Accelerometer sensor

Wednesday, 10 January 2007

Accelerometers are sensors or transducers that measure acceleration. Accelerometers generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. Accelerometers are useful in detecting motion in objects. An accelerometer measures force exerted by a body as a result of a change in the velocity of the body. A moving body possesses an inertia which tends to resist change in velocity. It is this resistance to change in velocity that is the source of the force exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated. The motion is detected in a sensitive portion of the accelerometer. This motion is indicative of motion in the larger object or application in which the accelerometer is mounted. Thus, a sensitive accelerometer can quickly detect motion in the application. Accelerometers have uses in many commercial, military, and scientific applications including inertial navigation, vehicular safety systems such as airbags, ride comfort control, platform stabilization, tilt sensing, and vibration monitoring. Accelerometers find use in automobile suspension systems, vehicle air bag systems, anti-lock brake systems (ABS), vibrometers, computer hard disc drivers, smart detonation systems for bombs and missiles and machine vibration monitors. A disk drive employs an accelerometer to detect disturbances affecting an actuator arm while attempting to maintain a head over a centerline of a track. The output of the accelerometer is used as a feed-forward compensation signal in a servo control system to effectively reject the disturbance. Accelerometers have been employed to help determine the acceleration or deceleration of a ship or plane, to monitor the forces being applied to an apparatus or device, such as a car, train, bus, and the like. Accelerometers are used as a GPS-aid to obtain position information when the GPS receivers lose their line-of-sight with the satellites. Accelerometers are widely used to monitor the vibration of electrical motors, pumps and the like in industrial applications, especially in continuous production operations. Changes in vibration levels, particularly in rotating machinery, provide an advance warning of problems such as excessive wear or an approaching bearing failure. Electromechanical accelerometers have been used in washing machines to detect an unbalanced load by sensing the sharp accelerations and decelerations of the spinning tub as it rocks back and forth. An accelerometer can measure changes in a patient's physical activity. The physical changes are detected by the accelerometer and algorithmically interpreted by circuitry within the pulse generator to produce a modified therapy that is correct for the current activity level. The accelerometer is placed within the implantable medical device. Accelerometers are also used to detect and record environmental data. In particular, accelerometers are often used in seismic applications to gather seismic data. In the area of oilfield investigation and earth formation characterization, accelerometers may be deployed in wireline applications, logging while drilling applications, or using coiled tubing.

Typical accelerometer sensors include a pendulous reaction mass, often referred to as a proof mass, suspended from a stationary frame by, for example, a flexural suspension member or some other form of pivot mechanism. An accelerometer may be viewed as a mass-spring transducer housed in a sensor case with the sensor case attached to a moving object whose motion is inferred from the relative motion between the mass and the sensor case. The relative displacement of the mass is directly proportional to the acceleration of the case and therefore the moving object. The heart of an accelerometer is a mechanical proof-mass. Pendulous accelerometers, for example, vibrating beam accelerometers, capacitive accelerometers, capacitive rebalance accelerometers, and translational mass accelerometers comprise a reaction mass. The proof-mass is connected to a substrate by a suspension. Under an applied acceleration, the proof-mass moves with respect to the substrate. The accelerometer is attached to the moving object, and as the object accelerates, inertia causes the proof mass to lag behind as its housing accelerates with the object. The force exerted on the proof mass is balanced by the spring, and because the displacement allowed by the spring is itself proportional to applied force, the acceleration of the object is proportional to the displacement of the proof mass. In a typical accelerometer sensor mechanism the pendulous reaction mass is suspended on a flexural suspension member inside an external support frame. Isolation is typically provided by mounting the supporting frame itself inside an isolation feature supported from a final exterior frame which provides mounting both to sensor covers and to the accelerometer housing. In an accelerometer, acceleration is usually measured at a measurement point in the accelerometer, along a sensitive axis of the accelerometer. Generally, the magnitude of an applied acceleration is communicatively coupled to external instruments or circuits as an electrical impulse having amplitude proportional to the magnitude of the applied acceleration. The electrical impulse comprises the measured acceleration and is processed by the external circuits as required for a variety of applications. The electrical impulse output of an accelerometer is proportional to the acceleration, applied at the measurement point along the sensitive axis of the accelerometer. A sense-element may be operated either open-loop, or placed into a force-feedback loop. Enclosure of a sense-element in a force-feedback loop is commonly called force-balancing or force-rebalancing. Accelerometers require either a frequency or voltage reference. Voltage references hardened against radiation are not available so that frequency referenced accelerometers are preferred for strategic applications. Frequency based accelerometers include silicon micromachined devices and quartz devices. An accelerometer generates an output signal which has an amplitude which is related to the acceleration that is applied to the accelerometer. It is often necessary to calibrate the accelerometer, that is, to determine the amplitude of its output signal as a function of the magnitude of the applied acceleration and of its frequency. The process of calibrating an accelerometer consists of computing a constant of proportionality, referred to as a scale factor of the accelerometer. The scale factor of an accelerometer precisely relates the amplitude of the electrical impulses comprising the measured acceleration to the magnitude of a corresponding acceleration applied at the measurement point, along the sensitive axis of the accelerometer.

Various accelerometers capable of measuring acceleration are being developed. The accelerometers are mainly fabricated through the semiconductor process, and classified into piezoelectric, piezoresistant and capacitance accelerometers. Often piezoelectric materials, piezoresistive materials, or air-gap capacitors are used in conjunction with an electrical position-sense interface to detect proof-mass displacements. Piezoelectric based electronic accelerometer suffer from several major drawbacks when faced with the continuing stricter demands of the industry. Most higher performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of sensors is not only cumbersome but tends to occur at significant increase in weight and volume of an accelerometer array. Capacitance accelerometers employ a capacitor between the mass and a support structure, and measure the variable capacitance between the two. An acceleration of the mass causes a change in the space between moving and fixed plates of the capacitor. The change in the space or displacement of the moving plate relative to the fixed plate is inversely proportional to the charge on the capacitor. In general, capacitive accelerometers change electrical capacitance in response to acceleration forces and vary the output of an energized circuit. A capacitive accelerometer shows a small level of characteristic change according to temperature variation, allows a field effect transistor of a high integrity to constitute a signal processing circuit without additional processes, and can be prepared at low cost. Capacitive accelerometer systems generally include sensing elements, such as capacitors, oscillators, and detection circuits. Angular accelerometers are employed to measure the second derivative of angular rotation with respect to time. In some machine controlled applications, a measurement of angular acceleration is often needed as a direct input to a control system. Rotary accelerometers may be used to provide closed loop motion control of a load through the use of feedback techniques. The acceleration signal from an accelerometer may be used to electronically simulate larger, smaller or varying system inertia. Pendulous gyroscopic accelerometer is a type of high precision accelerometers based on a rebalance mechanism. An unbalance is created, for example by adding a pendulous mass along a spin axis. An input acceleration creates a torque, which is counterbalanced by a torque in the opposite direction resulting from the rotation of the gyroscope about its input axis. The velocity of rotation of the gyroscope is used to determine the acceleration being sensed by the accelerometer. Linear accelerometers measure linear acceleration along a particular sensing axis. Linear accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to linear acceleration for use in a vehicle control system. A multi-axis accelerometer device can measure acceleration along multiple sensitive axes. This can be a combination of one or more accelerometers, with one or more axes of sensitivity each, and a common frame of reference with respect to which each of these accelerometers and their respective measurement points and sensitive axes remains fixed at all times. Optical accelerometers or displacement devices operate through a connection of an optical element to a mass usually positioned inside of a housing. As a force acts on the mass, the mass moves within the housing, thereby imparting a stress to the optical element indicative of the force. The optical element in such devices is typically an optical fiber, perhaps containing a fiber bragg grating (FBG). Thermal accelerometers are known that comprise an enclosure in which a central filament is disposed that is connected to a power supply member delivering electricity, and that lies between two detector filaments connected to a member for comparing the temperatures of the detector filaments. A resonant accelerometer is a sensor that responds to an acceleration force by producing a frequency shifted output signal. Quartz-based resonant accelerometers have been used in many commercial applications, including navigation-grade precision accelerometers.

Silicon micromachined acceleration sensors are beginning to replace mechanical acceleration switches. Microaccelerometers typically include a sensor for sensing a proof mass and movements thereof. Micro-accelerometers can be classified as an electric capacity sensing type, a piezoresistance sensing type, a piezoelectricity sensing type and an optical sensing type accelerometer according to the constructions and sensing methods thereof. Advancements in micromachining and other microfabrication techniques and associated processes have enabled manufacture of a wide variety of microelectromechanical (MEMS) devices. MEMS devices indicate microscale mechanical devices that are electrically controlled and measured, in which the MEMS is a technique for fabricating mechanical and electrical devices through the semiconductor process. One advantage of microfabricated sensors is the possibility of large scale production and ensuing lower costs. Another advantage is the small size and weight of the accelerometer. An MEMS accelerometer includes, among other component parts, a proof mass that is resiliently suspended by one or more suspension springs. The proof mass moves when the MEMS accelerometer experiences acceleration. The motion of the proof mass may then be converted into an electrical signal having a parameter magnitude that is proportional to the acceleration. Another type of MEMS accelerometer that is used to sense acceleration is commonly referred to as a teeter-totter capacitive acceleration transducer, or a teeter totter accelerometer. A typical teeter totter accelerometer includes an unbalanced proof mass suspended over a substrate using a fulcrum or other axis. Micromachined structures are frequently used as sensors or actuators and micromachined accelerometers. Among micromachined accelerometers, the differential capacitor type is typically used. A differential-capacitor based accelerometer typically includes a micromachined sensor and its excitation and readout electronics. The micromachined sensor typically includes several primary micromachined elements; a movable mass, support springs, and capacitor plates for sensing the displacement of the movable mass. Often, additional actuator plates are provided to implement a self-test function. The sensitivity of a micromachined accelerometer is determined by a variety of factors, including spring constant, mass of certain elements (e.g., proof mass), sense and parasitic capacitances, and electronic gain.

Related Information about Accelerometer sensor