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INDUSTRIAL CONTROL HANDBOOK - 0.4 COMPONENTS IN AUTOMATION

When we discuss automation in this text, we will always mean controlled automation. A circuit that allows you to control a heater is a labor-saving device, but without components to ensure that the room temperature remains comfortable, it is not an automated system.

Figure 0.3 illustrates the essential components in a controlled automated system:

  • the actuator (which does the work)
  • the controller (which "tells" the actuator to do work)
  • and the sensor (which provides feedback to the controller so that it knows the actuator is doing work)

 

0.4.1 The Controller as an Automation Component

A controlled system may be a simple digital system. An example is shown in figure 0.4, in which the actuator consists of a pneumatic valve and a pneumatic cylinder that must be either fully extended or retracted. The controller is a PLC that has been programmed to extend the cylinder during some more complicated process, and to go on to the next step in the process only after the cylinder extends.

Fig. 0.3 Components of a simple controlled automation systemFig. 0.3 Components of a simple controlled automation system

When it is time to extend the cylinder, the PLC supplies voltage to the valve, which should open to provide air to the cylinder, which should then extend. If all goes well, after a short time the PLC will receive a change in voltage level from the limit switch, allowing it to execute the next step in the process. If the voltage from the switch does not change for any reason (faulty valve or cylinder or switch, break in a wire, obstruction preventing full cylinder extension, etc.), the PLC will not execute the next step. The PLC may even be programmed to turn on a "fault" light when such a delay occurs.

A controlled system might be an analog system, as illustrated in figure 0.5. In this system, the actuator is a hydraulic servovalve, and a fluid motor. The servovalve opens proportionally with the voltage it receives from the controller, and the fluid motor rotates faster if it receives more hydraulic fluid. There is a speed sensor connected to the motor shaft, which outputs a voltage signal proportional to the shaft speed. The controller is programmed to move the output shaft at a given speed until a load is at a given position. When the program requires the move to take place, the controller outputs an approximately correct voltage to the servovalve, then monitors the sensor's feedback signal. If the speed sensor's output is different than expected (indicating wrong motor speed), the controller increases or decreases the voltage supplied to the servovalve until the correct feedback voltage is achieved. The motor speed is controlled until the move finishes. As with the digital control example, the program may include a function to notify a human operator if speed control isn't working.

Digital and analog controllers are available "off the shelf" so that systems can be constructed inexpensively (depending on your definition of "inexpensive"), and with little specialized knowledge required.

Fig. 0.4 A digital controlled systemFig. 0.4 A digital controlled system


Fig. 0.5 An analog controlled system Fig. 0.5 An analog controlled system

Most controllers include communication ports so that they can send or receive signals and instructions from other computers. This allows individual controllers to be used as parts of distributed control systems, in which several controllers are interconnected. In distributed control, individual controllers are often slaves of other controllers, and may control slave controllers of their own.

 

0.4.2 Sensors as Automation Components

Obviously, controlled automation requires devices to sense system output. Sensors also can be used so that a controller can detect (and respond to) changing conditions in its working environment. Figure 0.6 shows some conditions that a fluid flow control system might have to monitor. Sensors are required to sense three settings for values that have to be controlled (flow rate, system pressure, and tank level) and to measure the actual values of those three variables. Another sensor measures the uncontrolled input pressure, so that the valve opening can be adjusted to compensate.

A wide range of sensors exists. Some sensors, known as switches, detect when a measured condition exceeds a preset level (e.g., closes when a workpiece is close enough to work on). Other sensors, called transducers, can describe a measured condition (e.g., output increased voltage as a workpiece approaches the working zone).

Sensors exist that can be used to measure such variables as:

  • presence or nearness of an object
  • speed, acceleration, or rate of flow of an object
  • force or pressure acting on an object
  • temperature of an object
  • size, shape, or mass of an object
  • optical properties of an object
  • electrical or magnetic properties of an object

 

Fig. 0.6 Sensing in an automated systemFig. 0.6 Sensing in an automated system

The operating principles of several sensors will be examined.

Sensors may be selected for the types of output they supply. Some sensors output DC voltage. Other sensors output current proportional to the measured condition. Still other sensors have AC output. Some sensors, which we will not examine in this text, provide non-electrical outputs. Since a sensor's output may not be appropriate for the controller that receives it, the signal may have to be altered or "conditioned." Some sensor suppliers also sell signal conditioning units. Signal conditioning will be discussed later in this chapter, and in more detail.

 

0.4.3 Actuators as Automation Components

An actuator is controlled by the "controller." The actuator, in turn, changes the output of the automated process. The "actuator" in an automated process may in fact be several actuators, each of which provides an output that drives another in the series of actuators. An example can be found in figure 0.7, in which an hydraulic actuator controls the position of a load.

The controller outputs a low current DC signal. The signal goes to the first stage of actuator, the amplifier, which outputs increased voltage and current. The large DC power supply and the amplifier are considered part of the actuator. The amplified DC causes the second stage of the actuator, the hydraulic servovalve, to open and allow hydraulic fluid flow, proportional to the DC it received. The servovalve, hydraulic fluid, pump, filter, receiving tank and supply lines are all components in the actuator. The fluid output from the valve drives the third actuator stage, the hydraulic cylinder, which moves the load.

A servovalve is called a servovalve because it contains a complete closed-loop position control system. The internal control system ensures that the valve opens proportionally with the DC signal it receives. The three-stage actuator we have been discussing as a component of a closed loop control system, therefore, includes another closed-loop control system. Some actuators can only be turned on or off. A heater fan helps to control temperature when it is turned on or off by a temperature control system. Pneumatic cylinders are usually either fully extended ("on") or fully retracted ("off"). Other actuators respond proportionally with the signal they receive from a controller. A variable speed motor is an actuator of this type. Hydraulic cylinders can be controlled so that they move to positions between fully extended and fully retracted.

Fig. 0.7 Hydraulic actuated position controlFig. 0.7 Hydraulic actuated position control

Actuators can be purchased to change such variables as:

  • presence or nearness of an object
  • speed, acceleration, or rate of flow of an object
  • force or pressure acting on an object
  • temperature of an object final machined dimensions of an object
  • The operating principles of several actuators will be examined.

Actuators can be selected for the types of inputs they require. Some actuators respond to DC voltage or current. Other actuators require AC power to operate. Some actuators have two sets of input contacts: one set for connecting a power supply, and the other set for a low power signal that tells the actuator how much of the large power supply to use. A hydraulic servovalve is an actuator of this type. It may he connected to a 5000 PSI hydraulic fluid supply, capable of delivering fluid at 10 gallons per minute, and also to a 0 to 24 volt DC signal which controls how much fluid the valve actually allows through.

As with sensors, there may be incompatibilities between the signal requirement of an actuator and a controller's inherent output signals. Signal conditioning circuitry may be required.

GO TO NEXT PAGE: INDUSTRIAL CONTROL HANDBOOK - 0.5 INTERFACING AND SIGNAL CONDITIONING

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