T HREE -P OSITION D ISCRETE C ONTROLLERS

A three-position controller provides three output levels, instead of just two output levels like a two-position controller. Basically, this controller has an additional ON setting at 50% of the full ON range. The use of a three-position controller tends to reduce the cycling behavior of the process variable because it provides an intermediate output level, rather than just the two level settings of an ON/OFF controller. The controller stops at the intermediate 50% setting when the set point is achieved. In fact, a controller’s output is usually designed so that its half output, or 50%, coincides with the level required by the process to maintain the process variable at the set point, minimizing the error in the system. Figure 15-18 illustrates a three-position, direct-acting controller’s output (CV) according to the error present in the system. This output can be represented mathematically as:

CV = 100 % IF error >+ ∆ E

CV = 50 % IF − ∆ E < error <+ ∆ E

CV = 0 %

IF error <− ∆ E

Because three-position field devices are not as widely available as two- position ON/OFF devices, analog field devices are often used to implement three-position control. A PLC may also implement a three-position output using a contact output interface (see Figure 15-19). The incoming sides of

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Figure 15-18. Three-position, direct-acting controller’s output in response to error.

4A To three-position mode field device

Inside Module

Figure 15-19. Contact output interface implementing three-position control.

three of the contact module’s terminals are connected to 0%, 50%, and 100% power signals, respectively. The other sides of the contacts join together and connect to the field device (e.g., valve). In this configuration, a PLC with a contact output module can be interfaced with analog signals set at 0%, 50%, and 100% of the full range of the field control device to implement three- position control. Discrete-mode controllers with more than three positions (e.g., five positions, multipositions, etc.) may also be implemented this way in some control applications. Since output field devices with more than three positions are not readily available, analog output devices are often used in

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conjunction with multiple output contact cards to obtain multiposition control. Figure 15-20 illustrates an example of this type of direct-acting control system with five possible output settings.

Analog Analog

Voltage Voltage 0% Reference 25% Reference 50% Reference 75% Reference 100% Reference

5B To analog control field device

Inside Module

Figure 15-20. (a)

A discrete mode, multiposition controller and (b) its output.

E X AM PLE 1 5 -3

Graphically illustrate the reaction of a three-position controller output to the steam heating system shown in Figure 15-21. Include the effect of the controller’s lag.

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Steam Return Hp

V2 V1 Steam

Figure 15-21. Steam heating system controlled by a three-position controller.

S OLU T I ON

Figure 15-22 shows the controller output ( CV ) for this three-position discrete-mode controller. Note that the response plot (see Figure 15- 22a) shows that an overshoot is present, indicating a lag in steam actuation. The same lag also creates an undershoot condition when both V1 and V2 are ON. As shown, the system overshoot and undershoot pass the deadband during the lag periods.

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PV PV = 210 ° F

(E = +10) (a) PV = SP = 200 ° F

ON ON 100%

t Figure 15-22. (a) Response plot and (b) controller output of the heating system in

OFF OFF 0%

Lag

Figure 15-21.

T HREE -P OSITION D ISCRETE C ONTROLLERS