Hall Effect Sensor Based Portable Tachometer for RPM Measurement

Abstract
Hall effect sensors has a wide variety of applications in position sensing, angular measurement, speed measurement...Etc. Reference[1] is a velocity measurement by using linear hall effect sensor. Basically Hall Effect sensor is a magnetic field detector which detects the changes in field strength .This paper tries to explain a tachometer based on Hall Effect sensor which is accurate, portable and contactless. By using latched Hall Effect sensor, rotational speed can be measured by counting the number of pulses per revolution. The tachometer shaft is made in contact with the motor shaft whose speed is to be measured and a number of magnets are mounted on tachometer shaft. Hall sensor is positioned stationary in such a way that it will closely face the rotating magnets. Since the output is in the form of pulses it becomes easy to manipulate the data. We could develop a model which counts the rotation and which handle high speeds up to 5000-6000 rpm. With the help of a microcontroller speed can be calculated and displayed. From the conventional analog tachometer the Hall Effect based tachometer is expected to give wide ranges of operating speed and increased accuracy.
Keyword:Hall Effect, Lorentz Force, Tachometer, Revolution per Minute.
I.Introduction
Hall Effect is a phenomenon that occurs in conductor carrying a current when it is placed in a magnetic field perpendicular to the current. The charge carrier in the conductor become deflected by the magnetic field and give rise to an electric field(Hall field)that is perpendicular to both current and magnetic field. If current density ‘Jx’ is along X and the magnetic field, ‘Bz’, is along Z, then the Hall field is either along +Y or –Y depending on the polarity of the charge carriers in the material. The force exerted on the charge carriers is given by Lorentz equation. The Lorentz force is the force experienced by a moving charge in a magnetic field. When a charge q is moving with a velocity v in a magnetic field B, within the presence of an electric field E, then it experience a force F, that is proportional t o magnitude of the charge q, its velocity v, and the field B such that,
Explains the Hall Effect theory. Now here arises a question why we go for Hall Effect sensor based tachometer. To understand why, it requires some knowledge about other kinds of RPM detectors. Now consider a normal tachogenerator which gives analog response and requires a A /D converter to manipulate the signal and to display .But when we consider a Hall effect sensor it gives a direct digital (square or rectangular) output, which can be directly taken in to a microcontroller .When going for product like manufacturing this tachometer is expected to be less bulky. Optical and stroboscopic tachometers are the other options, but light beams are highly sensitive to vibrations .Normally a tachometer is expected to operate in a harmonic environment so precision in sensing is questioned .stroboscopic is the better choice of all but expensive. So Hall Effect based tachometer is an attempt to introduce a new RPM measuring device, which is precise less bulky and cost effective.
The System Design:
Considering the design consideration of tachometer, the major priority was given for making a portable device, compromising the noise effect by contact. But it can be hopefully reduced by optimum and stable mechanical design. The prototype gives a similarity with normal tachogenerator which is contactable and a rotating shaft inside. Fig.2 shows the side view and Fig.3 shows the isometric view gives the overall design of the proposed device. A 6mm shaft which is supported and rotating over two bearings takes the speed in to the device. Over the shaft magnets are mounted axially like in figure. The hall element is is positioned stationary over the circuit board and the magnets are closely faced to the sensor. The sensor used is a digital Hall Effect sensor which gives square pulses, a unipolar sensor AH 44E, ALLEGRO make.