The following are some examples of control systems designed to illustrate the use of a range of input and output devices.
A Conveyor Belt
Consider a conveyor belt that is to be used to transport goods from a loading machine to apackaging area. When an item is loaded onto the conveyor belt, a contact switch might be used to indicate that the item is on the belt and to start the conveyor motor.
The motor then has to keep running until the item reaches the far end of the conveyor and falls off into the packaging area. When it does this, a switch might be activated that has the effect of switching off the conveyor motor. The motor is then to remain off until the next item is loaded onto the belt. Thus the inputs to a PLC controlling the conveyor are from two switches and the output is to a motor.
Consider a simple goods lift to move items from one level to another. For example, it might lift bricks from the ground level to the height where some bricklayers are working. The lift is to move upward when a push button is pressed at the ground level to send the lift up wardor a push button is pressed at the upper level to request the lift to move upward, but inboth cases there is a condition that has to be met that a limit switch indicates that the access gate to the lift platform is closed. The lift is to move downward when a push button is pressed at the upper level to send the lift downward or a push button is pressed at the lower level to request the lift to move downward, but in both cases there is a condition that has to be met that a limit switch indicates that the access gate to the lift platform is closed. Thus the inputs to the control system are electrical on/off signals from push but tonswitches and limit switches. The output from the control system is the signal to control the motor.
A Robot Control System
Figure shows how directional control valves can be used for a control system of arobot. When there is an input to solenoid A of valve 1, the piston moves to the right and causes the gripper to close. If solenoid B is energized with A deenergized, the pistonmoves to the left and the gripper opens. When both solenoids are deenergized, no air passesto either side of the piston in the cylinder and the piston keeps its position without change.
Likewise, inputs to the solenoids of valve 2 are used to extend or retract the arm. Inputs to the solenoids of valve 3 are used to move the arm up or down. Inputs to the solenoids of valve 4 are used to rotate the base in either a clockwise or anticlockwise direction.
Figure shows a method that could be used to give an on/off signal when the liquid in acontainer reaches a critical level. A magnetic float, a ring circling the sensor probe, falls as the liquid level falls and opens a reed switch when the critical level is reached. The reedswitch is in series with a 39 O resistor so that this is switched in parallel with a 1 kO resistor by the action of the reed switch. Opening the reed switch thus increases the resistance from about 37 O to 1 kO. Such a resistance change can be transformed by signal conditioning to give suitable on/off signals.
Packages on Conveyor Belt Systems
In some situations, the requirement is to check whether there is a nontransparent item on the belt at a particular position. This can be done using a light emitter on one side of the belt anda photoelectric sensor on the other, there then being an interruption of the light beam when the item is at the required position. If the item had been transparent, such as a bottle, the photo electric sensor might have been positioned to pick up reflected light to determine when the item is in the required position.
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