Response Electronics CL6011B Especificaciones descargar pdf (Pagina 28)

3Ć8 Theory of Operation: Point Processing

Original Ć June 1990CE4.2:CL6211

3.3.1.4 Proportional-Derivative with Bias

The Proportional-Derivative with Bias (P/PD with Bias) PCA provides a

standard one or two mode control function. The P/PD with Bias PCA is

normally used as a PD controller, but it also allows the derivative term to

be tuned to zero to create a proportional-only controller.

The control algorithm can be viewed as a dual input, single output math

function whose output is derived from the periodic execution of a set of

positional control equations. The DCP's process variable is used as the

first input, the DCP's set point is used as the second input, and the result

of the control function becomes the IVP of the point.

The P/PD with Bias control algorithm is based on the following transfer

function:

IVP(s)

error(s)

Ă +Ă

K(T

Ăs ) 1)

aĂT

Ăs ) 1

where: T

= rate time constant

a = rate action limiter; 0.125

K = proportional gain

By using the intermediate term DN(s), the transfer function can be

rearranged to:

DN(s)

PV(s)

Ă +Ă

Ăs ) 1

aĂT

Ăs ) 1

IVP(s)

SP(s) * DN(s)

Ă +Ă K

These equations can be represented in the time domain as:

DN(s)Ă +Ă T

dPV

) PV * aĂT

dDN

IVPĂ +Ă K(SP * DN)

The equations shown are for reverse acting loops. The signs of the SP

and DN terms are inverted for direct acting loops. Note that the SP term

has been isolated so that rate action occurs only on changes to the PV,

which is necessary to obtain good control action in response to set point

changes. Also note that the rate action has been filtered to prevent the

algorithm from over reacting to high frequency noise.

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Response Electronics CL6011B Especificaciones Pagina 28

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