AS5510 AppNote extended range

Application Note
DN[Document ID]
AS5510
Extending the useful measurement
range
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
Content Guide
1
Overview..........................................................................................................................3
2
Using the AS5510 as a Position Sensor ...........................................................................3
3
Designing a system ..........................................................................................................4
4
Choosing a magnet ..........................................................................................................4
5
Example with Single AS5510............................................................................................4
6
Calibration........................................................................................................................7
7
Determining Absolute Position from Two AS5510 .............................................................8
8
Conclusion .......................................................................................................................9
9
Programming Example ................................................................................................... 10
10
Schematic and PCB Details ........................................................................................... 11
11
Contact Information ........................................................................................................ 12
12
Copyrights & Disclaimer ................................................................................................. 13
13
Revision Information ...................................................................................................... 13
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
1
Overview
The AS5510 is a Hall Effect sensor with a single Hall element in a 1.1 x 1.8 mm chip scale package.
It contains an integrated 10 bit ADC and uses I2C communication. In combination with a small
magnet the AS5510 can be used as a linear position sensor with a travel range of any arbitrary
length. However, longer travel ranges necessarily require longer magnets and larger gaps with the
result that system volume increases much more rapidly than useful range. As an example, a sensor
with 0.5mm linear range can occupy less than 4 mm 3 while a system with 3.0 mm range can require
more than 125 mm3. This application note describes how to use multiple AS5510 with a single
magnet to create more compact systems than would be possible with a single sensor. Specifically,
we detail a system using two AS5510 providing 3 mm linear range and occupying only 42 mm 3.
2
Using the AS5510 as a Position Sensor
The Hall Effect element in the AS5510 is only sensitive to magnetic fields that are perpendicular to
the mounting surface of the device; the z axis. The field at the sensor can be in any direction, but
only the z component will be measured.
When queried, the AS5510 will return a 10 bit integer in the range 0 to 1023 that is proportional to
the z component of the field at its current location. Figure 1 shows an example of the reported value
over a range of positions. The important thing to notice is that there is a region in the center where
the curve is nearly a straight line; the position of the magnet relative to the sensor is linearly
proportional to the field as reported by the sensor. We will assume linearity when calculating
position from the numbers reported by the AS5510.
Figure 1: The values reported by the AS5510 when scanned near a 4 mm long magnet.
10 Bit Value Reported by AS5510
Raw Value Returned From AS5510
1024
768
512
256
0
-3
-2
-1
0
1
2
3
Position Relative to Magnet (mm)
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
3
Designing a system
Designing a position sensor system around the AS5510 involves establishing three requirements:
1. Total range of motion over which good linearity is required
2. Required resolution. That is the smallest increment of position that must be detected.
3. Geometric constraints – how much space is available for the entire sensor?
Once the requirements are established magnet material, size, shape and gap can be chosen.
4
Choosing a magnet
The following "rules of thumb" can be used as a first determination of magnet dimensions when
sintered NdFeB is used. Other magnet materials generally require larger magnets.
1. The length of the magnet should be 2.5 to 3 times the required travel range. Longer
magnets can provide greater linearity over the important range, however they also need to
be correspondingly stronger (thicker) and will necessitate a larger gap. This increases the
overall space requirements.
2. Thickness from 20% to 50% of the length.
3. The gap from the magnet to the internal Hall sensor will be 1 to 2 times the magnet
thickness. Subtract the thickness of the AS5510 package (0.36 mm) to estimate the gap
from magnet to the top of the AS5510.
4. Width should be at least as great as the thickness.
In cases where long range, high resolution, and small size are all required, experimentation will be
necessary. FEA calculations can reduce the window of uncertainty, but manufacturing tolerances in
magnet material and system geometry might still require some prototyping to achieve the desired
goal.
5
Example with Single AS5510
As an example we will design a sensor with the following requirements:
1. Travel range:
1.5 mm
2. Resolution:
< 5.0 um per encoder count
3. Volume:
Must fit within 8mm x 5mm x 2mm volume
From this we can estimate magnet dimensions:
1. Length:
3.75 mm to 4.5 mm
2. Thickness:
0.75 mm to 2.25 mm
3. Gap:
0.15 mm to 3.9 mm from magnet to top of AS5510
4. Width:
0.75 mm to 2.25 mm (or greater)
A search of commercially available NdFeB shows that the AS5000-MA4x2H-1 is a possible
candidate. This magnet is 4 mm long, 1 mm thick, and 2 mm wide. This is the longest and widest
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
magnet that will fit the space requirements. The 5 mm height limitation leaves room for up to 3.4
mm gap with 1 mm thick magnet and 0.60 mm thick AS5510.
A test setup was constructed that moves an AS5510 sensor linearly under an AS5000-MA4x2H-1
magnet. Figure 2 shows the experimental setup and figure 3 shows a close view of just the PCB
and magnet. The raw 10 bit value reported by the AS5510 is shown in figure 4. This data was
obtained with the sensor sensitivity set to ±50 mT.
Figure 2: The test setup used to collect data
Figure 3: A close view of the PCB and magnet used.
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
Figure 4: Scan of a 4x1x2 mm Magnet
Scan of 4 x 1 x 2 mm Magnet (AS5000-MA4x2H-1)
1024
Value Returned From AS5510
896
768
640
0.25 mm gap
0.50 mm gap
512
0.75 mm gap
384
1.00 mm gap
256
1.25 mm gap
128
0
-2
-1,5
-1
-0,5
0
0,5
1
1,5
2
Position (mm)
Also shown in figure 4 are least-square linear fits to the center 1.5 mm of each curve. The straight
line represents the position that would be calculated from AS5510 readings. Calculation error is
seen as the horizontal distance between the curve and straight line. It is apparent by inspection that
small gaps will result in larger errors.
The resolution at each gap can be calculated as Δposition / ΔAS5510 reading.
Figure 5: Δposition / ΔAS5510 reading
Gap (mm)
Change in Position
(mm)
Change in ADC Value
(counts)
Resolution (um /
count)
0.25
1.5
756 – 148 = 608
2.5
0.50
1.5
712 – 210 = 502
3.0
0.75
1.5
667 – 270 = 397
3.8
1.00
1.5
636 – 318 = 318
4.7
1.25
1.5
621 – 340 = 281
5.3
1.0 mm and 1.25 mm gaps both use less than half of the available dynamic range of 1024 counts.
In this application a good solution will be to use 1.00 mm gap and increase the sensitivity of the
AS5510s from ±50 mT to ±25 mT. This will double the fraction of available dynamic range used and
consequently improve resolution by a factor of 2.
Figure 6 shows data acquired by two AS5510 spaced 1.5 mm apart with gaps of 1.00 mm.
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
Figure 6: Data of 2 AS5510 spaced 1.5mm to each other at 1mm airgap
Two Sensors 1.5 mm apart, 1.00 mm gap
1024
Bmax
Amax
AS5510 Output
768
512
A
B
256
Bmin
Amin
0
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
1,50
2,00
Measured Position (mm)
Sensor A will be used between -1.5 and 0 mm while sensor B will be used between 0 and +1.5 mm.
Notice that each takes a maximum and minimum value within its respective region. Each saturates
outside of its valid region, but that is not important.
6
Calibration
The values returned by the AS5510s are dimensionless. Converting those values to physical units
(i.e. um) requires calibration. Recall that position will be calculated from AS5510 readings. To this
end, the data in figure 4 is plotted in figure 7 with sensor readings as the independent variable.
Advantage has been taken of a spreadsheet application to calculate the slope and intercept of each
line by least squares fit.
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[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
Figure 7: Position as a function of sensor reading
Position as a Function of Sensor Reading
2,0
1,5
Position = -0.00233B + 1.9617
Measured Position (mm)
1,0
0,5
0,0
A
B
-0,5
-1,0
Position = -0.00235A + 0.4284
-1,5
-2,0
0
256
512
768
1024
Reported by AS5510
In the formulas written on the graph, the first term (the slope) is a conversion coefficient that we will
use to calculate position from the raw numbers reported by the AS5510's, the second term (the y
intercept) puts the 0 position in the center of travel. Other values can be substituted to put the
"home" position anywhere desired.
Here we have used a table of data and linear regression to calculate the equation for the line. In
fact, we could have found any two points on the line (preferably far apart) and calculated the slope
as ΔPosition/ΔAS5510 reading. The simpler method might be more practical when calibrating
sensor systems that are not on a lab bench but integrated into a larger system, perhaps on an
assembly line.
7
Determining Absolute Position from Two AS5510
A problem remains: which of the two sensor readings should be used at any given position? The
solution involves examining both. Refer to figure 6 where the two sensors readings are labeled A
and B. Notice that A is valid between -1.5 and 0 mm while B is valid between 0 and +1.5 mm. In
their respective regions the numbers reported by each AS5510 is bounded by a minimum and
maximum value. It is necessary only to read both sensors and determine which is within its valid
bounds.
With the coefficients above (slope and y-intercept) available, we can use the 10 bit values reported
by both AS5510 to calculate position. Figure 8 shows calculated position and error as a function of
ams Application Note
[v1-02] 2014-Aug-19
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AS5510
Extending the useful measurement range
measured position. The error is calculated as the difference between measured and calculated
position. No more than 20 um error over 1.5 mm travel is found in this case.
Figure 8: Calculated Position and Error
2,0
20
1,5
15
1,0
10
0,5
5
0,0
0
-0,5
-5
-1,0
-10
-1,5
-15
-2,0
-20
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
Error (um)
Calculated Position (mm)
Calculated Position and Error
Position
Error
2,0
Measured Position (mm)
8
Conclusion
A sensor system was designed using two AS5510 and one magnet. The initial specification and
final results are shown in figure 9.
Figure 9: Specification and result
Specification
Result
Travel Range (mm)
1.5
1.5
Resolution (um / encoder count)
< 5.0
< 2.4
Space
8x5x2
8 x 2.6 x 2
Accuracy
ams Application Note
[v1-02] 2014-Aug-19
< 20 um
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AS5510
Extending the useful measurement range
9
Programming Example
The following example in the C programming language is an example of how to calculate absolute
position from the two sensor readings.
float Get_Position(float SlopeA, float SlopeB,
float OffsetA, float OffsetB,
int Amin, int Amax, int Bmin, int Bmax)
{
unsigned char I2C_AdrA = 0x56, I2C_AdrB = 0x57; // the I2C addresses
unsigned char Data_LSB, Data_MSB;
unsigned int ValueA, ValueB; // 10-bit output value (0~1023)
float Position;
// Query both sensors
Data_LSB = I2C_Read8(I2C_AdrA, 0x00); // Read D7..0
Data_MSB = I2C_Read8(I2C_AdrA, 0x01); // Read D9..8 + OCF + Parity
ValueA = ((Data_MSB & 0x03)<<8) + Data_LSB;
if ( (Data_MSB & 0x08) == 0 )
ValueA = 1025;
// Offset Compensation not complete
// Outside the valid range AS5510
Data_LSB = I2C_Read8(I2C_AdrB, 0x00); // Read D7..0
Data_MSB = I2C_Read8(I2C_AdrB, 0x01); // Read D9..8 + OCF + Parity
ValueB = ((Data_MSB & 0x03)<<8) + Data_LSB;
if ( (Data_MSB & 0x08) == 0 )
ValueB = 1025;
// Offset Compensation not complete
// Outside the valid range AS5510
// determine which sensor to use and calculate absolute position
if (ValueA >= Amin && ValueA <= Amax)
{
Position = SlopeA * ValueA + OffsetA;
}
else if (ValueB >= Bmin && ValueB <= Bmax)
{
Position = SlopeB * ValueB + OffsetB;
}
else
{
error_message("encoder error");
Position = -100;
}
return (Position);
}
ams Application Note
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AS5510
Extending the useful measurement range
10
Schematic and PCB Details
Figure 10: Schematic of the PCB used to acquire all data in this document.
AS5510
B1
B2
B3
SDA VDD
SCL
ADR
TST
VSS
U1
AS5510
A3
B1
A2
B2
A1
B3
SDA VDD
SCL
ADR
TST
VSS
A3
A2
A1
U2
J6
C0
C1
C2
10uF
0.1uF
0.1uF
SCL
SDA
GND
VCC
4
3
2
1
Header 4
Figure 11: The PCB. Top layer copper is red while bottom layer copper is blue. Extra traces
on the top layer were added for alignment purposes.
Figure 12: Close-up view of the region of the PCB containing U1, U2, C1, and C2
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AS5510
Extending the useful measurement range
11 Contact Information
Buy our products or get free samples online at:
www.ams.com/ICdirect
Technical Support is available at:
www.ams.com/Technical-Support
Provide feedback about this document at:
www.ams.com/Document-Feedback
For further information and requests, e-mail us at:
[email protected]
For sales offices, distributors and representatives, please visit:
www.ams.com/contact
Headquarters
ams AG
Tobelbaderstrasse 30
8141 Unterpremstaetten
Austria, Europe
Tel: +43 (0) 3136 500 0
Website: www.ams.com
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AS5510
Extending the useful measurement range
12 Copyrights & Disclaimer
Copyright ams AG, Tobelbader Strasse 30, 8141 Unterpremstaetten, Austria-Europe. Trademarks
Registered. All rights reserved. The material herein may not be reproduced, adapted, merged,
translated, stored, or used without the prior written consent of the copyright owner.
Information in this document is believed to be accurate and reliable. However, ams AG does not
give any representations or warranties, expressed or implied, as to the accuracy or completeness of
such information and shall have no liability for the consequences of use of such information.
Applications that are described herein are for illustrative purposes only. ams AG makes no
representation or warranty that such applications will be appropriate for the specified use without
further testing or modification. ams AG takes no responsibility for the design, operation and testing
of the applications and end-products as well as assistance with the applications or end-product
designs when using ams AG products. ams AG is not liable for the suitability and fit of ams AG
products in applications and end-products planned.
ams AG shall not be liable to recipient or any third party for any damages, including but not limited
to personal injury, property damage, loss of profits, loss of use, interruption of business or indirect,
special, incidental or consequential damages, of any kind, in connection with or arising out of the
furnishing, performance or use of the technical data or applications described herein. No obligation
or liability to recipient or any third party shall arise or flow out of ams AG rendering of technical or
other services.
ams AG reserves the right to change information in this document at any time and without notice.
13 Revision Information
Revision History
Revision
Date
Owner
Description
1.0
15.07.2014
D. Cigna
Initial Revision
1.01
12.08.2014
D. Cigna
Added images for PCB and magnet
1.02
19.08.2014
A. Zenz
Updated to new template
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[v1-02] 2014-Aug-19
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