Figure 1 - The Control System
The automatic control system process.
II. Sensors
Thermistor
A non-linear device whose resistance varies with temperature. Thermistors are used at temperatures under 500°F. Fragility limits their use in industrial applications.
Resistance Temperature Detector (RTD)
Changes in temperature vary the resistance of an element, normally a thin platinum wire. Platinum RTDs find application where high accuracy and low drift are required. 3-wire sensors are used where the distance between the process and the controller is more than several feet. The third wire is used for leadwire resistance compensation.
Thermocouple
A junction of two dissimilar metals produces a millivolt signal whose amplitude is dependent on (a) the junction metals; (b) the temperature under measurement. Thermocouples require cold-end compensation whereas connections between thermocouple wire and copper at the controller's terminal block produce voltages that are not related to the process temperature. Thermocouple voltage outputs are non-linear with respect to the range of temperatures being measured and, therefore, require linearization for accuracy. Thermocouple junctions are usually made by welding the dissimilar metals together to form a bead. Different thermocouple types are used for various temperature measurements as shown in Table 1. Thermocouples are the most commonly used industrial sensor because of low cost and durability.
|
Thermocouple Type |
Wire Color |
Useful Temperature Range °F |
|
J K T R/S |
White Yellow Blue Black |
32 to 1300 -328 to 2200 -328 to 650 -32 to 2642 |
Table 1
Other
Other temperature sensors include non-contact infrared pyrometers and thermopiles. These are used where the process is in motion or cannot be accessed with a fixed sensor.
III. Sensor Placement
Reduction of transfer lag is essential for accurate temperature control using simple temperature controllers. The sensor, heater and work load should be grouped as closely as possible. Sensors placed downstream in pipes, thermowells or loose-fitting platen holes will not yield optimum control. Gas and air flow processes must be sensed with an open element probe to minimize lag. Remember that the controller can only respond to the information it receives from its sensor.
IV. Process Load Characteristics
Thermal lag is the product of thermal resistance and thermal capacity. A single lag process has one resistance and one capacity. Thermal resistance is present at the heater/water interface. Capacity is the storage capacity of the water being heated. Sometimes the sensor location is distant from the heated process and this introduces dead time. (See Figure 2a) Introduction of additional capacities and thermal resistance changes the process to multi-lag. (See Figure 2b and 2c)
Figure 2 - Various Processes
V. Control Modes
On-Off
On-Off control has two states, fully off and fully on. To prevent rapid cycling, some hysteresis is added to the switching function. In operation, the controller output is on from start-up until temperature set value is achieved. After overshoot, the temperature then falls to the hysteresis limit and power is reapplied.
On-Off control can be used where:
The process is underpowered
and the heater has very little storage capacity. Where some temperature oscillation
is permissible. On electromechanical systems
(compressors) where cycling must be minimized.
Figure 3 - On/Off Control
Proportional
Proportional controllers modulate power to the process by adjusting their output power within a proportional band. The proportional band is expressed as a percentage of the instrument span and is centered over the setpoint. At the lower proportional band edge and below, power output is 100%. As the temperature rises through the band, power is proportionately reduced so that at the upper band edge and above, power output is 0%.
Figure 4 - Proportional Control
Proportional controllers can have two adjustments:
Manual Reset
Allows positioning the band with respect to the setpoint so that more or less power is applied at setpoint to eliminate the offset error inherent in proportional control.
Figure 5 - Manual Reset
Bandwidth (Gain)
Permits changing the modulating bandwidth to accommodate various process characteristics. High-gain, fast processes require a wide band for good control without oscillation. Low-gain, slow-moving processes can be managed well with narrow band to on-off control.
Figure 6 - Proportional Bandwith
The relationship between gain and bandwidth is expressed inversely:
Proportional-only controllers may be used where the process load is fairly constant and the setpoint is not frequently changed.
Proportional with Integral (PI), Automatic Reset
Integral action moves the proportional band to increase or decrease power in response to temperature deviation from setpoint. The integrator slowly changes power output until zero deviation is achieved. Integral action cannot be faster than process response time or oscillation will occur.
Figure 7
Proportional with Derivative (PD), Rate Action
Derivative moves the proportional band to provide more or less output power in response to rapidly changing temperature. Its effect is to add lead during temperature change. It also reduces overshoot on start-up.
Proportional Integral Derivative (PID)
This type of control is useful on difficult processes. Its integral action eliminates offset error, while derivative action rapidly changes output in response to load changes.
VI. Proportional Outputs
Load power can be switched by four different proportioning means:
Current Proportional
A 4-20 mA signal is generated in response to the heating % requirement. This signal is used to drive SCR power controllers and motor-operated valve positioners.
Figure 8 - Control Current vs. Power Output
Phase Angle
This method of modulating permits applying a portion of an ac sine wave to the load. The effect is similar to light dimmer function.
Figure 9 - Phase Angle
Time Proportioning
A clock produces pulses with a variable duty cycle. Outputs are either direct- or reverse-acting. Direct-acting is used for cooling; reverse-acting for heating.
Figure 10 - Time Proportioning
Cycle Time
In time proportioning control the cycle time is normally adjustable to accommodate various load sizes. A low mass radiant or air heater requires a very fast cycle time to prevent temperature cycling. Larger heaters and heater load combinations can operate satisfactorily with longer cycle times. Use the longest cycle time consistent with ripple-free control.
VII. Power Handlers
Power is switched to an electric heating load through the final control element. Small, single-phase 120/240 V loads may be connected directly to the temperature controller. Larger, higher voltage heaters must be switched through an external power handler. Power handlers are either large relays (contactors), solid-state contactors or power controllers.
Mechanical Contactors
They are probably the most widely used power handlers. They:
Are rugged. Have fuses to protect against
burnout due to shorts. Will wear out in time due to
contact arcing. Cannot be fast-cycled for low-mass
loads. Produce RF switching noise.
Solid-State Contactors
They are often used on loads requiring fast switching times. They need heat sinking and I2T fuse protection. SCR power controllers. These devices switch ac power by means of thyristors (SCRs). These are solid-state devices that are turned on by gate pulses. They have unlimited life and require no maintenance. SCR controllers are available for switching single- or three-phase loads in zero crossing/burst firing (Figure 12) or phase-angle modes (Figure 10).
SCR power control selection by switching method can be simplified, as follows:Use zero crossing for all standard heater applications.
Specify phase angle:
When soft start (ramp voltage
to peak) is required on high inrush heater loads. If voltage limit is needed to
clamp the maximum output voltage to a level lower than the supply voltage.
Figure 11
VIII. Heater and Power Control Connections
Power controls are connected to the control signal and load. (Figure 12) The control signal to the power controller may originate from a manual potentiometer, PLC or temperature controller. This signal is normally 4-20 mA, but can be other currents or voltages. An increase in the signal level produces a corresponding increase in power controller output. Calculation of SCR size for various voltages and heater sizes is as follows:
Equations to Calculate Power and Current
watts = total heater watts
volts = line voltage
amps = total line current
SCRs should not be sized at exactly the heater current requirement because heaters have resistance tolerances as do line supplies.
Example:
A single-phase 240 volt heater is rated at 7.2 kW
7,200 ÷ 4240 = 30 A
If the heater is 10% low on resistance, at 240 V, the heater will draw 33 amperes. Damage to fuses will result. Power controllers must be properly cooled and, therefore, the mounting location should be in a cool area. SCRs dissipate approximately 2 watts per ampere per phase.
Proper fusing is essential to protect the SCR devices from damage due to load short circuits. The type of fuse is marked I2T or semiconductor.
Only SCRs designed to drive transformers should be used for that purpose. SCR power controllers must never be used as disconnects in high-limit applications.
Figure 12
