Types of Position Sensors
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Single-point sensing offers a binary yes/no response regarding an object's position. The most straightforward example is a switch triggered by an object's movement. A common illustration is a microswitch with a lever arm. A slight shift in the arm can close or open the switch. Microswitches are frequently employed as limit switches to prevent mechanisms from overtraveling. However, physical contact with the object being detected is necessary, making this approach unsuitable for many applications.
A simple non-contact position detector uses a reed switch, which activates when exposed to a magnetic field. A typical use case is detecting an open door or window within a home security system. The accompanying image shows a reed switch (with three screw terminals) and its activating magnet. The switch is mounted on the frame, while the magnet is attached to the door. Reed switches are also commonly used in float-type level detectors. Here, a reed switch inside a tube is activated as a float containing a magnet rises to a preset level. This image demonstrates a level switch featuring two floats for high/low level detection.
For scenarios where attaching a magnet to the moving object isn't feasible, inductive, capacitive, and optical techniques offer viable alternatives. All these methods rely on electronic circuitry and require an external power source. An inductive proximity sensor (on the left) uses a coil to detect eddy currents generated by a conductive metal object. Typical detection ranges span from 1 to 5 millimeters. The voltage output can be either pull-up (connected to the positive supply voltage) or pull-down (connected to the negative supply voltage). Although microswitches and reed switches can handle currents of 1 ampere or more, inductive sensors usually have a maximum current capacity of less than 100 milliamps. Their maximum operational frequency is typically above 500 hertz, significantly higher than mechanical switches. Capacitive sensors utilize an electric field to detect metallic objects. Their housing design resembles that of inductive sensors. However, they are more susceptible to dirt and are less popular as single-point sensors in industrial settings.
For applications requiring longer detection ranges, optical (photoelectric) sensors are a preferred choice. These sensors typically employ a light-emitting diode (LED) or laser as the source and a phototransistor as the detector. The emitted beam can be visible (commonly red) or infrared. The detector may be housed together with the source or in a separate unit. This image shows a combined housing (on the right). There are two primary operational modes for this configuration. Depending on the object's shape, color, and surface, it may reflect light back to the detector. Alternatively, the object can block the light returning to the detector from a reflector located behind it. If the source and detector are housed separately, the detector can be positioned behind the object instead of the reflector. A separate detector can also be set up to capture a beam reflected off the target object at an angle. Optical sensors boast detection ranges from less than 10 centimeters to over 10 meters and operational frequencies up to 500 hertz. Some models incorporate modulated beams to minimize sensitivity to ambient light or polarized beams to reduce false triggers from shiny surfaces. Optical and other externally powered sensors might include supplementary features like time delay, sensitivity adjustment, and activation indicators.
Continuous Sensing
Continuous sensors measure the position or displacement of an object. Instead of providing an on/off output, they deliver 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 glides 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 lifespan.
Non-contact sensing techniques provide exceptionally long service life. A linear variable differential transformer (LVDT) detects the position of a ferromagnetic core as it moves inside primary and secondary coils. A connecting rod links the core to the moving object. Units with travel distances ranging from 1 millimeter to 1 meter are available. This high-precision sensor operates across a wide temperature range and can detect minute position changes. However, the LVDT requires specialized AC signal conditioning circuitry, which increases complexity and can pose challenges in industrial settings. A linear variable differential transducer (DC LVDT) integrates oscillator, demodulator, and amplifier circuits into the sensor body. It runs on DC power (usually 10-28 volts) and produces a scaled DC voltage or current 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.
Similarly, sensing techniques can be adapted to measure rotation. When connected to a DC voltage source, a circular potentiometer readily converts shaft position into a DC voltage level. The resolution and accuracy are determined by the potentiometer specifications. A large-diameter potentiometer, such as a Helipot, features a substantial resistive element, providing high linearity and resolution. Most pots have stops limiting rotation to less than 300 degrees. Some without end stops allow continuous rotation. By incorporating an appropriate mechanism, these pots can also measure linear motion. A common example is monitoring gasoline levels in a car's fuel tank. A float and lever assembly rotates 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 degrees but can accurately measure angles down to 0.001 degrees. RVDT applications are diverse. One type of electronic viscometer uses an RVDT as the sensing element.
Rotary encoders (on the right) monitor shaft position. Many encoders produce a pulse output that can be read by a counter or rate meter. Some have a digital output that directly interfaces with a controller or computer. Absolute encoders report the shaft position relative to a fixed reference point. Incremental encoders track the position relative to a variable starting point. Both types can be used on shafts that rotate only a few degrees or turn at thousands of revolutions per minute.
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 illustrates the working principle of a Tyco string pot. It features 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 ensures the cable remains taut as it extends and retracts. A rotary sensor connected to the spool tracks the cable's position. Internal electronics convert this data into a DC voltage, current, or digital output signal. Measurement ranges from less than 1 meter to greater than 30 meters are available.
This article has covered some of the more popular position sensors utilized in industrial applications. Even within these categories, a vast array of products exists. Most have outputs that can be easily read by analog, digital, and bargraph meters. Weschler lists electronic sensors under the Position Sensor and Level Sensor categories. Tachometer Measurements offers additional information on optical, inductive, and Hall-effect sensing techniques for rotation detection.
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