One-Chip Linear Position Measurement with Hall SensorsAugust 1, 2006 By: David Lin Sensors
Check out this Hall effect sensor design that allows you to use magnetic tape of any length for error-free, easy assembly.
Logging linear movement can be a complicated and expensive business. A variety of measuring systems can be used, depending on the accuracy required and distance to be measured. The simplest of these is the sliding potentiometer, which supplies a variable resistance proportional to the distance of travel. Incremental or absolute linear encoders can have accuracies of <1 µm at lengths of several meters. Simple applications, such as level controllers, lift magnets, seat adjusters, motion sensors, or noncontact sliding switches, however, require the measurement of distances from a few millimeters to several hundred centimeters—as inexpensively as possible and without contact between the components. Magnetic encoders based on Hall sensors have long been successfully deployed for the detection of rotational motion .
The operating principle of rotational magnetic encoders is based on detecting the magnetic field of a cylindrical, diametrically magnetized permanent magnet positioned normal to the IC. The four Hall sensors measuring the rotating magnetic field are arranged in pairs orthogonally. These provide suitable electrical sine and cosine signals from which the angle of rotation of the permanent magnet can be clearly derived. For sensing linear position, the new iC-ML has the four Hall sensors placed in a line and configured for a magnetic pole pitch of 2.56 mm. The circuit allows a suitable magnetic tape of any length to be used and also permits error-free assembly, thus reducing the time and effort required for this part of the production process.
Figure 1 shows the iC-ML in relation to the magnetic tape. The X coordinate lies in the direction of the tape's orientation, which alternates between north and south magnetization, and parallel to the chip's surface. The sine and cosine signals shown are the electrical output voltages in analog operating mode. The origin of the magnetic tape coordinate system is set to the center of any one of the north poles.
Figure 1. The principle of magnetic position sensing using integrated Hall sensors
Mechanically, the iC-ML's longitudinal shift is then defined by the distance xd between the outermost Hall sensor on the left and the origin of the magnetic tape. If the circuit is moved along the X axis, the signal pattern shown in Figure 1 is generated at the device outputs. With each magnetic period of 5.12 mm an electrical signal period is produced. Within this period the absolute position can be determined.
The circuitry integrated into the iC-ML contains a Hall signal amplification unit with automatic gain control, compensation for changes in temperature, magnetic field strength, and supply voltage, as well as an A/D and a D/A converter for digitized and converted analog output signals. The differential magnetic field evaluation makes the circuitry insensitive to external magnetic DC fields. If the magnetic field strength drops below an acceptable threshold, depending on the configuration of the iC-ML this is either shown as an analog value (GAIN output) or digital signal (NERR output).
The signal conditioning unit of the iC-ML supports a resolution accuracy of 20 µm at a linear speed of up to 5 m/s. In addition to the sine and cosine signals, which are kept at a constant amplitude of 2 Vp-p, the IC provides digital signals in the form of incremental pulses and ABZ quadrature signals with resolutions of 6, 7, and 8 bits per magnetic period. A further option is to generate sawtooth and triangle signals by converting these digital signals back to analog.
Modes of Operation
Various modes of operation are coded in tri-state logic via three inputs. The three states are low, high, and Vcc/2 level, which sets itself automatically with an open input. A total of 26 operational modes can be selected using the three inputs:
- 1. Analog sine and cosine signal (unipolar or differential)
- 2. Incremental A/B signals with a zero pulse
- 3. Counter pulse for up/down binary counters
- 4. Analog sawtooth or triangle signal with a settable amplitude
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