Output Control Device

A variety of output control devices can be operated by the PLC output to control traditional industrial processes. These devices include pilot lights, control relays, motor starters, alarms, heaters, solenoids, solenoid valves, small motors, and horns. Similar electrical symbols are used to represent these devices both on relay schematics and PLC output connection diagrams. Figure shows common electrical symbols used for various output devices. Although these symbols are generally acceptable, some differences among manufacturers do exist.

An actuator, in the electrical sense, is any device that converts an electrical signal into mechanical movement. An electromechanical solenoid is an actuator that uses electrical energy to magnetically cause mechanical control action. A solenoid consists of a coil, frame, and plunger (or armature, as it is sometimes called). Figure shows the basic construction and operation of a solenoid. Its operation can be summarized as follows:

• The coil and frame form the fixed part.
• When the coil is energized, it produces a magnetic field that attracts the plunger, pulling it into the frame and thus creating mechanical motion.
• When the coil is de-energized the plunger returns to its normal position through gravity or assistance from spring assemblies within the solenoid.
• The frame and plunger of an AC-operated solenoid are constructed with laminated pieces instead of a solid piece of iron to limit eddy currents induced by the magnetic field.

Solenoid valves are electromechanical devices that work by passing an electrical current through a solenoid, thereby changing the state of the valve. Normally, there is a mechanical element, which is often a spring, that holds the valve in its default position. A solenoid valve is a combination of a solenoid coil operator and valve, which controls the flow of liquids, gases, steam, and other media.

When electrically energized, they open, shut off, or direct the flow of media.

Figure  illustrates the construction and principle of operation of a typical fluid solenoid valve. Its operation can be summarized as follows:

• The valve body contains an orifice in which a disk or plug is positioned to restrict or allow flow.
• Flow through the orifice is either restricted or allowed depending on whether the solenoid coil is energized or de-energized.
• When the coil is energized, the core is drawn into the solenoid coil to open the valve.
• The spring returns the valve to its original closed position when the current coil is de-energized.
• A valve must be installed with direction of flow in accordance with the arrow cast on the side of the valve body.

Stepper motors operate differently than standard types, which rotate continuously when voltage is applied to their terminals. The shaft of a stepper motor rotates in discrete increments when electrical command pulses are applied to it in the proper sequence. Every revolution is divided into a number of steps, and the motor must be sent a voltage pulse for each step. The amount of rotation is directly proportional to the number of pulses, and the speed of rotation is relative to the frequency of those pulses. A 1-degree-per-step motor will require 360 pulses to move through one revolution; the degrees per step are known as the resolution. When stopped, a stepper motor inherently holds its position. Stepper systems are used most often in “open-loop” control systems, where the controller tells the motor only how many steps to move and how fast to move, but does not have any way of knowing what position the motor is at.

The movement created by each pulse is precise and repeatable, which is why stepper motors are so

effective for load-positioning applications. Conversion of rotary to linear motion inside a linear actuator is accomplished through a threaded nut and lead screw. Generally, stepper motors produce less than 1 hp and are therefore frequently used in low-power position control applications. Figure shows a stepper motor/drive unit along with typical rotary and linear applications.

All servo motors operate in closed-loop mode, whereas most stepper motors operate in open-loop

mode. Closed-loop and open-loop control schemes are illustrated in Figure. Open loop is control without feedback, for example, when the controller tells the stepper motor how many steps to move and how fast to move, but does not verify where the motor is. Closed loop control compares speed or position feedback with the commanded speed or position and generates a modified command to make the error smaller. The error is the difference between the required speed or position and the actual speed or position.

Figure illustrates a closed-loop servo motor system. The motor controller directs operation of the servo motor by sending speed or position command signals to the amplifier, which drives the servo motor. A feedback device such as an encoder for position and a tachometer for speed are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor’s position and speed feedback information that the controller compares to its programmed motion profile and uses to alter its position or speed.