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Programming a Back Biased Switch for Proximity Sensing
Proximity Sensing Programming Technique

This applications note assumes the user is programming an Allegro fully integrated back biased Hall effect switch or that the user has designed an appropriate magnetic circuit to accommodate the programming range of a programmable unipolar Hall effect switch. Programming is accomplished by controlling the offset of the Schmitt trigger, as seen in the functional block diagram in Figure #1. The method of programming the device is given in the appropriate datasheet.

A graph of Magnetic Flux Density of the Reference Target vs. Position (Rotational Degrees) helps to illustrate programming and its effects on the performance of the sensor, see Figure #2. Each curve represents a different installation air gap for the sensor; only one tooth is used to represent a typical ferrous target used for proximity sensing. A piece of metal that translates instead of rotating would generate a similar graph with a horizontal axis in millimeters instead of degrees. As proximity sensing applications can use either rotating or translating targets, this paper uses rotating as a convention. A detailed description of the Reference Target can be found in Appendix A.

Notice that an arbitrary Schmitt threshold would represent two horizontal lines on the graph, separated by the typical hysteresis of 20 Gauss. The more positive switch-point is called B_OP (Magnetic Operate Point) and the more negative is called B_RP (Magnetic Release Point). The output switches at B_OP when the signal goes from the valley to the tooth and on B_RP when the signal goes from tooth to valley. (Reference Figure #3)

Figure #4 is a further zoom of the B_OP switchpoint moving from valley to tooth. Two vertical lines are drawn to show the switch-point location at installation air gaps of 0.75mm and 2.25mm for the given B_OP. Notice that the difference between the air gaps is approximately 1.5° over the air gap range 0.75mm to 2.25mm. This is the relative accuracy that is expected over installation air gap for the given arbitrary threshold.

Proximity Sensing

In the proximity sensor case the switchpoint is programmed to achieve the desired position, which may be in millimeters or degrees on the horizontal axis of figures #2, 3, and 4. Programming makes it possible to compensate for mechanical offsets generated during manufacturing, which allows for tight control of the switching position. For example: if the desired switchpoint in figure #4 is 12° and the device is installed at 0.75mm air gap, then B_OP in should be programmed at approximately 400 Gauss, a higher level than that shown in the figure. If the device were installed at 2.25mm air gap then B_OP should be programmed at just over 200 Gauss or slightly lower than that shown in figure #4.

Note: care must be taken not to adjust the switch-point too close to either the tooth or the valley field as the sensor switch-point will vary over temperature according to the specification. If the switching threshold drifts above the tooth signal or below the valley signal at any given installation air gap then the device will not switch.

Reference Target Dimensions

Target	Diameter (Do)	Thickness (F)	Tooth Width (T)	Valley Width (Pc - T)	Valley Depth (ht)
Reference Target	120mm	6mm	23.5mm	23.5mm	5mm

Gear Parameters for Correct Operation

Characteristic	Description	Limits
		Min.	Typ.	Max.	Units
Valley Depth (ht)	Depth of Target Valley	5	—	—	mm
Valley Width (Pc - T)	Length and Width of Target Valley	30	—	—	mm
Tooth Width (T)	Width of Target Tooth	5	—	—	mm
Thickness (F)	Thickness or Length of Target Tooth	5	—	—	mm

Suggested Devices

Allegro Part Number	Temperature Range(s)	Package Type(s)	Tape & Reel Available
A3250	J, L	LT, UA	Yes
A3251	J, L	LT, UA	Yes
ATS635	J	SB	No

Possible Applications

• Proximity sensor
• Vane interrupter switch
• Speed sensor
• Cam sensor
• Rotary Encoder
• Brake light switch
• Revolution counting
• Flow meter
• Level meter
• Non-contact limit switch
• Brushless motor commutation
• Headlight position sensor

Please visit the Allegro's website, www.allegromicro.com, for magnet suppliers.

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