Over the last 30 years, there has been a significant evolvement of ladder logic for industrial controllers. It now supports advanced functionality that includes data acquisition, networking, data manipulation, motion control, and process control. Designers starting a new control system are faced with many basic functions this tried-and-trusted language needs to perform, and it does so, almost effortlessly.
Detecting the presence of an object is one of the most pervasive functions a control system is asked to perform. Whether it involves detecting an object passing by on a conveyor, the closing of a gate, or locating a machine part as it revolves, object detection is a staple function in the automation industry. Objects can be sensed in myriad ways, and these include mechanical, inductive, capacitive, optical, and ultrasonic techniques and devices to detect proximity or nearness.
The limit switch is a most basic sensor, mostly an electromechanical device detecting the absence or the presence of an object. As the actuator of the switch touches the sensed object, it operates a set of contacts. Depending on the actuator style, the specific means of contact may vary from plungers to wands, springs, levers, and rollers. However, the moving parts are prone to damage and wear, and it may not always be possible or desirable to make physical contact with the object to be sensed.
Inductive Proximity Switches
Non-contact sensing technology uses inductive proximity switches to detect the absence or presence of metallic objects without actually making contact with it. These are the most common and inexpensive devices. Inside the proximity switch is an oscillator driven coil. The magnetic field created by the oscillator appears at the active face of the switch. As soon as a metal target approaches this area, the electromagnetic field reduces, and this turns the switch on or off.
For objects that have a dielectric constant different from air, detecting them is easy with a capacitive sensor. Unlike the inductive proximity sensors that sense only metal objects, capacitive sensors can detect plastic, liquids, and wood. Although their method of operation is similar to that of the inductive proximity sensors, capacitive sensors detect objects based on an electrostatic field rather than an electromagnetic field.
Ultrasonic Proximity Sensors
These sensors measure the time of flight of a burst of sound impulse from the source until detecting the echo signal returned by a reflection from the detected object. Almost all materials reflect ultrasonic sound, which remains unaffected by color, transparency, or polish of the object.
These sensors use a light beam as the detecting medium. Most popular photoelectric sensors are the diffuse, reflective, and through-beam types. The sensing distance depends on the type of light used—laser, LED, infrared, or visible.
Diffuse sensors contain both the receiver and emitter in the same unit. The optical beams may be either slightly diverging, or parallel. When an object appears in the luminous beam, it causes diffused reflection. The receiver detects the reflections from the object.
The ladder logic represents the contacts of the sensor and the PLC CPU assigns it a memory location. The CPU represents them in the Normally Open or Normally Closed in the ladder logic.