursive) transfer equation presented below:
y(k) = i(k − 1) + K
p

(1 + K
i
T
a
+ K
d
/T
a
)e(k) − (K
d
/T a)E(k − 1)

x 10
Where:
y(k): present output of the PID controller; it may vary from 0.0 to 100.0 %.
i(k-1): integral value in the previous state of the PID controller.
K
p
: Proportional gain = P931.
K
i
: Integral gain = P932 = [1 / T
i
(s)].
K
d
: Differential gain = P933 = [T
d
(s)].
T
a
: sampling period of the PID controller = P934.
e(k): present error, being [SP(k) – PV(k)] for direct action, and [PV(k)].– SP(k)] for reverse action.
e(k-1): previous error, being [SP(k-1) – PV(k-1)] for direct action, and [PV(k-1)].– SP(k-1)] for reverse action.
SP: present control setpoint of the PID controller.
PV: process variable of the PID controller, read through the analog inputs (AI1 and AI2).
14.1.3 Parameters
See below the description of the parameters related to the PID controller application.
Micro Mini Drives | 14-5