PID control

Question 1

Circle

Answer 1

Discrete control element located in and visible in the field. transmitters and valves

Question 2

Circle with box

Answer 2

Control element connected to a control system but visible in the field

Question 3

circle with line

Answer 3

Discrete control element located in the field, but visible in the control room. indicator, alarm, switch

Question 4

Circle with box and line

Answer 4

Control element connected to a control system and visible in the control room. controller

Question 5

cascade control

Answer 5

Two or more controllers with one nested inside the other (master and slave). output from the master controller is the set point for the slave controller

Question 6

Cascade control sequence

Answer 6

1. start on manual
2. change to auto SP set by operator
3. When settled change to cascade, where master SP set by controller

Question 7

Ratio controll

Answer 7

Applied when two flow rates need to be kept to a constant ratio. Ensures that flow set points of two loops are kept in proportion

Question 8

Feed forward control

Answer 8

Set point of upstream flow determines the required flow of stream downstream e.g outlet temp set point we can calculate the required steam flow to heat it up to that point.

Question 9

Interacting control loops

Answer 9

Heat exchange with changing flow rate and temps. loops interact - flow affects temp

1. feed forward
2. decoupling math
3. set up control loops with different control loops with different speed of response.

Question 10

Inverse response

Answer 10

opposite response than what is expected. e.g adding more steam into column expect to decrease liquid level as more is evaporated. However the extra gas results in more overflow as more liquid is pushed off the trays and increases level in re-boiler.

Question 11

Valve position control

Answer 11

used for fine adjustment, large valve set to 50% open with another smaller valve controlled by pHC to make fine adjustments.

Question 12

V-notch weir control

Answer 12

non - linear, need to think about best and worst case conditions, overall no good reason to use one

Question 13

Conical tank control

Answer 13

very non-linear, at low h dh/dt is very high. Don’t install in real process bad for control only need conical shape at the base of a tank.

Question 14

design for good control

Answer 14

Minimise dead time - install measurements as close as possible to the process

Disturbance rejection - balance tank (small), good flow control.

Environmental upstream control - control inputs into a process as well as possible. don’t rely on feedback control

Avoid non-linear elements - no conical tanks, v-weir notch

consider effect of process recycle - recycle disturbances less stable control.

Question 15

Derivative control + noise

Answer 15

derivative action causes the noise (random error) in the PV signal to be amplified and reflected in the controller output. (Bad)

Question 16

PID control

Answer 16

The three terms of a properly tuned PID controller thus work together to provide a rapid response to error (proportional term), to eliminate offset (integral term), and to minimize oscillations in the PV (derivative term).

Question 17

Proportional control (P)

Answer 17

Minimise error

Question 18

Integral term (I)

Answer 18

Eliminate offset

Question 19

Derivative control (D)

Answer 19

minimize oscillations in the PV

Question 20

bumpless transfer

Answer 20

bumpless transfer aims to remove the “jump” in valve signal when we transition between manual to automatic. we want u_manual = u_auto at change over time
=IF(D19(u)=”manual”, u-manual - u_P - u_D, u_I + kc/Tiet)

Question 21

phase angle

Answer 21

• deadtime / period x 360 (negative due to lag)

Question 22

Amplitude ratio

Answer 22

output amplitude / input amplitude