Types of Position Sensors
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The ability to determine the existence or location of an object can be achieved through various sensing methods. These techniques generally fall into two main categories: single-point sensing and continuous sensing.
**Single-Point Sensing**
Single-point sensing offers a binary indication—yes or no—regarding an object's position. The simplest form is a switch triggered by an object's movement. A common example is a microswitch with a lever arm. A small movement of the lever arm can close or open the switch. Microswitches are frequently employed as limit switches to prevent overtravel of a mechanism. Physical contact with the object being sensed is necessary, which makes this approach unsuitable for many applications.
A straightforward non-contact position detector uses a reed switch. This switch activates when exposed to a magnetic field. A typical application is detecting an open door or window in a home security system. The image shows a reed switch (with three screw terminals) and the activating magnet. The switch is mounted on the frame, while the magnet is attached to the door. Reed switches are also widely used in float-type level detectors. A reed switch inside a tube is triggered as the float, containing a magnet, rises to the preset level. This picture illustrates a level switch with two floats for high/low level detection.
In numerous applications, attaching a magnet to the moving object isn't feasible. For such cases, inductive, capacitive, and optical methods are viable alternatives. All of these require electronic circuitry and external power. An inductive proximity sensor (on the left) uses a coil to detect eddy currents generated by a conductive metal object. Typical detection ranges are between 1 to 5 mm. The voltage output can either be pull-up (to the positive supply voltage) or pull-down (to the negative supply voltage). While microswitches and reed switches can handle currents of 1A or more, inductive sensors are usually limited to less than 100 mA. Their maximum operating frequency is typically greater than 500 Hz, significantly higher than mechanical switches. Capacitive sensors use an electric field to detect metallic objects. Their housing resembles inductive sensors, but they are more susceptible to dirt and less commonly used as single-point sensors in industrial settings.
For applications requiring a longer detection range, optical (photoelectric) sensors are a preferred choice. These typically employ a light-emitting diode (LED) or laser as the source and a phototransistor as the detector. The emitted beam can be visible (usually red) or infrared. The detector can be housed with the source or separately. This image shows a combined housing (on the right). There are two operational modes for this configuration. Depending on the object's shape, color, and surface, it might reflect light back to the detector. Alternatively, the object can block the light returning to the detector from a reflector positioned behind it. If the source and detector are in separate housings, the detector can be placed behind the object instead of the reflector. A separate detector can also be positioned to capture a beam reflected off the target object at an angle. Optical sensors offer detection ranges from less than 10 cm to over 10 m and operating frequencies up to 500 Hz. Some units use a modulated beam to reduce sensitivity to ambient light or a polarized beam to minimize false triggers from shiny objects. Optical and other externally powered sensors may include additional features such as time delay, sensitivity adjustment, and activation indicators.
**Continuous Sensing**
Continuous sensors measure the position or displacement of an object. Instead of an on/off output, they provide a continuous (typically analog) output signal. The linear potentiometer (at the top) is a cost-effective transducer. It generates an analog electrical signal directly proportional to the position of the wiper as it moves along a resistive element. A common application is in a light dimmer, where a person's hand serves as the moving mechanism. The resolution and linearity depend on the resistor composition. Wear on the wiper contact limits the operating life.
Non-contact sensing methods provide extremely long lifespans. A linear variable differential transformer (LVDT) detects the position of a ferromagnetic core as it slides within a primary and two secondary coils. A connecting rod links the core to the moving object. LVDTs with travel ranges from 1 mm to 1 m are available. This high-precision sensor operates across a wide temperature range and can detect minute positional changes. However, the LVDT requires specialized AC signal conditioning circuitry, which adds complexity and can be challenging to implement in an industrial setting. A linear variable differential transducer (DC LVDT) integrates oscillator, demodulator, and amplifier circuits within the sensor body. It operates on DC power (typically 10-28 V) and produces a scaled DC voltage or mA output. This versatile sensor can also measure other mechanical parameters. For instance, with a spring-loaded rod in contact with a moving object, it can measure flatness or runout.
Similar sensing techniques can be applied to measure rotation. When connected to a DC voltage source, a circular potentiometer easily translates shaft position into a DC voltage level. Resolution and accuracy depend on the potentiometer specifications. A large-diameter pot, like a Helipot, features a substantial resistive element, offering high linearity and resolution. Most pots have stops limiting rotation to less than 300°. Some without end stops allow continuous rotation. With the addition of a suitable mechanism, these pots can also measure linear motion. A common example is the gasoline level in a car's fuel tank, where a float and lever assembly rotate the pot as the fuel level changes.
The rotary variable differential transducer (RVDT) employs a non-contact method to detect rotation. This sensor (in the middle) has a ferromagnetic core that rotates between primary and secondary coils. DC-powered RVDTs are restricted to a sensing range of less than ±75° but can accurately measure less than 0.001°. RVDT applications are diverse. One model of an electronic viscometer uses an RVDT as the sensing element.
Rotary encoders (on the right) monitor shaft position. Many produce a pulse output that can be read by a counter or rate meter. Some have a digital output that interfaces directly with a controller or computer. An absolute encoder specifies the shaft position relative to a fixed reference point. An incremental encoder tracks the position compared to a relative (variable) starting point. Either type can be used on a shaft that rotates only a few degrees or turns at thousands of RPM.
A mechanism is sometimes added to one of these basic sensing techniques to create another type of sensor. A string potentiometer (also known as a cable-actuated position sensor) can measure extensive movements along one axis. This diagram shows the working principle of a Tyco string pot. It includes a flexible measuring cable wrapped around a spring-loaded spool. The free end of the cable is attached to the moving object. A tension spring keeps the cable taut as it extends and retracts. A rotary sensor attached to the spool tracks the cable's position. Internal electronics convert this into a DC voltage, mA, or digital output signal. Measurement ranges from less than 1 meter to more than 30 meters are available.
This article discussed only some of the more popular types of position sensors used in industrial applications. Even within these categories, a wide variety of products exist. Most have outputs that can be easily read by analog, digital, and bargraph meters. Weschler lists electronic sensors in the Position Sensor and Level Sensor categories. Tachometer Measurements provides further information on optical, inductive, and Hall sensing techniques for rotation detection.
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