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Hitachi SJ 300 User Manual

Page 6

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SJ100/L100 / PID / 6

2-2 PID Gain Adjustments & Control Characteristics

The optimal gain factors of PID vary from condition to condition, and from system to system. That

means it is necessary to set those parameters by taking into account the individual control characteristics

of your particular system. The following are the characteristics that are required for a good PID control:

l Stable performance

l Quick response

l Small steady-state deviation

You adjust each parameter K

p

, T

i

and K

d

inside the stable performance area. Generally, when you

increase each gain (K

p

, K

i

, K

d

) parameter (= decrease Integration time : T

i

), you can obtain quick

response. But if you increase them too much, the control will be unstable, because the feedback value is

continuously increasing and decreasing, which leads to an oscillation of the control. In the worst case the

system is led to a divergence mode. (Refer to Fig. 2-4)

Following are the procedures to adjust each parameter.

(1) After changing target,

response is slow

--- Increase P-gain (K

p

)

response is quick but unstable

--- Decrease P-gain (K

p

)

(2) Target and feedback

do not become equal

--- Decrease Integration time (T

i

)

become equal after unstable vibration

--- Increase Integration time (T

i

)

(3) Even after increasing Kp, response is still slow

--- Increase D-gain (K

d

)

it is still unstable

--- Decrease D-gain (K

d

)

Controlled
object

time

Target

Controlled
object

NG : Divergence

Fig. 2-4 Example of good control and bad control (in case of step response)

Controlled
object

Target

NG : Damped oscillation

time

Controlled
object

Target

Good Control

time

Target

NG : Slow response, big

steady state deviation

time

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