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  • 1. Introduction To TemperatureControllers On/Off An on-off controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. An on-off controller will switch the output only when the temperature crosses the setpoint. For heating control, the output is on when the temperature is below the setpoint, and off above setpoint. Z Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. In cases where this cycling occurs rapidly, and to prevent damage to contactors and valves, an on- off differential, or “hysteresis,“ is added to the controller operations. This differential requires that the temperature exceed setpoint by a certain amount before the output will turn off or on again. On-off differential prevents the output from “chattering” (that is, engaging in fast, continual switching if the temperature’s cycling above and below the setpoint occurs very rapidly). On-off control is usually used where a precise control is not necessary, in systems which cannot handle the energy’s being turned on and off frequently, where theThe Miniature CN77000 is a full featured mass of the system is so great that temperaturesmicroprocessor-based controller in a 1/16 DIN change extremely slowly, or for a temperature alarm.package. One special type of on-off control used for alarm is aHow Can I Control My Process Temperature Accurately limit controller. This controller uses a latching relay,and Reliably? which must be manually reset, and is used to shut down a process when a certain temperature is reached.To accurately control process temperature withoutextensive operator involvement, a temperature controlsystem relies upon a controller, which accepts atemperature sensor such as a thermocouple or RTD as ON ON ON ONinput. It compares the actual temperature to the desiredcontrol temperature, or setpoint, and provides an output Heaterto a control element. OFF OFF OFF OFFThe controller is one part of the entire control system, Temperatureand the whole system should be analyzed in selectingthe proper controller. The following items should beconsidered when selecting a controller: 1. Type of input sensor (thermocouple, RTD) and temperature range 2. Type of output required (electromechanical relay, SSR, analog output) 3 Control algorithm needed (on/off, proportional, PID) 4 Number and type of outputs (heat, cool, alarm, Setpoint limit) On-OffWhat Are the Different Types of Controllers, and How DifferentialDo They Work? (Deadband)There are three basic types of controllers: on-off, Timeproportional and PID. Depending upon the system to becontrolled, the operator will be able to use one type or ON/Off Temperature Control Actionanother to control the process. Z-110
  • 2. Introduction To Temperature Controllers Cont’dProportional temperature difference. If the temperature is below setpoint, the output will be on longer; if the temperatureProportional controls are designed to eliminate the is too high, the output will be off longer.cycling associated with on-off control. A proportionalcontroller decreases the average power being supplied The proportional band is usually expressed as ato the heater as the temperature approaches setpoint. percent of full scale, or degrees. It may also be referredThis has the effect of slowing down the heater, so that it to as gain, which is the reciprocal of the band. Note,will not overshoot the setpoint but will approach the that in time proportioning control, full power is applied tosetpoint and maintain a stable temperature. This the heater, but is cycled on and off, so the average timeproportioning action can be accomplished by turning the is varied. In most units, the cycle time and/oroutput on and off for short intervals. This “time proportional band are adjustable, so that the controllerproportioning “ varies the ratio of ‘on’ time to ‘off‘ time to may better match a particular process.control the temperature. The proportioning actionoccurs within a “proportional band” around the setpoint In addition to electromechanical and solid state relaytemperature. Outside this band, the controller functions outputs, proportional controllers are also available withas an on-off unit, with the output either fully on (below proportional analog outputs, such as 4 to 20 mA or 0 tothe band) or fully off (above the band). However, within 5 Vdc. With these outputs, the actual output level isthe band, the output is turned on and off in the ratio of varied, rather than the on and off times, as with a relaythe measurement difference from the setpoint. At the output controller.setpoint (the midpoint of the proportional band), the One of the advantages of proportional control isoutput on:off ratio is 1:1; that is, the on-time and off-time simplicity of operation. It may require an operator toare equal. If the temperature is further from the setpoint, make a small adjustment (manual reset) to bring thethe on- and off-times vary in proportion to the temperature to setpoint on initial startup, or if the process conditions change significantly. 15 Sec. On 5 Off Time Proportional 4-20 mA Proportional Percent On Time Off Time Temp Output Percent On Seconds Seconds (ºF) Level Output Repetitive 0.0 0.0 20.0 over 540 4 mA 0.0 20 sec. Cycle 0.0 0.0 20.0 540.0 4 mA 0.0 Time 12.5 2.5 17.5 530.0 6 mA 12.5 25.0 5.0 15.0 520.0 8 mA 25.0 Time Proportioning at 75% Output Level 37.5 7.5 12.5 510.0 10 mA 37.5 50.0 10.0 10.0 500.0 12 mA 50.0 62.5 12.5 7.5 490.0 14 mA 62.5 75.0 15.0 5.0 480.0 16 mA 75.0 87.5 17.5 2.5 470.0 18 mA 87.5 100.0 20.0 0.0 460.0 20 mA 100.0 100.0 20.0 0.0 under 460 20 mA 100.0 Proportional Bandwith Example: heating Setpoint: 500˚F Proportional Band: 80˚F (±40˚F) Systems that are subject to wide temperature cycling will also need proportional controllers. Depending upon the process and the precision required, either a simple proportional control or one with PID may be required. Processes with long time lags and large maximum rate of rise (e.g., a heat exchanger), require wide proportional bands to eliminate oscillation. The wide band can result in large offsets with changes in the load. To eliminate these offsets, automatic reset (integral) can be used. Derivative (rate) action can beThe CN2010 controller features ramp and soak, the ability used on processes with long time delays, to speedto control temperature over time. recovery after a process disturbance. Z-111
  • 3. There are also other features to consider when process temperature back to setpoint,selecting a controller. These include auto- or self-tuning, where the instrument will automatically calculate The rate or derivative function provides the controllerthe proper proportional band, rate and reset values for with the ability to shift the proportional band, toprecise control; serial communications, where the unit compensate for rapidly changing temperature. Thecan “talk” to a host computer for data storage, analysis, amount of shift is proportional to the rate of temperatureand tuning; alarms, that can be latching (manual reset) change.or non-latching (automatic reset), set to trigger on high A PID, or three-mode controller, combines theor low process temperatures or if a deviation from proportional, integral (reset) and derivative (rate)setpoint is observed; timers/event indicators which can actions, and is usually required to control difficultmark elapsed time or the end/beginning of an event. In processes. These controllers can also be made withaddition, relay or triac output units can be used with two proportional outputs, one for heating and anotherexternal switches, such as SSR solid state relays ormagnetic contactors, in order to switch large loads up to for cooling. This type of controller is required for Z processes which may require heat to start up, but then75 A. generate excess heat at some time during operation.PID What are the Different Output Types That Are AvailableThe third controller type provides proportional with for Controllers?integral and derivative control, or PID. This controllercombines proportional control with two additionaladjustments, which helps the unit automatically The output from the controller may take one of severalcompensate for changes in the system. These forms. The most common forms are time proportionaladjustments, integral and derivative, are expressed in and analog proportional. A time proportional output applies power to the load for a percentage of a fixed cycle time. For example, with a 10 second cycle time, if the controller output were set for 60%, the relay would Offset be energized (closed, power applied) for 6 seconds, PB SP and de-energized (open, no power applied) for 4 Temp. seconds. Time proportional outputs are available in three different forms: electromechanical relay, triac or ac solid state relay, or a dc voltage pulse (to drive an external solid state relay). The electromechanical relay Time is generally the most economical type, and is usually chosen on systems with cycle times greater than 10 Process with Temperature Offset seconds, and relatively small loads.time-based units; they are also referred to by their An ac solid state relay or dc voltage pulse are chosenreciprocals, RESET and RATE, respectively. for reliability, since they contain no moving parts. Recommended for processes requiring short cycleThe proportional, integral and derivative terms must be times, they need an additional relay, external to theindividually adjusted or “tuned” to a particular system, controller, to handle the typical load required by ausing a “trial and error” method. It provides the most heating element. These external solid state relays areaccurate and stable control of the three controller types, usually used with an ac control signal for ac solid stateand is best used in systems which have a relatively relay output controllers, or with a dc control signal for dcsmall mass, those which react quickly to changes in voltage pulse output controllers.energy added to the process. It is recommended insystems where the load changes often, and the An analog proportional output is usually an analogcontroller is expected to compensate automatically due voltage (0 to 5 Vdc) or current (4 to 20 mA). The outputto frequent changes in setpoint, the amount of energy level from this output type is also set by the controller; ifavailable, or the mass to be controlled. the output were set at 60%, the output level would be 60% of 5 V, or 3 V. With a 4 to 20 mA output (a 16 mAWhat Do Rate and Reset Do, and How Do They Work? span), 60% is equal to (0.6 x 16) + 4, or 13.6 mA.Rate and reset are methods used by controllers to These controllers are usually used with proportioningcompensate for offsets and shifts in temperature. When valves or power controllers.using a proportional controller, it is very rare that the What Should I Consider When Selecting a Controller forheat input to maintain the setpoint temperature will be My Application?50%; the temperature will either increase or decreasefrom the setpoint, until a stable temperature is obtained. When you choose a controller, the main considerationsThe difference between this stable temperature and the include the precision of control that is necessary, andsetpoint is called offset. This offset can be how difficult the process is to control. For easiest tuningcompensated for manually or automatically. Using and lowest initial cost, the simplest controller which willmanual reset, the user will shift the proportional band so produce the desired results should be selected.that the process will stabilize at the setpointtemperature. Automatic reset, also known as integral, Simple processes with a well matched heater (not over-will integrate the deviation signal with respect to time, or undersized) and without rapid cycling can possiblyand the integral is summed with the deviation signal to use on-off controllers. For those systems subject toshift the proportional band. The output power is thus cycling, or with an unmatched heater (either over- orautomatically increased or decreased to bring the undersized), a proportional controller is needed. ® Z-112
  • 4. Temperature ControllersSelection ConsiderationsCONTROLLABILITY OF CONTROL ALGORITHM (MODE) output is turned on and off in the ratio ofELECTRIC HEAT This refers to the method in which the the measurement difference from theThe basic function of a controller is to controller attempts to restore system setpoint. At the setpoint (the midpoint ofcompare the actual temperature with its temperature to the desired level. The two the proportional band), the output on/offsetpoint and produce an output that will most common methods are two-position ratio is 1:1, that is, the on-time and off-maintain the setpoint. (on-off) and proportioning (throttling) time are equal. If the temperature isThe controller is one part of the entire controls. further from the setpoint, the on- and off-control system, and the whole system ON/OFF CONTROL times vary in proportion to theshould be analyzed in selecting the On/off control has the simplest of control temperature difference. If theproper controller. The following items modes. It has a deadband (differential) temperature is below setpoint, the outputshould be considered when selecting a expressed as a percentage of the input will be on longer. If the temperature is toocontroller. span. The setpoint is usually in the high, the output will be off longer.1. Type of input sensor (thermocouple, center of the deadband. Therefore, if the RTD, card, and temperature range) input is 0 to 1000°F, the deadband is 1% and the setpoint is set at 500°F, the2. Placement of sensor output will be full on when the3. Control algorithm needed (on/off, temperature is 495°F or below and will proportional, PID, autotune PID) stay full on until the temperature reaches Above Temp.4. Type of output hardware required 505°F, at which time the output will be (electromechanical relay, SSR, analog full off. It will stay full off until the output signal) temperature drops to 495°F. On Off5. Additional outputs or requirements of If the process has a fast rate of Time Time system (display required of response, the cycling between 495 and At 505°F will be fast. The faster the rate of Setpoint temperature and/or setpoint, cooling Temp. outputs, alarms, limit, computer response of the process, the greater the communication, etc.) overshoot and undershoot and the faster the cycling of the contactor when used asTYPE OF INPUT a final control element. BelowThe type of input sensor will depend on Temp.the temperature range required, the On/off control is usually used where aresolution and accuracy of the precise control is not necessary, formeasurement required, and how and example, in systems that cannot handle having the energy turned on and off Figure 1: Proportional controlwhere the sensor is to be mounted. frequently, where the mass of the systemPLACEMENT OF THE SENSOR is so great that the temperature changesThe correct placement of the sensing The proportional band is usually extremely slowly, or for a temperature expressed as a percentage of full inputelement with respect to the work and alarm.heat source is of the utmost importance range scale, or in degrees. It may also befor good control. If all three can be One special type of on/off control used referred to as gain, which is thelocated in close proximity, a high degree for alarm is a limit controller. This reciprocal of the band. In many units, theof accuracy, up to the limit of the controller uses a latching relay, which cycle time and/or proportional bandwidthcontroller, is relatively easy to achieve. must be manually reset, and is used to are adjustable, so that the controller canHowever, if the heat source is located shut down a process when a certain be better matched to a particularsome distance from the work, widely temperature is reached. process.different accuracies can be obtained just PROPORTIONAL Proportional controllers have a manualby locating the sensing element at Proportional controls are designed to reset (trim) adjustment, which can bevarious places between the heater and eliminate the cycling associated with used to adjust for an offset between thethe work. on/off control. A proportional controller steady state temperature and theBefore selecting the location for the decreases the average power being setpoint.sensing element, determine whether the supplied to the heater as the temperature In addition to electromechanical and solidheat demand will be predominantly approaches setpoint. This has the effect state relay outputs, proportionalsteady or variable. If the heat demand is of slowing down the heater so that it will controllers are also available withrelatively steady, placement of the not overshoot the setpoint, but will proportional analog signal outputs, suchsensing element near the heat source approach the setpoint and maintain a as 4 to 20 mA or 0 to 5 Vdc. With thesewill hold the temperature change at the stable temperature. This proportioning outputs, the actual output level amplitudework to a minimum. action can be accomplished by turning is varied, rather than the proportion of on the output on and off for short intervals. and off times.On the other hand, placing the sensing This “time proportioning” varies the ratioelement near the work, when heat of “on” time to “off” time to control the PROPORTIONAL PLUS INTEGRALdemand is variable, will enable it to more temperature. PLUS DERIVATIVE CONTROL MODEquickly sense a change in heat (PID):requirements. However, because of the The time period between two successive This controller operates the same way aincrease in thermal lag between the turn-ons is known as the “cycle time” or proportional controller does, except thatheater and the sensing elements, more “duty cycle.” The proportioning action the function of the trim adjustment isovershoot and undershoot can occur, occurs within a “proportional band” performed automatically by the integralcausing a greater spread between around the setpoint temperature. Outside function (automatic reset). Thus, loadmaximum and minimum temperature. this band, the controller functions as an changes are compensated forThis spread can be reduced by selecting on/off unit, with the output either fully on automatically and the temperaturea PID controller. (below the band) or fully off (above the agrees with the setpoint under all band). However, within the band, the operating conditions. Offset is eliminated. Z-113
  • 5. Temperature ControllersSelection Considerations terms must be “tuned,” i.e., adjusted to a The types of hardware available, external particular process. This is done by trial to the controller, to allow it to handle the and error. Some controllers called load, are as follows: Autotune controllers attempt to adjust the 1) Mechanical Contactor Low Rate PID parameters automatically. 2) ac controlled solid state relay Setting TYPE OF CONTROL OUTPUT 3) dc controlled solid state relay HARDWARE 4) Zero crossover SCR power controller The output hardware in a temperature 5) Phase angle fired SCR power controller Z Set controller may take one of several forms. Mechanical contactors are externalPoint Deciding on the type of control hardware relays, which can be used when a higher Rate Set to be used depends on the heater used amperage than can be handled by the Properly and power available, the control relay in the controller is required, or for algorithm chosen, and the hardware some three-phase systems. They are not external to the controller available to recommended for cycle times shorter handle the heater load. The most than 15 seconds. commonly used controller output Solid state relays have the advantage hardware is as follows: over mechanical contactors, in that they Time Proportional or On/Off have no moving parts, and thus can beFigure 2: Rate function compensates for rapid 1) Mechanical Relay used with short cycle times. The shorterchanges. 2) Triac (ac solid state relay) the cycle time, the less dead lag and the 3) dc Solid State Relay Driver (pulse) better the control. The “switching” takesThe derivative function (rate action) Analog Proportional place at the zero voltage crossover pointcompensates for load changes which 1) 4-20 mA dc of the alternating current cycle; thus, notake place rapidly. An example is a 2) 0-5 Vdc or 0-10 Vdc appreciable electrical noise is generated.traveling belt oven where the product is An ac controlled solid state relay is usedfed intermittently. When the product A time proportional output applies power with either a mechanical relay or triacenters the oven, there is a sharp rise in to the load for a percentage of a fixed output from the controller, and isthe demand for heat, and when it stops, cycle time. For example, with a 10 available for currents up to 90 amps atthere is an excess of heat. Derivative second cycle time, if the controller output voltages of up to 480 Vac. DC solid stateaction reduces the undershoot and were set for 60%, the relay would be relays are used with dc solid state driverovershoot of temperature under these energized (closed, power applied) for 6 (pulse) outputs. The “turn on” signal canconditions and prevents bad product due seconds, and de-energized (open, no be from 3 to 32 Vdc and models are power applied) for 4 seconds. available to control up to 90 amps at up The electromechanical relay is generally to 480 Vac. the most economical output type, and is Zero crossover SCR power controllers usually chosen on systems with cycle are used to control single or three-phase times greater than 10 seconds and power for even larger loads. They can be relatively small loads. used for currents up to 200 amps at Choose an ac solid state relay or dc 480 volts. A 4-20 mA dc control signal is Set voltage pulse to drive an external SSR usually required from the controller. The Point with reliability, since they contain no zero crossover SCR power controllers moving parts. They are also convert the analog output signal to a time recommended for processes requiring proportional signal with a cycle time of Offset short cycle times. External solid state about two seconds or less, and also relays may require an ac or dc control provide switching at the zero crossover signal. point to avoid generating electrical noise. An amplitude proportional output is Phase angle SCR power controllers also usually an analog voltage (0 to 5 Vdc) or are operated by a 4-20 mA dc controller current (4 to 20 mA). The output level output. Power to the load is controlled by from this output type is also set by the governing the point of turn on (firing) of controller. If the output were set at 60%, each half cycle of a full ac sine wave.Figure 3: Reset function eliminates offset. the output level would be 60% of 5 V, or This has the effect of varying the voltageto over or under curing. 3 V. With a 4 to 20 mA output (a 16 mA within a single 0.0167 second period. By span), 60% is equal to (0.6 x 16) + 4, or comparison, time proportional controllersPID control provides more accurate and 13.6 mA. These controllers are usually vary the average power over the cyclestable control than on/off or proportional used with SCR power controllers or time, usually more than 1 second, andcontroller types. It is best used in proportioning valves. often more than 15 seconds. Phasesystems that have a relatively small angle SCR’s are only recommended formass and which react quickly to changes The power used by an electrical resistance heater will usually be given in low mass heating elements such asin energy added to the process. It is infrared lamps or hot wire heaters.recommended in systems where the load watts. The capacity of a relay is given inchanges often. The controller is expected amps. A common formula to determineto automatically compensate the amount the safe relay rating requirements is:of energy available or the mass to be W = V(A)(1.5) or A = W/(V)(1.5)controlled, due to frequent changes in Where A = relay rating in ampssetpoint. W = heater capacity in wattsThe proportional, integral and derivative V = voltage used 1.5 = safety factor Z-114
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