Measure Of Angular Velocity 4pu5w

Measure of Angular Velocity The main application of angular velocity measurement is in speed control systems. They also provide the usual means of measuring translational velocities, which are transformed into rotational motions for measurement purposes by suitable gearing. Many different instruments and techniques are available for measuring rotational velocity. Some type of sensor is configured to produce a known number of pulses per revolution of the device being monitored. By counting pulses, the number of revolutions or angular position may be determined. In conjunction with this if time is also measured angular speed may be measured. There are three major types of instruments used for measurement of rotary motion, namely: a. Electrical b. Optical c. Mechanical Digital tachometers, or to give them their proper title, digital tachometric generators, are usually non- instruments that sense the age of equally spaced marks on the surface of a rotating disc or shaft. Measurement resolution is governed by the number of marks around the circumference. Various types of sensor are used, such as optical, inductive and magnetic ones. As each mark is sensed, a pulse is generated and input to an electronic pulse counter. Usually, velocity is calculated in of the pulse count in unit time, which of course only yields information about the mean velocity. If the velocity is changing, instantaneous velocity can be calculated at each instant of time that an output pulse occurs. In this circuit, the pulses from the transducer gate the train of pulses from a 1MHz clock into a counter. Control logic resets the counter and updates the digital output value after receipt of each pulse from the transducer. The measurement resolution of this system is highest when the speed of rotation is low. Fig: Scheme to measure angular velocity

Optical sensing Digital tachometers with optical sensors are often known as optical tachometers. Optical pulses can be generated by one of the two alternative photoelectric techniques. The pulses are produced as the windows in a slotted disc in sequence between a light source and a detector. Another way is to have both light source and detector mounted on the same side of a reflective disc which has black sectors painted onto it at regular angular intervals. Light sources are normally either lasers or LEDs, with photodiodes and phototransistors being used as detectors. Optical tachometers yield better accuracy than other forms of digital tachometer but are not as reliable because dust and dirt can block light paths.

Variable Reluctance Transducers Variable reluctance velocity transducers, also known as induction tachometers, are a form of digital tachometer that uses inductive sensing. They are widely used in the automotive industry within anti-skid devices, anti-lock braking systems (ABS) and traction control. A more sophisticated and the most recently used type has a rotating disc that is constructed from a bonded-fibre material into which soft iron poles are inserted at regular intervals around its periphery. The sensor consists of a permanent magnet with a shaped pole piece, which carries a wound coil. The distance between the pick-up and the outer perimeter of the disc is around 0.5 mm. As the disc rotates, the soft iron inserts on the disc move in turn past the pick-up unit. As each iron insert moves towards the pole piece, the reluctance of the magnetic circuit increases and hence the flux in the pole piece also increases. Similarly, the fluxes in the pole piece decreases as each iron insert moves away from the sensor. The changing magnetic flux inside the pick-up coil causes a voltage to be induced in the coil whose magnitude is proportional to the rate of

change of flux. This voltage is positive whilst the flux is increasing and negative whilst it is decreasing. Thus, the output is a sequence of positive and negative pulses whose frequency is proportional to the rotational velocity of the disc. The maximum angular velocity that the instrument can measure is limited to about 10 000 rpm because of the finite width of the induced pulses. As the velocity increases, the distance between the pulses is reduced, and at a certain velocity, the pulses start to overlap. At this point, the pulse counter ceases to be able to distinguish the separate pulses. The optical tachometer has significant advantages in this respect, since the pulse width is much narrower, allowing measurement of higher velocities. A simpler and cheaper form of variable reluctance transducer also exists that uses a ferromagnetic gear wheel in place of a fibre disc. The motion of the tip of each gear tooth towards and away from the pick-up unit causes a similar variation in the flux pattern to that produced by the iron inserts in the fibre disc. The pulses produced by these means are less sharp, however, and consequently the maximum angular velocity measurable is lower. Fig: Variable Reluctance Transducer

Hall Effect Sensors The Hall Effect is the appearance of a transverse voltage difference on a conductor carrying a current perpendicular to a magnetic field. This voltage is directly proportional to the magnetic field strength. When the current is flowing through a plate (because of an emf of battery) there is zero voltage displayed in the transverse direction. However when there is a magnetic field acting perpendicular to the current, the electrons are deflected by the magnetic force and create a negative charge on one end. The other side has accumulation of positive charge. A voltage across the transverse ends of plate is displayed on the meter. The

hall effect is present in any conductor carrying current in the magnetic field, but it is much more predominant in semiconductors than in metals. Thus most hall effect transducers use a semi conducting material as the conductor.

The rotating element in Hall-effect or magnetostrictive tachometers has a very simple design in the form of a toothed metal gearwheel. The sensor is a solid-state, Hall-effect device that is placed between the gear wheel and a permanent magnet. When an intertooth gap on the gear wheel is adjacent to the sensor, the full magnetic field from the Fig: Hall Effect Sensor magnet es through it. Later, as a tooth approaches the sensor, the tooth diverts some of the magnetic field, and so the field through the sensor is reduced. This causes the sensor to produce an output voltage that is proportional to the rotational speed of the gear wheel.

Tachogenerators

The measurement of speed can also be done with the help of tachogenerator. The tachogenerators are of two types, namely: DC & AC D.C. Tachogenerator -The armature of the d c tachogenerator is kept in the permanent magnetic field. The armature is coupled with the shaft whose speed is to be measured. When the shaft revolves, the armature also revolves in the magnetic field producing an E.M.F, which is proportional to the magnetic flux and speed of rotation.  As the field of permanent magnet is fixed, the E.M.F generated is proportional to speed. The E.M.F induced is measured using moving coil voltmeter, the scale of which is calibrated in speed directly. The circuit has commutator and carbon brush arrangement to receive current from induced E.M.F. 

The advantages of D.C. Tachogenerator are as follows  The output voltage is small enough to measure it with conventional d.c. voltmeters  The polarity of output voltage directly indicates the direction of rotation. Disadvantages  Due to variation in resistances at brushes, considerable error is introduced in output voltage. Regular maintenance of commutator & brushes is required.

A.C. Tachogenerator – In this, the coil is wound on stator itself. The magnetic field is thus rotating. The magnetic field can be obtained using permanent magnet or electromagnet. Due to rotation of magnet, an emf is generated in the stator coil, which has amplitude and frequency directly proportional to the speed of rotation. Thus speed of rotation can be calibrated in of either amplitude or frequency of the induced voltage. – In the above circuit shown in fig., the amplitude is used to measure speed. Using rectifier and filter A. C. voltage is converted to D.C and measured with the help of moving coil voltmeter.

– Advantages of A.C. Tachometers are as follows – The output can be calibrated in of two parameters namely amplitude and frequency of induced voltage. – Commutator and brush resistance problems are eliminated as the coil is wound on stator. – Disadvantages – For very low speeds rotation frequency of induced voltage is also very low, thus ripples in the output increases. To overcome this number of poles of stator is increased so that induced voltage frequency is higher even at low speeds. – For high speeds, the frequency of induced voltage is also very high. Thus the coil impedance increases and the linearity of output is affected. To maintain the good linearity the display device should have higher input impedance than that of the coil.

Optical Encoders Optical instruments for angular velocity measurement include. – Optical Shaft encoder – Non optical tachometer – Stroboscope. Optical shaft encoders are based on circular binary encoders which are used for angular displacement measurement can also be used for angular velocity measurement. Similar to the above disc can be made with optical encoding for optical tachometer.

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A Photo interrupter consists of an LED light source (often infrared) and a photo detector. Mechanical interruption of the light path can be used for various purposes such as counting, triggering etc. Photo interrupter type sensors are used in automobiles to monitor the angular position and the rate of change of steering angle. The sensor comprises a slit disc attached to steering column and rotates with the steering wheel and a fixed U shape photo interrupter. When the column is turned the rotating slotted disc alternatively exposes and blocks the light emitting beams directed toward photodiode receptor, this interruption of the light beam generated a series of pulses

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Other non optical tachometers are available as handheld units. A light beam from tachometer is reflected from the moving pattern on the rotating surface, which may be something as simple as a piece of reflective tape. By counting the reflected light pulses, rotary speed may be determined to high accuracy.

Stroboscope

• Early version of mechanical stroboscope is shown in fig. above. (which will help understanding the principle). It consists of a whirling disc and a stationary mask having opening of same shape & size. When the two openings on the disc & stationary mask coincide, an observer can catch glimpses of the object behind the mask. • If the object is rotating and the speed of disc is synchronized with the motion of object. The object would appear to be motionless. Also if the disc is rotating at slightly lesser speed than object the object would appear to creep forward and if the disc is rotating at slightly faster speed the rotating object would appear to creep backward. • Modern stroboscopes operate with slightly different setup. Instead of the whirling disc, a controllable, intense flashing light source is used. Repeated short duration (10-40 ms) light flashes of adjustable frequency are supplied by the light source. The required flashing rate frequency for synchronization with the motion of object is read as the angular speed of the object. These devices are often called strobe lights. • To properly view the rotating object in the flash light it is required to place a distinctive mark on the rotating object (e.g. mark on one of the spokes of moving wheel). This mark is taken as reference to look for one-on-one synchronization. • Stroboscopes are used to check speeds ranging from 60 to 1,000,000 rpm.

• The stroboscope is highly versatile; it uses no power from the moving object that is being measured. •

It has an accuracy as close as 0.1%

• Most stroboscopes consist of an oscillator, a reed and a flasher as shown above. The oscillator provides trigger pulses to flasher mechanism to control the flashing rate. The vibrating reed serves as a reference for accurately calibrating the stroboscope. The reed is driven from ac lines and vibrates at 7200 times per minute. This steady rate is used to calibrate the flasher. Flasher produces illumination required for the measurements. The flasher lamp can be a very bright LED or an inner gas filled tube which gives illumination on ionisation of the gas. • When the frequency of rotation of object exactly matches the stroboscope frequency the moving mark on the object is viewed stationery as it is at the same position every time when flash light illuminates. Such stationery image also appears when speed of rotation is some exact multiple of the stroboscope frequency. The highest of the strobe frequency that produces stationery image is the fundamental frequency. e.g. when object is rotating at 1200 rpm and the flashing rate is 600 m the identification mark will be in the same position during two consecutive flashes and would appear stationary. Similarly at 400 m & 300 m flash rates also it would appear stationery as in these cases the identification mark will be in the same position during three & four consecutive flashes respectively. To determine the correct frequency corresponding to rotary speed following procedure is used • Determine a flashing rate f1 that freezes the motion. Slowly reduce the rate until the motion is frozen once more, note this rate f2. Then actual rotational speed fo of the object is given by expression.

Thus it is not necessary to always obtain one-to-one synchronization. Also note that above procedure makes it possible to extend the upper limit of the device beyond stroboscope’s normal frequency range. Flash rates range from 110 to 150,000 m enabling measuring speeds upto 1,000,000 rpm.

Mechanical Tachometers • Mechanical Tachometers are either 1) Direct counters OR 2) Centrifugal speed indicators •

Mechanical counters may be of the direct counting digital type or maybe counters with a gear reducer. In the latter case, angular motion available at shaft end is reduced by a worm and gear, and the output is indicated by rotating scales.

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In both examples rpm is measured by simply counting the revolutions for length of time as measured with stop watch and calculating the rpm. Sometimes timing mechanism is also incorporated in the counter. The timer is used to actuate a internal clutch that controls the time interval during which count is measured.

Centrifugal rpm indicators

Centrifugal tachometer. (Centrifugal rpm indicators ) use the “fly-ball governor” principle, which balance centrifugal force against a mechanical spring. An appropriate mechanism transmits the resulting displacement to a pointer, which indicates speed on a calibrated scale. Description of working : Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it. Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the spring tension, causes an increase in the indication on the dial face. When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to 500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).