On—off control

The most rudimentary form of regulatory control is on-off control. This type of control is primarily intended for use with FCEs that are non-throttling in nature, i.e. some type of switch as opposed to a valve. An excellent example of on-off control is a home heating system. Whenever the temperature goes above the set point, the heating plant shuts off, and whenever the tenperature drops below the set point, the heating plant turns on. This behaviour is shown by Equation 4.1  

The controller output MV is equal to 0 per cent or off whenever PV exceeds the set point SP. Whenever the process variable is below the set point, the controller output is equal to 100 per cent or on. 

In this example, at time t = 0, PV is less than SP, and MV is equal to 100 per cent. When PV crosses the set point, MV becomes 0 per cent. The temperature rises somewhat 

Before completing this section, it is right that we briefly examine the use of on-off control -also called bang-bang control. While primarily used for temperature control in domestic 

Although the controller has a SP, there must be a deadband around this value within which no control action takes place. Without this the MV would be switched on and off at an unsustainable frequency. In domestic situations this deadband occurs almost accidentally as a result of the mechanism involved. Ternperamre is generally measured using a bimetallic strip which, as it bends, makes or breaks a contact The distance it has to move between contacts provides the small deadband necessary. If this were not the case, then a deadband would need to be dehberately designed into the controller. 

On-off control is thus only applicable on industrial processes where tight control is not necessary and where deadtime is negligible. This restricts its use primarily to some level controllers. Typically it would be implemented using high and low level limit switches that would activate a solenoid valve. It can be emulated with a high gain proportional-only controller with a deadband, but care needs to be taken that excessive control action does not damage the control valve. 

There are basically two ways to keep the level of a variable within certain limits the on-off method and the modulating or continuous adjustment method. The on-off control is probably the simplest type of automatic control. 

In on-off control of temperature, the power to the extruder is fuii-on when the measured temperature is below the setpoint and completely off when the measured temperature is above the setpoint. This situation is shown in Fig. 4.22. 

The temperature in most homes is controlled by thermostats using the on-off principle. On-off control is widely used in the industry to control temperature and other variables. In fact, some missile guidance systems use on-off control, indicating that accurate control in sophisticated systems can be achieved by on-off control. That is, of course, if the system characteristics lend themselves to such type of control. 

One of the problems of applying on-off control to the heating and cooling of extruders is the significant thermal lag in these machines. There is an inherent delay between the time the controller calls for heat from the heater band, and the time when the heat actually reaches the sensor. The same holds true when the controller turns off. This thermal lag can be several minutes (about 5 minutes for a 90 mm 

Time/power-temperature diagram for on-off control with hysteresis 

---

The actuator contains the final orifice and a finger pad or mechanical linkage for on—off control. The spray pattern is largely affected by the constmction of the actuator, particularly by the chamber preceding the orifice. Actuators are often termed mechanical breakup and nonmechanical breakup depending upon the complexity of this chamber. Mechanical breakup actuators are of more expensive two-piece constmction. Actuators are usually molded from polyethylene or polypropylene the breakup insert may be almost any material, including metal. 

On/Off Control An on/off controller is used for manipulated variables having only two states. They commonly control temperatures in homes, electric water-heaters and refrigerators, and pressure and liquid level in pumped storage systems. On7off control is satisfac-toiy where slow cychng is acceptable because it always leads to cycling when the load hes between the two states of the manipulated variable. The cycle will be positioned symmetrically about the set point only if the normal value of the load is equidistant between the two states of the manipulated variable. The period of the symmetrical cycle will be approximately 40, where 0 is the deadtime in the loop. If the load is not centered between the states of the manipulated variable, the period will tend to increase, and the cycle follows a sawtooth pattern. 

A three-state controller is used to drive either a pair of independent on/off actuators such as heating and cooling valves, or a bidirectional motorized actuator. The controller is actually two on/off controllers, each with deadband, separated by a dead zone. When the controlled variable lies within the dead zone, neither output is energized. This controller can drive a motorized valve to the point where the manipulated variable matches the load, thereby avoiding cychng. 

The Ziegler and Nichols closed-loop method requires forcing the loop to cycle uniformly under proportional control. The natural period of the cycle—the proportional controller contributes no phase shift to alter it—is used to set the optimum integral and derivative time constants. The optimum proportional band is set relative to the undamped proportional band P , which produced the uniform oscillation. Table 8-4 lists the tuning rules for a lag-dominant process. A uniform cycle can also be forced using on/off control to cycle the manipulated variable between two limits. The period of the cycle will be close to if the cycle is symmetrical the peak-to-peak amphtude of the controlled variable divided by the difference between the output limits A, is a measure of process gain at that period and is therefore related to for the proportional cycle ... 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Solenoid Valves The electric solenoid valve has tw o output states. Wlien sufficient electric current is supplied to the coil, an internal armature moves against a spring to an extreme position. This motion causes an attached pneumatic or hvdraiilic valve to operate. Wlien current is removed, the spring returns the armature and the attached solenoid valve to the deenergized position. An intermediate pilot stage is sometimes used when additional force is required to operate the main solenoid valve. Generallv, solenoid valves are used to pressurize or vent the actuator casing for on/off control-valve application and safetv shutdown applications. 

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

Ail compressor anti-surge (ON - OFF) control circuit, comprising transmitters, computers and pneumatic blo-w-off control valve Reverse flow protection (on axial compressors only) as supplementary protection device against surging, v. orkmg indepenciently of the control ciicuit Blow-off. silencer Expander emergency stop valve with pneumatic actuator and solenoid valve... 

On/off control A simple two-position control system that is only capable of performing these two functions (on and off). 

Valves have two main functions in a pipeline to control the amount of flow, or to stop the flow completely. There are many different types but the most commonly used are the gate valve and the globe valve. The gate valve contains a disk that slides at right angles to the flow direction. This type of valve is used primarily for on-off control of a liquid flow. Because small lateral adjustments of the disk cause e.xtreme changes in the flow cross-sectional area, this type of valve is not suitable for adjusting flow rates. 

On-off control of steam systems can result in the formation of a partial vacuum, leading to condensate locking... 

Since the low-pressure float needs a solenoid valve for tight closure, this valve can be used as an on-off control in conjunction with a pre-set orifice and controlled by a float switch (Figure 8.2). 

Further unintended variations will occur with the flow of the primary cooling medium. With two-step (on-off) control of the compressor within an air-conditioning unit, the temperature will slowly rise while the compressor is off until the compressor restarts. 

Set the on/off control of the balance to the on position and record the weight shown on the digital display if the balance is linked to a printer, confirm that the printed result agrees with the digital display. Return the control to the off position. 

The traditional arrangement of simple spherical glassware and Isomantles with full-power on-off controllers monitored by mercury thermometers, would still be widely recognised. So too would be the plug-shot piston pumps set up and monitored by use of measuring cylinders. Although tried and tested this hardware system requires constant attention by a skilled lab. technician to achieve control and reproducibility of even the first-order process parameters manual data collection is hardly feasible at better than 10-15 minute intervals. 

The simplest and, despite its several drawbacks, the most widely used type of control is the on/off control system. An example is a contact thermometer, which closes or opens a heater circuit. The designation on/off means that the controller output, or the manipulated variable (electric current) is either fully on or completely off. To avoid oscillations around the setpoint, the real on/off controller has built into it, a small interval on either side of the setpoint, within which the controller does not respond, and which is called the differential gap or deadzone. When the controlled variable moves outside the deadzone, the manipulated variable is set either on or off. This is illustrated in Fig. 2.30. Such shifts from the set point are known as offset. 

Figure 2.30. On/off controller with differential gap or dead zone.

The oscillatory nature of the action and the offset make the resulting control rather imperfect, but the use of on/off control can be justified by its simplicity and low price, and the reasonable control obtained, especially for systems which respond slowly. 

On/off control 96 Operating line equation 595 Optimisation 108 Order of reaction 53, 323 Oscillating tank reactor 350 Oscillations 507, 518, 663... 

Figure 10-13 illustrates the flow versus valve stem travel characteristic for various typical valve trim functions (Fisher Controls, 1987). The quick opening characteristic provides the maximum change in flow rate at low opening or stem travel, with a fairly linear relationship. As the valve approaches the wide open position, the change in flow with travel approaches zero. This is best suited for on-off control but is... 

Controller load performance, (a) On-off controller ii proportional (P) (c) proportional-integral (J) proportional-integral-derivative (PID),... 

The length of the alkyl side-chain of alkyl methacrylate (RMA) was influential in controlling the thickness and density of the surface skin layer. Poly (I P A A-co-H M A) or poly(IPAAm-co-LMA), were H is hexyl and L is lauryl, was able to form a thin and dense surface skin layer which was favorable for rapid on-off control of drug release and for maintaining a constant release rate. 

Figure 13.3 illustrates a typical control action (below) and the response of a process variable (above) in an on - off control system. As a result of on - off control, the response of a process variable inevitably becomes oscillatory around a set-point (overshoot and hunting). Thus, the precise control of a process variable is difficult to achieve using an on-off control system, and, therefore, the on-off control can be applicable only when the minimum and maximum values of the response are acceptable for successful operation of the bioprocess. To decrease the oscillation around a set-point, a differential gap (or a dead zone) is normally used to determine a threshold value. 

Figure 13.3 Control action and response of an on-off controller with differential gaps.

The on-off control is a simple and cheap algorithm for manipulating a control variable, and, therefore, it has been apphed to bioprocess controls such as temperature control and pH control. 

Although the SCADA system is capable of on/off control functions such as opening and closing valves, supervisory setpoint control, and scanning of large... 

The vessel is supplied with a temperature-indicator (the probe, a Pt 100Q, is located inside the vessel) and a temperature ON-OFF controller (see Fig. 13) it is heated by an electric resistance (R2), manually regulated. The vessel, as well as the resistance, is thermally insulated to avoid heat dispersion. 

---