Sensors are used in assembly operations to detect presence, position, orientation or movement of an object.

According to a survey conducted by the Freedonia Group Inc., sensor demand from the industrial sector will increase 6 percent per year through 2006 to $3.3 billion. These gains will be fueled by a healthy macroeconomic climate and steadily improving capital spending by process industries and manufacturers of industrial machinery. Even though technological innovations, such as fiber optic positioning sensors, have benefited the industrial sector, this is one of the more mature markets for sensors. It is not expected to register the type of gains expected in more dynamic markets, such as information technology and health care.

Six percent is a pretty significant increase for such an innocuous device. When considering assembly equipment, sensors are not the first items that come to mind. Large capital equipment, such as conveyors, are more easily recognizable. So what does a sensor do?

What Are Sensors?

Sensors enhance and replace various human senses in automated assembly equipment. They respond to light, electromagnetic fields, sound or pressure.

Sensor use is varied. Depending on the application, sensors can increase productivity and safety. Although an application?s sensing requirements can be satisfied in many ways, usually one sensing mode is best for the situation.

In general, most sensors are analog or digital. An analog sensor is wired into a circuit so that it will have an output that falls within a certain range. Then, the value can assume any possible value within that range.

Digital sensors generate a discrete signal. This means that the sensor can output a range of values, but the value must increase in steps. There is a known relationship between any value and the values preceding and following it. Discrete signals typically have a stair-step appearance when graphed on chart.

Sensors for Assembly Machines

In automated assembly machines, sensors are used to detect the presence, position and orientation of parts. Sensors are also used for simple inspection tasks, and they can tell other devices, such as presses, when to activate. Sensors used with automated assembly systems include mechanical and limit switches, height gauges, laser sensors, photoelectric sensors and proximity sensors.

Which sensor to use depends on the application, size constraints, response time, the material being sensed, electrical interface requirements, reliability, resolution, cost and environment.

Mechanical and Limit Switches

The switch, which is one of the most basic of all sensors, comes in two types?normally open and normally closed. Prior to advances in sensor technology, mechanical switches were used extensively in control applications. Due to improved reliability and performance, mechanical switches are still used for this purpose, but they are primarily used where switch actuation and wear are minimal. Although many types of switches are available, mechanical switches are used most often for controlling an automated assembly process.

The standard limit switch is a mechanical device that uses physical contact to detect the target. A typical limit switch consists of a switch body and an operating head. The switch body contains electrical contacts to energize or de-energize a circuit. The operating head incorporates a lever arm or plunger. This is also called an actuator. The actuator rotates when the target applies force. This movement changes the state of contacts within the switch body.

An extensive choice of actuator profiles enables limit switches to be specified for a variety of applications. Several types of actuators are available. The roller actuator is used for most rotary lever applications. It is generally available in various fixed lengths or as an adjustable-length actuator.

The fork-style actuator must be physically reset after each operation and is suitable for transverse movement control.

Flexible loop and spring rod actuators can be actuated from all directions, making them suitable for applications where the direction of approach is constantly changing.

Plunger-type actuators are ideal where short, controlled machine movements are present, or where space or mounting does not permit a lever-type actuator. The plunger can be activated in the direction of plunger stroke, or at a right angle to its axis.

Contact with the target differentiates a limit switch from other sensors. Other sensors change their output when an object is present, but not touching the sensor. Because a limit switch requires contact, it should be mounted away from machine component and operator movements. An important aspect of limit switch mounting is cam design. Incorrect cam design can lead to premature switch failure.

Height Gauges

Height gauges measure distances based on the amount of physical displacement. They use capacitive pulsing, optical linear encoder, linear variable differential transformer and even laser encoder principles to determine the displacement. Measurement resolution accuracy is typically 0.0005 to 0.00005 inch, depending on the type of height gauge.

They measure the height or thickness of a part, and occasionally to indicate the presence of a part. Height gauges may also be coupled to measure the presence of a small or thin component on an assembly. One gauge serves as a reference, and the other measures the component.

Photoelectric Sensors

Photoelectric sensors detect objects with a modulated light beam that is either broken or reflected by the target. The sensor consists of a light source, a receiver to detect the emitted light and components to evaluate and amplify the detected signal. Common applications include area scanning, counting and detection of clear glass and plastics.

Sensing modes for photoelectric sensors include through-beam, diffuse and retroreflective.

The through-beam sensing mode requires separate emitter and receiver units. The units are aligned so that the greatest possible amount of pulsed light from the transmitter reaches the receiver. When the target is placed in the path of the light beam, it blocks the light to the receiver. This causes the receiver?s output to change state. When the target no longer blocks the light path, the receiver?s output returns to its normal state.

Through-beam sensing is suitable for detecting opaque or reflective objects. It is not suitable for detecting transparent objects. Additionally, vibration can cause alignment problems.

Retroreflective sensors contain the emitter and receiver in one unit. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. When a target blocks the light path, the output of the sensor changes state. When the target no longer blocks the light path, the sensor returns to its normal state.

Unlike through-beam sensors, retro-reflective sensors cannot detect shiny objects. Shiny objects reflect light back to the sensor. Therefore, the sensor is unable to differentiate between light reflected from the shiny object and light reflected from a reflector.

Like the retroreflective mode, the diffuse mode contains the emitter and receiver in one unit. However, light from the emitter strikes the target, and the reflected light is diffused from the surface at all angles. If the receiver receives enough reflected light, the output will switch states. When no light is reflected back to the receiver, the output returns to its original state.

Light sources for photoelectric sensors include fiber optics and lasers. Fiber optic sensors use an emitter, receiver and a flexible cable packed with tiny fibers that transmit light. There are two types of fiber optic assemblies: individual and bifurcated. An individual fiber optic assembly guides light from an emitter to a location, or from a location to the receiver. A bifurcated fiber optic assembly combines emitted light with received light in the same assembly.

Fiber optics has many advantages. By mounting the sensor in a separate location and running fiber optic cables into the area to be detected, many negative factors that can affect a sensor?s performance—such as electromagnetic interference and radio frequency interference noise—can be circumvented.

Lasers can also be used as sensor light sources. These high-precision sensors have a high-intensity light, which makes setup and adjustment easy. Laser sensors detect extremely small objects at long distances. Laser sensor applications include exact positioning, speed detection or checking thread thickness of 0.1 millimeter and over.

Proximity Sensors

Proximity sensors are used for sensing the closeness of objects. Their range varies with the type of sensor, its sensitivity and the material being sensed. There are three basic types of proximity sensors: inductive, capacitive and ultrasonic.

Inductive proximity switches are ideal for industrial applications with sensing distances up to 20 millimeters. They incorporate an electromagnetic coil, which senses conductive materials, such as steel, stainless steel, lead, brass, aluminum, copper and other metals. These coils are wound in ferrite cores and can be shielded or unshielded.

With a shielded proximity sensor, the side face of the sensing coil is encased in metal shielding and can be embedded in a metal case. With the nonshielded version, the side face of the sensing coil is not metal-shielded. This type provides a longer sensing distance compared with the shielded type. However, special cautions must be taken for the mounting position, because this type is easily affected by surrounding metal.

Typical inductive proximity sensor applications include parts positioning, cap detection, broken drill detection and cam follower.

Capacitive proximity sensors are similar to inductive proximity sensors and can also be shielded and unshielded. The main difference between the two types is that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field. Capacitive proximity sensors will sense metal and various nonmetal materials through glass or plastic container walls. Typical capacitive sensor applications include bin-level control, carton detection, liquid-level detection and sight glass level.

An ultrasonic proximity sensor uses a transducer to send and receive high-frequency sound signals. It is very similar to radar. When a target enters the beam, the sound is reflected back to the switch, causing it to energize or de-energize the output circuit. Ultrasonic sensors are not affected by target surface, color or translucency, and they function extremely well in harsh environments.

Applications include equipment positioning, industrial automation (applications that monitor, control or automate a repetitive process), tank measurement and control, collision avoidance and environmental monitoring (fresh or wastewater flow monitoring, stream staging, and reservoir and snow level).

The Future of Sensors—Smarter and Smaller

Smart sensors have been used in the process industry for several years. But now they are moving into other industries. But what are smart sensors and what can they do?

A smart sensor can monitor parameters such as voltage, radiation, temperature and humidity, and process this information within the sensor itself. It can identify threshold limits, process and manipulate data, and activate alarms. The sensor works off an electrical bus, which eliminates the need for large wiring harnesses. A basic sensor usually does not deliver a linear signal. Linearity, however, is the goal in process control.

Besides getting smarter, sensors are also getting smaller. This is due, in part, to the increased demand for automated production processes.

As automation increases, so does the need for sensors. This means there is less space for installation. Decreased installation space means that sensors are installed in inaccessible places.

Miniature sensors are specially designed for precision sensing in small areas previously accessible only to remote sensors and fiber optic cable. Typical applications include mounting on compact conveyors, packaging machines, circuit board and semi-conductor wafer handling equipment, document handling equipment, robot end-effectors, feeder bowls, between the rollers of narrow conveyors, and as replacements for damaged small-diameter inductive proximity sensors. They are also an excellent choice for in-die and on-die sensing applications, including press performance monitoring, feed detection, feeding fault detection, material positioning, part out verification, slug stacking, die protection, quality validation, transfer press finger sensing, positioning and parts nesting.