Position sensors

POSITION SENSOR

Position sensors are used to electronically monitor the position or movement of a mechanical component. The position sensor produces data that may be expressed as an electrical signal that varies as the position of the mechanical component changes. Position sensors are typically used in machines to monitor the physical state of a moving mechanical component of an automated system.
Position sensing devices detect the movement and position of the control device and translate that movement and position into a control signal that may be further processed and used to control the movement of a vehicle or equipment. Position sensors are frequently employed in automotive vehicles to sense and monitor the position of an object that may travel through various positions.
For example, position sensors are commonly employed to monitor the position of an electrically powered door so as to determine whether the door is in the open position or closed position.
Automotive vehicles increasingly are equipped with electrically powered devices such as side entry doors and rear entry doors, each of which are powered by an electric motor that receives electric current from the vehicle battery. Each of these power doors also typically employs an absolute position sensor in order to determine the absolute position of the door.
In a power door application , the use of an absolute position sensor allows for the determination of the absolute position of the power door relative to the open and closed positions. It is also known to use a position sensor device to detect the movement and location of a control device, such as a controller, joystick control, vehicle throttle control, and an accelerator device.
A position sensor may be used in a computer interface device to detect a user's manipulation of the device. The detected manipulation may then be used to provide input to a computer system to control computer-generated objects and environments, to control physical objects, and/or to instruct the computer to perform tasks.
In one application, a user interacts with a computer-generated environment, such as a game, a surgical simulation, a graphical user interface, or other environment generated in response to an application program, by manipulating an object such as a mouse, joystick, trackball, gamepad, rotary knob, or three dimensionally translatable object to control a graphical image within a graphical environment or to otherwise control the operation of the computer.
Techniques for position sensing
There are a variety of known techniques for position sensing. Optical, resistive, electrical, electrostatic and magnetic fields are all used with apparatus to measure position.
Position sensing apparatus for using these energies for sensing include resistive contacting sensors, inductively coupled ratio detectors, variable reluctance devices, capacitively coupled ratio detectors, optical detectors using the Faraday effect, photo-activated ratio detectors, radio wave directional comparators, and electrostatic ratio detectors. Optical systems often use position sensing detectors to determine the position of an optical spot that is incident upon the active surface of a device. The optical spot may be a reflected laser beam from the surface of an optical recording medium.
The optical detector is constructed using photodetectors, such as photo-diodes or PIN-diodes. Signals required for the X and Y displacement of the spot is found by suitably subtracting currents from adjacent cells, followed by normalization to the total intensity of the optical spot. Optical sensors, however, are also susceptible to failures caused by problems inherent in the nature of their components.
For example, optical sensors tend to become unreliable as a result of large changes in ambient light, misalignments between the light source and the detector, reduced light levels caused by dirt or debris accumulation, reduced light levels caused by the aging of the internal light sources, and manufacturing differences in sensitivity between devices.
Capacitance-based position sensors are widely known. Many such sensors employ a variable capacitor having a value that varies with relative position of a pair of objects. In these systems, the relative position of the objects can be determined by measuring the capacitance. Single-electrode capacitive sensors for sensing the proximate presence of an object are commonly configured to provide a binary output and operate by measuring a value of electrical capacitance to an electric ground. If the sensor is configured as a "proximity sensor" it provides an output determinative of proximate presence when the value of the measured capacitance exceeds a predetermined threshold valve. If the sensor is configured as a "motion sensor" it provides the determinative output when the rate of change of capacitance exceeds a predetermined threshold value.
Among position sensing technologies, magnetic sensing is known to have a unique combination of long life components and excellent resistance to contaminants. Magnetic sensors typically rely upon permanent magnets to detect the presence or absence of a magnetically permeable object within a certain predefined detection zone relative to the sensor. In combination with the permanent magnet, some sensors of this type utilize Hall Effect and/or magnetoresistive components located at particular positions relative to the permanent magnet and other.
Generally, a magnet is used to create a magnetic field which is measured by an IC (integrated circuit) containing a magnetically sensitive feature. The magnet is connected to the element to be measured and moves relative to the IC. The changing magnetic field at the IC is converted into an output signal proportional to the movement.
A hall sensor is a type of magnetic sensor that uses the Hall effect to detect a magnetic field. The Hall effect occurs when a current-carrying conductor is placed into a magnetic field. A voltage is generated perpendicular to both the current and the field. The voltage is proportional to the strength of the magnetic field to which it is exposed. Hall position sensors are usually deployed as an integral part of closed loop feedback control systems which are used in a variety of fields such as automotive vehicle component testing and manufacturing, semiconductor manufacturing, industrial automation and robotics and the like. While Hall sensors are very reliable and have many useful applications, they are not as sensitive as magnetoresistive (MR) sensors. Hall sensors may also be more limited to the type of magnet used than an MR sensor.
Magnetoresistive sensors are a type of magnetic sensor that uses the magnetoresistive effect to detect a magnetic field. Ferromagnetic metals, such as the nickel-iron alloy commonly known as Permalloy, alter their resistivity in the presence of a magnetic field. When a current is passed through a thin ferromagnetic film in the presence of a magnetic field, the voltage will change. This change in voltage represents the strength or direction of the magnetic field. Some magnetic position sensors provide an indication of the displacement of the mechanical component by using a magnetic field sensing device which reports the intensity of a magnetic field from a magnet which is positioned on the mechanical component. The magnet is positioned and the magnetic field sensing device is located relative to the magnet in such a fashion as to cause the magnetic field to vary in the magnetic field sensing device as the magnet moves.
Magnetic position sensors are generally a non-contact type of sensors which are devices that generate change to an electronically interrogated physical parameter that is proportional to the movement of a structure, such as, for example, an actuator shaft operatively coupled to the sensor. This change is achieved without physical contact between the parameter and the interrogation device. Magnetic position sensors consist of a magnetic field sensing device which is usually stationary and a magnet that is attached to a moving component. As the magnet approaches the sensing device the magnetic field of the magnet is detected and the sensing device generates an electrical signal that is then used for counting, display, recording and/or control purposes. In magnetic position sensing, the magnitude of magnetic field strength is generally measured by an appropriate measuring device, such as a Hall-effect element or magneto-resistive element. The value of the measured field intensity is translated through the measuring device to a voltage or current value that is uniquely representative of the specific rotational position of the actuator shaft. Magnetic sensing devices have many applications, including navigation, position sensing, current sensing, vehicle detection, and rotational displacement. There are many types of magnetic sensors, but essentially they all provide at least one output signal that represents the magnetic field sensed by the device. One of the benefits of using magnetic sensors is that the output of the sensor is generated without the use of contacts. This is a benefit because over time contacts can degrade and cause system failures. Because such a position sensor bases positional detection on magnetic properties, this type of sensor inherently excels in resistance to exposure to common environmental contaminants such as water, oil, etc. A problem with such sensors is that they depend on movement of the magnet and they are not able to provide information as to the static position of a mechanical component.
Angular and linear position sensors are widely used in automatic control systems as feedback-sensing devices in one or more control loops of the system. Various types of angular position sensors are currently used in conjunction with vehicle steering wheels, or hand wheels, including relative, absolute, analog and digital angular position sensors. Known technologies that can be used to determine angular position include contact measurement, such as a resistance stripe, or non-contact measurement effects, based on inductance, capacitance, optical, or magnetic field.
A relative angular position sensor measures the angular position of a rotating object by either incrementing or decrementing a counter, depending upon the rotational direction of the object, and relating that information to an angular reference point.
Rotary position sensing is used in a number of applications, such as motor position feedback control and/or commutation, cam and crank shaft position sensing for controlling ignition timing, misfire detection, engine speed monitoring etc, robotics, and machine tool position control. Rotary position sensors utilize a magnetic field and a magnetosensitive device, such as a Hall effect device or a magnetoresistor located within the magnetic field. Absolute position sensors provide a sensed position signal which contains information about the absolute position relative to a predetermined position.
An absolute position sensor indicates very precisely the position of the moving components, so that they can be controlled and, above all, so that these components can be relocated when the system in which they are integrated is activated. An absolute position sensor delivers a number of output signals in the case of a parallel digital output signal, but in the case of a series digital output signal, the sensor delivers a single signal resulting from a shaping according to a data transmission protocol and executed from the signals in parallel described in therein.