Vacon 50X User Manual - page 110

109 

 vacon

fundamentals of pi control

Tel. +358 (0)201 2121 • Fax +358 (0)201 212205

 10

10.2.3   Parameter 852 (PI Prop Gain)

Parameter 852 (PI Prop Gain) is the proportional feedback gain for the process control loop. It determines 

the overall effect on the process for an incremental change in the feedback signal.

Generally, when configuring this parameter, you must observe the drive’s response to an incremental 

change in the feedback input, and then decide if this response is sufficient.

For example, if the feedback input changes 1 V (or 1 mA), what is the drive’s response? Is it enough or too 

much?

10.2.4   Parameter 853 (PI Int Gain)

Parameter 853 (PI Int Gain) is the integral feedback gain for the process control loop. This parameter de-

termines the short-term effects of a change in the feedback signal.

Generally, when configuring this parameter, you must observe the drive’s response to an incremental 

change in the feedback input over a certain length of time, and then decide if this response is acceptable.

For example, if the feedback input changed 1 V (or 1 mA) for 5 seconds, what is the drive’s response? Is 

it acceptable? Would you prefer to have the drive ignore a change over such a short time period, but still 

react to longer time durations (say, 8 to 10 seconds)? (If so, decreasing the integral gain by reducing the 

value for parameter 853 would have that effect.)

10.2.5   Parameter 854 (PI Feed Gain)

Parameter 854 (PI Feed Gain) is the feedback scaling factor. It is used to scale the signal supplied by the 

transducer — thereby optimizing the effect of the signal on the drive.

10.2.6   Parameter 850 (PI Configure)

Parameter 850 (PI Configure) determines the characteristics of the process control loop – direct-acting 

or inverse-acting (also known as reverse-acting), the rate of response (fast or slow), whether feed-for-

ward is enabled, and whether the loop is operated via digital inputs. The following paragraphs discuss 

each of these characteristics in more detail:

•

Direct- or inverse-acting loop

In a direct-acting loop, as the process speed increases, the feedback signal will decrease and cause a 

corresponding decrease in the process speed as it approaches the regulation point. This type is typically 

employed in pump applications where the level control is the process variable.

Conversely, in an inverse-acting loop, as the process speed increases, the feedback signal increases but 

causes a corresponding decrease in the process speed as it approaches the regulation point. This type is 

typically employed in supply pump applications where the pressure is the process variable.

•

Slow or fast rate of response

A slow rate of response (over 10 s, usually) is most often selected for processes with long time constraints 

(for example, thermal and fluid level controls). On the other hand, a fast response rate is utilized for pro-

cesses with short time constraints (such as mechanical systems and pressure loops). Most industrial 

systems require a slow rate of response.

•

Whether feed-forward is enabled

Feed-forward is usually enabled when there is very little difference between the process speed and the 

feedback signal.

For example, feed-forward is useful in “speed regulation” situations, such as controlling motor speed in 

a closed loop. Note that feed-forward should be enabled when attempting to close a speed loop.

Feed-forward is not suited to applications such as pressure regulation systems because generally the 

process speed and the process variable are vastly different.

•

Whether PI control is enabled via a digital input

A digital input, when properly configured via the corresponding parameter, may be used to toggle PI con-

trol.