AS5311

Data Sheet

www.austriamicrosystems.com

Revision 1.01

20 - 23

12 AS5311 Off-axis Rotary Applications

The AS5311 can also be used as an off-axis rotary encoder, as shown in Figure 3. In such applications, the

multi-pole magnetic strip is replaced by a multi-pole magnetic ring. The ring can have radial or axial

magnetization.

Figure 16: Angular Resolution and Maximum Speed versus Ring Diameter

In off-axis rotary applications, very high

angular resolutions are possible with the

AS5311.

The number of steps per revolution

increases linearly with ring diameter.

Due to the increasing number of pulses per

revolution, the maximum speed decreases

with increasing ring diameter.

Example:

a magnetic ring with 41.7mm diameter has

a resolution of 65536 steps per revolution

(16-bit) and a maximum speed of 305 rpm

The number of incremental steps per revolution can be calculated as:

polepairs

nbr

steps

l

incrementa

_

*

1024

_

=

2

*

*

1024

_

π

d

steps

l

incrementa

=

Note that the circumference (d*π) must be a multiple of one polepair = 2mm, hence the diameter of the magnet

ring may need to be adjusted accordingly:

π

mm

polepairs

nbr

d

2

*

_

=

The maximum rotational speed can be calculated as:

π

π

*

39000

*

60

*

_

max_

_

max_

d

d

speed

lin

speed

rot

=

=

where nbr_polepairs

= the number of pole pairs at the magnet ring

d

= diameter of the ring in mm; the diameter is taken at the locus of the Hall elements

underneath the magnet

max_rot_speed = maximum rotational speed in revolutions per minute rpm :

max_lin_speed = maximum linear speed in mm/sec (=650 mm/s for AS5311)

Note:

further examples are shown in the “Magnet Selection Guide”, available for download from the

austriamicrosystems website

http://www.austriamicrosystems.com/eng/Products/Magnetic-Encoders/Linear-Encoders

Res

[bit]

Steps /

Rev.

Ring

Diameter

[mm]

Max

Speed

[rpm]

15

32768

20.9

609

16

65536

41.7

305

17

131072

83.4

152

AS5311 off-axis rotary resolution & speed

0

20000

40000

60000

80000

100000

120000

140000

160000

20

40

60

80

100

ring diame ter [mm]

0

100

200

300

400

500

600

700

resolution

speed rpm