as5311 austriamicrosystems, as5311 Datasheet - Page 15

• Manufacturer Part Number: as5311
• Description: High Resolution Magnetic Linear Encoder
• Manufacturer: austriamicrosystems

AS5311
Data Sheet
Figure 12 shows the error caused by a mismatch of pole length. Note that this error is an additional error on top
of the chip-internal INL and DNL errors (see 5.5). For example, when using a multi-pole magnet with 1.2mm pole
length instead of 1.0mm, the AS5311 will provide 1024 incremental steps or 4096 absolute positions over
2.4mm, but with an additional linearity error of up to 50µm.
The curvature of ring magnets may cause linearity errors as well due to the fact that the Hall array on the chip is
a straight line while the poles on the multi-pole ring are curved. These errors decrease with increasing ring
diameter. It is therefore recommended to keep the ring diameter measured at the location of the Hall array at
20mm or higher.
9.2 Position of the Index Pulse
An index pulse is generated when the North and South poles are placed over the Hall array as shown in
Figure 14.
The incremental output count increases when the magnet is moving to the left, facing the chip with pin#1 at the
lower left corner (see Figure 14, top drawing). At the same time, the absolute position value increases.
Likewise, the position value decreases when the magnet is moved in the opposite direction.
9.3 Mounting the Magnet
9.3.1
As a rule of thumb, the gap between chip and magnet should be ½ of the pole length, that is Z=0.5mm for the
1.0mm pole length of the AS5311 magnets. However, the gap also depends on the strength of the magnet.
Typical gaps for AS5311 magnets range from 0.3 to 0.6mm (see 5.4).
The AS5311 automatically adjusts for fluctuating magnet strength by using an automatic gain control (AGC). The
vertical distance should be set such that the AS5311 is in the “green” range. See 6.5 for more details.
9.3.2
When aligning the magnet strip or ring to the AS5311, the centerline of the magnet strip should be placed
exactly over the Hall array. A lateral displacement in Y-direction (across the width of the magnet) is acceptable
as long as it is within the active area of the magnet. See Figure 14 for the position of the Hall array relative to
Pin #1.
Note: the active area in width is the area in which the magnetic field strength across the width of the magnet is
constant with reference to the centerline of the magnet (see Figure 13 ).
9.3.3
The lateral movement range (stroke) is limited by the area at which all Hall sensors of the IC are covered by the
magnet in either direction. The Hall array on the AS5311 has a length of 2.0mm, hence the total stroke is
Note: active area in length is defined as the area containing poles with the specified 1.0mm pole length. Shorter
poles at either edge of the magnet must be excluded from the active area (see Figure 13).
Figure 13: Active area of strip magnet
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maximum lateral Stroke = Length of active area – length of Hall array
Vertical Distance
Alignment of Multi-pole Magnet and IC
Lateral stroke of Multi-pole Strip Magnets
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