Industrial process control can be difficult, complex, and problematic. Yet, a smooth running process line is a site to behold; thanks to technological advances in control devices. In our discussion, the word "process" implies any industrial manufacturing, testing, or assembly operation in which an operator controls, depending on the position or state of the items made, tested, or packaged. Let's take a look at those devices that "see" the produced items and activate the required machines.

Motion control. One of the most important aspects of process control is controlling the motion of the items you're making or modifying. In most industrial processes, the position of an item dictates the operation. Think of an assembly line with dozens of operations. Each operation occurs in a certain location, with the item moving from one machine or process to the next. The goal is to supply accurately and reliably position information by providing an appropriate electrical signal.

Limit switches. The most commonly used process control device is the limit switch. It functions by opening or closing when an item reaches a pre-set location. We subdivide mechanical-type limit switches into those operated by linear and rotary motion. There are large and small switches that operate from linear motion. The precision limit switch is an example of a small switch. It varies from the larger size mainly in a lower operating force and shorter stroke. The operating force may be as low as 1 lb, and the stroke only a few thousandths of an inch.

We generally call limit switches operated by rotary motion rotating cam limit switches. These are control-circuit devices used with machinery having a repetitive cycle of operation; one correlated to shaft rotation. They're used to limit and control the movement of a rotating machine.

The switch assembly consists of one or more snap-action switches. Cams assembled on a shaft activate the snap-action switches. The shaft, in turn, is either d irect- or gear-driven by a rotary motion on the machine.

The cams are independently adjustable for operating at different locations within a complete 360 degrees rotation. In some cases, the number of total rotations available is limited. In others, rotation can continue at speeds up to 600 revolutions per minute (rpm).

When selecting a limit switch, you must determine its application in the electrical circuit and consider these factors: contact arrangement; current rating of the contacts; slow or snap action; isolated or common connection; spring return or maintained; and number of normally open (NO) and normally closed (NC) contacts required.

Contacts. In most cases, the switch consists of double-break, snap-action, silver-tipped, or solid silver contacts. The contact current rating will vary from 5A to 10A at 120VAC, continuous. The make-contact rating will be much higher, and the break-contact rating will be lower. Isolated NO and NC contacts are available.

Action. Here the operator has the major decision, to dictate the type of mechanical action available to operate the switch. Length of travel, speed, force available, accuracy, and type of mounting are some considerations.

In discussing the action of limit switches, you should become familiar with the following industry terms: Operating force: The amount of force applied to the switch to cause the "snap over" of the contacts. Release force: The amount of force still applied to the switch plunger at the instant of "snap back" of the contacts to the unoperated condition. Pre-travel or trip travel: The distance traveled in moving the plunger from its free or unoperated position to the operated position. Over-travel: The distance beyond operating position to the safe limit of travel (usually expressed as a minimum value). Differential travel: The actuator travel from the point where the contacts snap over to the point where they snap back. Total travel: The sum of the trip- and over-travel.

Most limit switch manufacturers list this information for certain switches in their specifications. Be careful: The accuracy of switch operators at the point of snap-over varies with different types and manufacturers. In general, it's in the range of 0.001 in. to 0.005 in.

The operator probably having greatest use is the roller lever. It's available in a variety of lever lengths and roller diameters. The next most frequently used operator is the push rod. It can come only as a rod or with a roller at the end. In most cases, particularly with the oil-tight machine-tool limit switch, you can rotate the head carrying the operator to four positions, each 90 degrees apart. You can also mount it either at its top or side. Two other operators used in machine control are the fork lever and the wobble stick.

Symbols. You use the limit switch symbol in applying the limit switch to the schematic or elementary circuit diagram. The symbol represents the switch in four different conditions. Usually, the switch has a NO contact and/or a NC contact. In some switches, there are two NO and two NC contacts.

Show the switch symbol as the switch being in the unoperated condition of the machine. This may result in the operated or unoperated condition.

There are times when you may use both contacts on a given limit switch in a circuit. Under this condition, it helps to join the two contact symbols with a broken line, which indicates they are contacts on the same limit switch.

Proximity switches. One of the most common control switches, the proximity switch is a device capable of acting as an electronic switch when in the presence or close proximity of an object. What differentiates it from a mechanical switch is it does not require physical contact with anything else to operate. The methods of achieving this operation are many: changes in RF fields, magnetic fields, capacitive fields, acoustic fields, and light rays. Objects having ferrous (iron) material content usually alter RF fields.

Magnetic fields can close reed switches. Bringing a magnet up close to the switch or introducing a magnetic material between the magnet and the switch does the activation. Magnets also can alter electrical fields in devices through a phenomenon called the Hall Effect.

Capacitive devices make use of a change in capacitance. This occurs when the sensed object (acting as one plate of a capacitor) passes by the sensor (acting as the other). The sensor "sees" an alternation in the dielectric between the "plates" when detecting nonmetallic objects.

Sonic devices use sound fields interrupted by a detected object. These also detect reflection of sound from objects.

Photoelectric devices work in a similar manner except they detect beams of light rather than sound waves.

All these methods have strengths and weaknesses in actual application. Certain processes use two-wire DC proximity switches because they eliminate the potential for wiring mistakes during installation. Basically, they're not sensitive to voltage polarity.

Inductive sensors. Proximity switches that use RF (radio frequency magnetic wave) field usually employ one half of a ferrite core, with the coil being part of an oscillator circuit. When a metallic object enters this field, the object at some point will absorb enough energy from the field to cause the oscillator to stop oscillating. It's this difference between oscillating and not oscillating that the switch detects as the difference between an object being present or not present. Several variables determine the distance at which this detection takes place, including:

  • The diameter of the core (distance varies directly with the core diameter);

  • Size of object to be sensed (distance varies directly with size);

  • Kind of metal (distance is greatest with iron, less for other metals); and

  • Circuit sensitivity (distance is set by circuit design).

Capacitive sensors. These sensors also have oscillators. However, their oscillators usually begin oscillating in the presence of the detected object. This happens when the object creates enough capacitance in a critical part of the oscillator circuit to cause oscillation. In both methods (capacitive and inductive), detection is in the form of the presence or absence of oscillation. You then use this information to do useful work by operating a load directly through a solid-state output circuit or indirectly through a relay.

Other types. The magnet-operated proximity limit switch operates by passing an external magnet near the face of the sensing head. This actuates a small, hermetically sealed reed switch. The 120VAC pilot-duty model also includes an epoxy-encapsulated triac output.

A mercury switch operates by passing a permanent magnet of sufficient strength past the switch.

The vane-operated limit switch actuates by passage of a separate steel vane through a recessed slot in the switch. You attach either vane or switch to the moving part of the machine. As the vane passes through the slot, it changes the balance of the magnetic field, causing the contacts to operate. The switch is available with either a NO or NC contact and can detect very high speeds of vane travel without detrimental effects, such as wear and breakage. There is no physical contact between vane and switch. Therefore, the upper limit on vane speed is governed by factors other than the switch. The vane switch offers excellent accuracy and response time. Repeatability is constant within plus or minus 0.0025 in. or less, provided the path of the vane through the slot is constant. Response time (after the vane has reached the operating point) is less than a millisecond.

Solid-state devices. These devices respond differently than electromechanical output devices, especially to excess currents and incorrect voltage polarities. While excessive currents (if not sustained) do not particularly affect an electrical-mechanical contact device, they might destroy a solid-state contact device. The latter device, however, can be designed so excessive current conditions cannot occur even when a short circuit occurs. In the same way, DCswitches, which might be destroyed by a wrong polarity connection, can be designed to not operate on reverse polarity and will not be damaged.

Other semiconductor devices. A photoelectric sensor is an electrical device that responds to a change in the intensity of the light falling upon it.

An LED (light emitting diode) is a solid-state semiconductor, similar electrically to a diode (except it emits a small amount of light when current flows through it in a forward direction). LEDs can be built to emit green, yellow, red, or infrared light; the latter being invisible to the human eye.

A phototransistor is the most widely used optical element in photoelectric sensors. It offers the best trade-off between light sensitivity and response speed compared to photo-resistive and other photo-junction devices.

You use photocells for greater sensitivity to visible wavelengths, as in some color registration and ambient light detection applications.

Photodiodes are reserved for applications requiring extremely fast response time or linear response over several magnitudes of light level change.

Modulated LED sensors. Unlike their incandescent equivalents, LEDs can be turned "ON" and "OFF" (or modulated) at a high rate of speed, typically at a frequency of several kHz. This modulating means you can tune the amplifier of the photo-transistor receiver to the frequency of modulation and amplify only light signals pulsing at that frequency. In our process control industry, we call the modulated LED light source of a photoelectric sensor the transmitter (or emitter), and the tuned photo-device the receiver.

An ambient light receiver is a commonly-used, non-modulated photoelectric device. Red-hot metals and glasses emit large amounts of infrared light. As long as these materials emit more light than the surrounding light level, you can reliably detect them with an ambient light receiver.

Ultrasonic sensors emit and receive sound energy at frequencies above the range of human hearing (above about 20 kHz). We categorize ultrasonic sensors by transducer type, which is either electrostatic or piezoelectric.

Electrostatic types can sense objects up to several feet away by reflection of ultrasound waves from the object's surface. Piezoelectric types generally sense at only shorter ranges.

Remote photoelectric sensors contain only the optical components of the sensing system. Circuitry for system power, amplification, and output are all at another location, typically in a control panel. Thus, remote sensors are generally smaller and more tolerant of hostile sensing environments than self-contained sensors.

Self contained photoelectric sensors contain the optics along with all the electronics. Their only requirement is a source of voltage for power. The sensor itself does all of the work, which includes modulation, demodulation, amplification, and output switching.

Optical fiber. There are many sensing situations where space is too restricted or the environment too hostile for even remote sensors. For such applications, photoelectric sensing technology offers optical fiber as a third alternative in sensor technology. These are transparent strands of glass or plastic used to conduct light into and out of such areas. Fiber light guides, used along with either remote or self-contained sensors, are purely passive components of the sensing system. Since they contain no electrical circuitry and have no moving parts, they can safely pipe light into and out of hazardous sensing locations and withstand hostile environmental conditions.