Study Questions

Apply Your Knowledge

What are the units of the PID tuning constants?

PID Algorithm

One constant for each mode.

KC, controller gain

check.gif (1210 bytes)

TI, integral time

check.gif (1210 bytes)

Td, derivative time check.gif (1210 bytes)











If the steady-state process gain (Kp) has a value of zero, the
controller gain (Kc) should be...

PID Algorithm








same sign as E = SP - CV.

feedback control is not possible.











Why is the
integral time (TI) in the denominator?

PID Algorithm




So that 

wpe355.gif (1111 bytes)

To prevent a division by zero.


An arbitrary convention.

Because the controller gain (KC) is in the numerator.












The PID controller...

PID Algorithm



is non-linear.


is linear (obeys proportionality and superposition)


obeys proportionality only.

obeys superposition only.












Zero steady-state offset is important because...

PID Algorithm

We want zero offset for most controlled variables!



it ensures stability.


the integral mode provides this.


the controlled variable ultimately returns to its set point after a disturbance.

the derivative of the controlled variable goes to zero.












What generally limits the "
aggressiveness" possible with the PID controller
( why must we observe limits, |KC|< max, TI > min )?

PID Algorithm



Floating point errors in the digital computers.


The valve going fully open or closed.


Instability occurring when the controller is too aggressive.





What is the effect of each mode of the PID controller the on steady-state offset for a step input?

PID Algorithm


1. Proportional

2. Integral

3. Derivative






What is the purpose of the
initialization constant "I" in the PID algorithm?

PID Algorithm



To ensure zero steady-state offset.


To prevent the valve from going fully open.


To provide initialization, i.e., "bumpless transfer".

To add a fourth mode.