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High-Performance Power ICs and Hall-Effect Sensors
Ring Magnet Speed Sensing for Electronic Power Steering 
by Dan Dwyer, Sensor Sysems Engineer
Adequate control of Electric Power Steering (EPS)
systems requires both speed and direction information from the steering
input shaft. This control will typically come from high-resolution
speed information and fairly coarse position information.
A dual multi-pole ring magnet can be used with a matrix of Hall effect dual output switches and latches to provide all of the required information. Figure 2 shows the configuration of the magnet with a high-resolution outer ring of alternating north and south poles and a low-resolution inner ring of alternating poles.
In order to determine the direction of the rotating magnet, a single Hall-effect sensor with dual outputs from two separate bipolar elements is utilized. Because the two Hall plates (A & B) are situated a distance apart on the surface of the IC, there is a phase lag in the signals generated by the rotating magnet. Refer to figure 3.
With proper magnet pole spacing, the resulting output
signals (Sensor A and Sensor B) are in quadrature and are easily processed
to provide two-state direction information. Refer to Figure 4. The
sensor-to-sensor spacing for the device used in this example is 1.5
mm (dual output bipolar switch). The optimum magnet pole spacing provides
a peak signal in sensor A and zero signal in sensor B. This spacing
corresponds to a dimension that is approximately equal to 3.0 mm between
the alternating poles or a pole period of 6 mm.
In order to obtain absolute position information, a state machine must be generated from the outputs of separate Hall-effect latching sensors. The same phase delay that is induced in the pair of signals of the dual Hall-effect sensor can be induced in separate sensor packages through proper package placement. If two sensor packages are placed at relative angular position that corresponds with the period, T, of the magnet poles, then the output of the two sensors will be exactly in phase. However, if the sensor package spacing is 1.25(T/2), where T is the magnet pole period, then the outputs will be in quadrature. This will hold true for any multiple of this period, i.e. 2.25(T/2), 3.25(T/2), 4.25(T/2).
To generate a matrix of sensor outputs that provide
a cascading phase delay, then each device must be placed at an increasing
fractional multiple of the magnet pole period. For instance, to get
three devices with cascading outputs, sensor #1 can be placed in any
location, sensor #2 can be placed 1.33(T/2) from sensor #1 and sensor
#3 can be placed 1.67(T/2) from sensor #1. Depending on the package
size and magnet size, however, the sensors may not be able to be placed
very close together. This restriction is not a problem if the magnet
poles are fairly consistent. With a repeatable magnet profile, the
fractional portion of the multiplication factor is the only pertinent
value for establishing sensor placement. Using the previous example
of three sensors, the desired cascading output can be realized with
a position of 1.33(T/2) for sensor #2 and 2.67(T/2) for sensor #3.
See figure 5.
With a coarse magnet profile of three alternating north and south pole pairs, the use of three separate Hall effect latches provides six discreet state combinations (A through F) that are repeated three times per magnet revolution. If a controller can track which of the 120° regions that a given sensor lies in, then the system position resolution is 20°. A benefit of this matrix is the ability to detect two fault conditions (LLL & HHH) that logically never occur. See Figures 6a & 6b.
| Angular position (°) | Sensor #1 | Sensor #2 | Sensor #3 | Zone |
|---|---|---|---|---|
| 0 - 20 | L | H | H | A |
| 20 - 40 | L | H | H | B |
| 40 - 60 | L | H | L | C |
| 60 - 80 | H | H | L | D |
| 80 - 100 | H | L | L | E |
| 100 - 120 | H | L | H | F |
| 120 - 140 | L | L | H | A |
| 140 - 160 | L | H | H | B |
| 160 - 180 | L | H | L | C |
| 180 - 200 | H | H | L | D |
| 200 - 220 | H | L | L | E |
| 220 - 240 | H | L | H | F |
| 240 - 260 | L | L | H | A |
| 260 - 280 | L | H | H | B |
| 280 - 300 | L | H | L | C |
| 300 - 320 | H | H | L | D |
| 320 - 340 | H | L | L | E |
| 340 - 360 | H | L | H | F |
| DNE | L | L | L | |
| DNE | H | H | H |
Figure 6b: State Diagram for three latch devices
Alternative Solution
Allegro also offers a complimentary device to the dual output bipolar switch. The A3422 internally processes the output signals from two hall elements and provides two separate signals that represent speed and direction, respectively. The use of the 3422 precludes external processing circuitry that would be required to establish a digital direction value.
Suggested Devices
| Allegro Part Number | Temperature Range(s) |
Package Type(s) | Tape & Reel Available |
Comments |
|---|---|---|---|---|
| A1212 | E, L | LT, UA | Yes | Sensitive latch |
| A1214 | E, L | LH, UA | Yes | Sensitive latch |
| A3280 | E, L | LH, UA | Yes | Very sensitive latch |
| A3281 | E, L | LH, UA | Yes | Sensitive latch |
| A3425 | E, L | K, L | Yes | Dual output bipolar switch |
Typical Applications
- Automotive EPS or EPAS
- Industrial machinery
- Recreational power steering