Tuesday, October 30, 2012

AI-Tek Speed Sensor Principles of Operation

Figure 1 Internal Configuration of Typical Sensors

Principles of Operation

The internal construction of the typical Al-Tek variable reluctance sensor is a magnet, pole piece and coil (See figure 1). A magnetic field (lines of flux) extends from the magnet, through the pole piece and coil out into the air space at the end of the sensor. The return path of the magnetic field is from the air space to the other end of the magnet. As a ferrous object approaches the tip of the pole piece, the magnetic field increases and then decreases as the object moves away from the pole piece. The snap or the rapid change in the magnetic field induces an AC voltage signal in the coil. With an ideal target and matching sensor, the induced voltage is in the shape of a sine wave.

As can be seen, the generated frequency signal is directly proportional to the number of ferrous objects passing the pole piece per unit time. The amplitude of the voltage output is proportional to the speed of the ferrous objects passing the pole piece.

Many applications of AI-Tek magnetic sensors use gears as targets. Typical sensor output wave forms with various targets are illustrated in Figure 3. Testing sensors with gears rather than other ferrous discontinuities such as sprockets, keyways, boltheads, etc. is because the output is predictable and repeatable. See Figure 2 for commonly used gear terminology.

Diametral Pitch = No. of Teeth + 2

                        Outside Dia. of Gear (in.)

Figure 3 Generated Voltage Waveforms

Figure 2 Common terms used in defining gears

 The performance of a sensor can be easily defined when using a gear for a target; it also allows for estimated performance with alternate targets. AI-Tek sensors are tested with AGMA standard gears; the performance curves are included in this catalog.
Al-TekInstruments differs from most sensor manufacturers in the presentation of performance curves and test parameters. Most existing data is specified at a surface speed of 1000 in/sec and 0.005 in. air gap; we feel that a 0.030 in. air gap and 500 in sec. surface speed (1800 RPM motor with 5 to 6 in. dia. gear) are more realistic parameters to specify performance.

Magnetic Sensor Selection

The following information is supplied for assistance in selecting the proper sensors for your particular applications. One of the fundamental questions to be answered is, “Will there be enough sensor output voltage at the lowest operating speed?”
The sensor output voltage depends on:
• Surface Speed - speed target passes pole piece
• Gap - distance between target and pole piece
• Target Size - geometric relationship of pole piece and target
• Load Impedance - connected to sensor
The surface speed of a gear depends upon its diameter and RPM. Surface speed is expressed in terms of inches per second (IPS).
Surface Speed (IPS) = RPM x Outside Dia. (in.) x p

Figure 4 Sensor output as a function of gear tooth size

There is an optimum pitch (or tooth size) to obtain the highest possible output from a sensor, but this is seldom necessary. Figure 4 illustrates the relationship of tooth size and spacing for optimum magnetic sensor output. Using a fine tooth gear, relative to a large pole piece diameter sensor, results in a lower generated voltage because the flux also passes into adjacent teeth, resulting in a lower total flux variation.
The relationship between pole piece diameter and gear pitch and its effect on the output of a sensor is described in Table A.

Table A Relative Output vs. Gear Pitch

The load impedance, with relation to the internal impedance of the sensor, dictates the amount of sensor output voltage that will be seen by that load. Magnetic sensors are designed with the lowest practical impedance consistent with providing maximum output. The load impedance should be high in relation to the impedance of the sensor to minimize the voltage drop across the coil and to deliver the maximum output to the load.
Most of the output voltages listed in the AI-Tek catalog are based on a load impedance of 100k ohms. To use a generality, the load impedance should be 10 times that of the sensor.
In order to assist you in selecting your sensor, AI-Tek Instruments has developed an output vs. speed curve for each sensor family. By looking at the application extremes of highest speed/lowest gap and lowest speed/highest gap, the full variation of sensor output can easily be determined. We also specify each family in two ways: Standard - minimum output voltage at 1000 IPS, 0.005 in. gap. Guarantee Point - minimum output voltage at 500 IPS, 0.030 in. gap. Sensors with .187” dia. pole piece are tested with an 8 D.P. gear, 100k ohms load; .106” dia. & smaller pole piece sensors are tested with a 20 D.P. gear, 100k ohms load. Sensors with connectors also use a 250 pf capacitor shunted across the load.
If you would like more information or pricing on AI-Tek Instrument products please contact Forberg Scientific Customer Service.
Toll Free: 855-288-5330