Note: A safety margin should be considered to ensure the air gap stays within its allowed range despite any parameter variation due to fabrication, lifetime drift, etc.
The application defines a minimum value of SNR which consequently defines an upper bound to the air gap, AGMAX(MAG). Since the air gap sets the level of the signal, it defines the signalto- noise ratio (SNR). For this purpose, Allegro has developed sensors such as ALS31300 with a different sensing range on the z axis. Generally, for joysticks that use only small tilt angles ( θ MAX ≪ 25°), the non-saturation constraint is more restrictive on the z-axis with respect to the x/y axes. Table 1: Joystick Magnetic Limitations on Air Gap
When considering a joystick consisting of a ball joint of 10 mm diameter, a cylindrical magnet of 1 T, diameter 5.4 mm, length 1 mm, and which can be tilted of θ MAX = 25°, simulations lead to the minimum air gap values shown in Table 1. In practice, this non-saturation condition provides an additional constraint on air gap, AG MIN(MAG), depending on the sensitivity of the sensor, the shape and remanent magnetic field of the magnet, and the maximum tilt angle, θ MAX. For correct behavior, it is important to ensure the sensor does not saturate during implementation. The sensor is typically able to sense a given range of magnetic field without experiencing saturation. Magnetic constraints arise from the signal level requirements. The mechanical lower bound should be considered when using a low sensitivity device. Minimum air gap, AGMIN(MECH) can be deduced by considering the limit contact case in Figure 4. This constraint ensures no contact between the rotatingįigure 4: Joystick Mechanical Limitation on Air Gap Mechanical constraints will provide a lower bound to the air gap for a cylindrical magnet that is not embedded in the ball of the joystick. This parameter must comply with the following mechanical and magnetic constraints. Air Gap ConstraintsĪir gap as defined in Figure 2 is a key parameter in the application that will both affect the selection of sensor and the final responsivity of the stick. This distance is commonly called air gap (AG).įor joystick applications, air gap is defined at no tilt, θ = 0°.
In practice, responsivity is also dependent on the orientation of the stick, φ, but this dependence can usually be excluded with respect to other parameters such as air gap.Īs will be demonstrated in the next section, responsivity is closely related to the distance from the magnet to the sensor since it can exacerbate or dampen magnet border effect, short-scaleĪsymmetry, etc. The responsivity of the stick position point in the position plot is defined as:
R can represent x (when φ = 0°) or y (when φ = 90°) or any combination of both when the direction is arbitrary.
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