Honeywell VRX180 user manual Set Point Profile Implementation, Example Set Point Profile

Programming and Operating Concepts

Set Point Profile Implementation

By definition, set point profiles are essentially user specified plots of process values against time. These plots are characterized by “segments” which are a series of intervals of varying time lengths that divide the plots into several segments. Within each segment, process values are typically drawn as straight lines that ramp up or down or stay constant at predetermined levels. An example of a simple five- segment set point profile is shown in Figure 3-23. Set point profiles with up to 63 segments can be specified using the instrument. Note that when a segment depicts the process value as sloping up or down, it is referred to as a “ramp.” The term “soak” is used to describe a segment when the process value is made to stay constant. In Figure 3-23, segments 1, 3, and 5 are ramps while segments 2 and 4 are soaks.

PROCESS

VALUE IN

ENGINEERING

UNITS

&	∋	(	)	∗
		TIME

Figure 3-23 Example Set Point Profile

To force a process value to vary linearly with time at various rates within successive time intervals is the job of a set point profiler, another class of function blocks available within the instrument. Be advised that use of set point profilers is typically observed in thermal or heat treat applications. For example, being able to vary temperature in accordance with a set point profile is vital in the tempering of metal or ceramic parts.

Refer to the application of Figure 3-11 discussed at the beginning of this section. This application dealt with controlling a furnace zone’s temperature by means of a 4 to 20 mA gas valve actuator. If the furnace zone temperature were to be manipulated so that it followed the ramps and soaks of a set point profile, the first step would be to implement the function block diagram established in Figure 3-16In general, the control configuration that holds a process value to a local set point, must be programmed and on line before allowing the process value to be characterized by a profile. With regard to the application at hand, a set point profiler function block programmed with a user defined set point profile may be brought into the configuration once the furnace zone’s basic temperature control loop is operational. Note that the output of the profiler function block will essentially be the set point profile.

From the cascade control strategy’s explanation, recall that all control loop function blocks within the instrument have a Setpoint #2 parameter that may be used as a remote set point input for connecting to the profiler’s output.

LP1 in the function block diagram of Figure 3-16 will make use of SP2’s remote set point functionality so that a set point profiler’s time varying set point may be applied to it. Refer to Figure 3-24.

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