MA7200 PLUS Operating Manual
TECO – Westinghouse Motor Company App-1 Main PID Control
Deviation = Set-point (Target) value - Detected value
Bn-16 (Detection gain).
Control output (P) = deviation
Bn-17 (Proportional gain).
The output of the (I) control block will increase with time (integral) and the output will be equal to the deviation
after time specified by parameter Bn-18 (Integral time).
The output of the (D) control block depends on the setting of PID parameter Sn-64 and is as follows.
When Sn-64 = 1,3,5 or 7: D control block output = PID error
(
Bn-19
)
5 m sec
When Sn-64 = 2,4,6,or 8: D control block output = PID feedback
(
Bn-19
)
5 m sec
The parameter Cn-55 (I Upper limit bound) prevents the calculated value of the integral control block from
exceeding a fixed amount. The value is limited to within 0-109% of the maximum output frequency (100%).
Increasing the value of Cn-55 improves integral control. If hunting or speed oscillation cannot be reduced
by decreasing the value of Bn-18 or increasing the value of Cn-56 (PID primary delay constant) , then Cn-55
must be decreased. However, if the setting of Cn-55 is set too small, the output error between the set-point and
the feedback value increases. So the various parameters need to be adjusted to achieve optimum results for a
particular application.
The parameter Cn-56 is a low-pass (integral) filter setting for the PID control output. If the viscous friction of
the mechanical system is high, or if the rigidity is low, causing the mechanical system to become unstable
(oscillate), increase the setting Cn-56 so that it is higher than the oscillation period. This will decrease the
response time of the system, but it will result in system stability.
The parameters Cn-64 (PID Target Upper Limit) and Cn-65 (PID Target Lower Limit) limit the value of the PID
set-point (target) signal. The actual frequency command is limited by parameters Cn-18 (Frequency command
upper bound) and Cn-19 (Frequency command lower bound). This is shown in Fig. 1.3.3 below.
.
PID Set-point (Target)
PID
Output
PID Feedback
Output
Frequency
PID
Function
Soft Start
Limit
Cn-18
&
Cn-19
Freq.
Cmd.
Limit
Cn-64 & Cn-65
Fig. 1.3.3 Cn-64 & Cn-65 and Cn-18 & Cn-19 Limits
1.4 PID Control Functions Using the Multi-function Input Terminals
Table.1.4.1 below lists of the settings for multi-function inputs (Sn-25 to Sn-28) for the PID function.
Table 1.4.1 Multi-function Input Settings for Parameters Sn-25 to Sn-28
Sn-25 to Sn-28
Setting
Function
Description
14
PID integration reset
ON: Reset PID integration
15
PID control invalid
ON: PID control not effective
30
PID control invalid 2
ON: PID control not effective, using An-16
as frequency command
PID Integral Reset (Setting: 14).
The integral can be reset to zero through one of the multi-function input terminals
-
(Sn-25 to 28 = 14).
Cont.
