CN-0341: Magnetoresistive Linear Position Measurement

Circuit Note
CN-0341
Devices Connected/Referenced
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0341.
AD7866
Dual Channel, 1MSPS, 12-bit,
Simultaneous Sampling SAR ADC
AD8227
Wide Supply Range, Rail-to-Rail,
Instrumentation Amplifier
AD8615
Low Offset, Low Noise, Precision Amplifier
Magnetoresistive Linear Position Measurement
The circuit provides all necessary signal conditioning including
instrumentation amplifiers, buffers, and a dual channel ADC
that efficiently process the AMR sensor low level bridge outputs.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0341 Circuit Evaluation Board (EVAL-CN0341-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The result is an industry leading position measurement solution
suitable for valve and flow measurement, machine tool speed
control, motor speed measurement, and other industrial or
automotive applications.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 provides a contactless, AMR
(anisotropic magnetoresistive) linear position measurement
solution with 2 mil (0.002 inch) accuracy over a 0.5 inch range.
The circuit is ideal for applications where high speed, accurate,
non-contact length and position measurements are critical.
5V
0.1µF
5V
5V
REF
0.1µF
10µF
2.96kΩ
10µF
2.5V
3.3V
5V
DVDD AVDD
REFSEL
RANGE
AD8227
AD8615
VDRIVE
VA1
VCC
SCLK
+VO1
AA745
5V
–VO1
+VO2
–VO2
GND1
SDP
2.5V
REF
2.96kΩ
AD7866
AD8227
DOUTA
5V
AD8615
CS
VB1
DGND AGND
DCAP A
470nF
DCAP B
470nF
VREF
5V
100nF
2.5V
12115-001
AD8615
Figure 1. Magnetoresistive Linear Position Measurement System (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. 0
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CN-0341
Circuit Note
The Sensitec AA745 is an AMR-based angular sensor containing
two galvanically separated Wheatstone bridges at a relative angle of
45° to each other. The AA745 offers minimal offset voltage
(±2 mV) and high signal amplitude (70 mV). A rotating magnetic
field stimulates the sensor, creating an output voltage of ±70 mV.
An AD8227 instrumentation amplifier amplifies the signal of
interest while rejecting the Wheatstone bridge common-mode
voltage of 2.5 V. The common-mode output voltage of the inamp is set to 2.5 V by driving the VREF pin to 2.5 V. A 2.96 kΩ
gain resistor sets the gain of 32. This creates an analog output
voltage of 0.2 V to 4.8 V for a bridge output of 2.5 V ±70 mV.
The circuit signal bandwidth is determined by the AD8227 that
has an approximate 100 kHz bandwidth for a gain of 32.
A unity gain AD8615 op amp buffers the in-amp output voltage
and connects directly to the ADC. This buffer has a rail-to-rail
output stage that swings to within 200 mV of the supply rails.
The AD7866 is a dual channel 12-bit 1 MSPS SAR ADC. The
polarity of the RANGE pin determines the analog input range
and output coding. If this pin is tied to a logic high when the chip
select goes low, the analog input range of the next conversion is 0 V
to 2VREF (0 V to 5 V), leaving approximately 200 mV headroom
for the 0.2 V to 4.8 V input signal from the buffer amplifier.
An example of the AMR effect is shown in Figure 2. A current (I),
flowing through a conductor, is subject to an external magnetic
field (HY). The resistance of the conductor changes as a function of
the angle (Ø) between the magnetization vector (M) and the
current flow vector (I). The magnetization vector is the net sum
of the internal magnetic field (HX) and the applied external
magnetic field (HY).
The maximum resistance occurs when the magnetization vector
(M) is parallel to the current vector (I). The minimum resistance
occurs when the magnetization vector (M) is perpendicular to
the current vector (I).
Effective utilization of the AMR effect requires the conductor to be
a material insensitive to mechanical stress but sensitive to magnetorestriction. For these reasons, permalloy (80% nickel, 20% iron)
is the most commonly used alloy in AMR sensor manufacturing.
Permalloy Properties
There are two properties of permalloy strips that provide design
challenges when creating angular measurement systems.
First, permalloy has a narrow linear operating region (see Figure 3).
Only when the angle (Ø) between the magnetization vector (M)
and current flow vector (I) becomes larger, does the response
become linear. Unfortunately, shortly after the response becomes
linear, it saturates.
Connecting the REFSEL pin low configures the ADC to use the
internal 2.5 V reference voltage. This voltage is available on the
VREF pin but requires a buffer before it can be used elsewhere in
the system. The DCAPA pin and DCAPB pin are decoupled with
470 nF capacitors to ensure proper operation of the ADC. The
reference voltage is buffered by the AD8615 and sets the
common-mode output voltage of the AD8227 in-amp.
The AD7866 simultaneously samples both channels of the
magnetoresistive sensor. The digital words are normally available
on DOUTA and DOUTB. Each data stream consists of one leading
zero followed by three status bits and then twelve bits of conversion
data. However, by holding the chip select low for an additional 16
clock cycles, both digital words are read from one channel, DOUTA.
An SPI interface allows access to both channels on one data line.
Magnetoresistive (MR) Theory
Magnetoresistivity is the ability of a material to change the value
of its resistance when subjected to an external magnetic field.
The most commonly used MR sensors are based on the
anisotropic magnetoresistive (AMR) effect.
M
Ø
HY
I
R
R0 + ΔR
R0
12115-003
CIRCUIT DESCRIPTION
HY
–1.0 –0.5
0
0.5
1.0
H0
Figure 3. Permalloy Resistance vs. Magnetic Field
Secondly, permalloy is insensitive to polarity. The resistance of a
permalloy strip decreases whether the angle (Ø) between the
magnetization vector (M) and the current flow vector (I) is
positive or negative.
Barber Poles
A common method used to improve both the linearity and polar
insensitivity of the permalloy strip is to add aluminum stripes
angled at 45° to the strip axis called barber poles, as shown in
Figure 4. Any current flowing between barber poles takes the
shortest path—the perpendicular path, and the angle between the
current flow vector (I) and magnetization vector (M) shifts by 45°.
ALUMINUM STRIPES
12115-002
I
M
Figure 2. Anisotropic Magnetoresistive Example
HX
HY = 0
Figure 4. Barber Pole Effect in a Permalloy Strip
Rev. 0 | Page 2 of 8
12115-004
HX
HY
Circuit Note
CN-0341
Figure 5 shows the result of adding barber poles to a permalloy
strip. The current flow vector shifts by 45°, but the magnetization
vector remains unchanged. Notice the linear behavior now
present in the middle of the graph.
R
R0 + ΔR
The maximum peak signal amplitude of the AA745 is 70 mV
(14 mV/VCC on a 5 V supply). The sensor offset voltage is ±10 mV
(±2 mV/VCC on a 5 V supply) giving a useable 2.5 V ±0.70 mV
output signal. A rotating magnetic field produces the sin (2ø)
and cos (2ø) outputs seen in Figure 8. Both signals are periodic
over a 180° range, making the detection of full 360° measurements
impossible without additional circuitry and components.
2.57
OUTPUT VOLTAGE (V)
2.55
–1.0 –0.5
0
0.5
H0
1.0
Figure 5. Barber Pole Permalloy Resistance vs. Magnetic Field
Magnetic Field Strength and Orientation
The AA745 magnetoresistive sensor requires a minimum
magnetic field strength of 25 kA/m to ensure the error
specification found in the data sheet. This stimulating magnetic
field must intersect the center of the sensor package.
When selecting a magnet, consider the air gap between the
sensor and the magnet as shown in Figure 6. The distance
between magnet and sensor should be equal to half of the
magnet length (d = 0.5 × L). It is critical to align the magnet
and sensor in three dimensions as accurately as possible. Any
misalignment introduces errors and create nonlinearities in the
signal of interest. Misalignment errors in any dimension are
discussed in the Test Results section.
L
12115-006
Figure 6. Magnet Orientation and Air Gap for Linear Position Measurement.
Sensor Basics
The standard AMR sensor consists of two Wheatstone bridges,
with one bridge at a relative angle of 45° with respect to the
other. Permalloy strips comprise each element of both bridges
and have nominal resistance values of 3.2 kΩ.
VCC1
GND1
+VO1
–VO2
VCC2
GND2
+VO2
COS (2Ø)
SIN (2Ø)
2.49
2.47
2.45
2.43
0
45
90
ANGLE (°)
135
180
Figure 8. Magnetoresistive Sensor Output Voltage
Channel Sensitivity
The sensor has a nominal sensitivity of 2.35 mV/° for each
channel. This means each degree of change between the
magnetization vector and the sensor orientation produces an
output voltage change of 2.35 mV. The sensitivity is not constant
with respect to the angle. The areas of decreased sensitivity are
the portions of each output where the slope of the line
approaches zero.
The software takes advantage of this, measuring the angle based
on whichever sensor is most accurate at the time. If channel one
is approaching 45°, channel two is used to calculate the angle
and maintain the system accuracy.
Test Results
The EVAL-CN0341-SDPZ PCB is tested by mounting a magnet
to the arm of a digital caliper. The EVAL-CN0341-SDPZ PCB
sits in position with the face of the AA745 AMR sensor (U5)
perpendicular to the face of the magnet. As the magnet moves,
the caliper displays the distance travelled accurate to 0.0005
inch. Simultaneously, the magnetic field lines intersect the
sensor and provide a useable output voltage. A functional
diagram of the setup is shown in Figure 9, and a photo of the
setup is shown in Figure 10.
12115-007
–VO1
2.51
Referring to Figure 8, channel one (the blue line) loses sensitivity as
the magnetization vector angle nears 45° or 135°. Similarly,
channel two (the red line) loses sensitivity around 0° and 90°.
Fortunately, when one channel has reduced sensitivity, the other
channel is in a region of high sensitivity.
d
SENSOR
2.53
12115-008
HY
12115-005
R0
Figure 7. AA745 Dual Wheatstone Bridge Configuration
Rev. 0 | Page 3 of 8
CN-0341
Circuit Note
Restricting the measurement range to 0.4 inches produces better
error results. Note that 0.4 inches coincides with the linear portion
of the trigonometric waves shown in Figure 8 and confines
measurement to a 30˚ range. Applying a new gain correction factor
for this modified range produces a ±1 mil error seen in Figure 12.
CALIPER
1.0005in
0.0010
MAGNET
+6V
0.0005
12115-009
EVALSDP-CB1Z
Figure 9. Data Collection Test Setup
0
–0.0005
–0.0010
–0.0015
–0.2
–0.1
0
0.1
DISTANCE TRAVELLED (Inches)
0.2
12115-012
USB
ERROR (Inches)
SENSOR
EVAL-CN0341-SDPZ
Figure 12. Magnetic Field Position Error: 0.4 Inch Range,
Data Shown for Four Boards
12115-010
The sensor is positioned so it sits in the middle of the body of
the magnet as seen in Figure 13. A common error source,
vertical misalignment, occurs when the sensor is moved up or
down with respect to the magnet.
Figure 10. Photo of Bench Test Setup
12115-013
The magnet used in testing is 2 inches long and is positioned
1 inch away from the sensor. Data is collected by moving the
magnet and comparing the evaluation software reading to the
caliper digital display reading. Figure 11 shows the output
position error recorded over a 1.0 inch range. The error is
±2 mil over the entire range.
0.002
Figure 13. Photo of Bench Test Setup: Vertical Misalignment
0.001
–0.001
–0.002
–0.003
–0.004
–0.005
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
DISTANCE TRAVELLED (Inches)
0.3
12115-011
ERROR (Inches)
0
Figure 11. Magnetic Field Position Error: 1.0 Inch Range,
Data Shown for Four Boards
Rev. 0 | Page 4 of 8
Circuit Note
CN-0341
Figure 14 shows the errors introduced by misaligning the sensor
and magnet vertically. This test consists of moving the PCB up
or down by 0.25 inch and 0.5 inch before collecting data. For a
measurement range of 1.0 inch, moving the 0.25 inch up or
down seriously degrades the performance, adding several mils
of error to the calculation. Moving the sensor 0.5 inch up or
down makes matters worse, adding tens of mils of error to the
original reading.
0.020
ERROR (Inches)
0.015
0.035
0.030
0
0.020
–0.010
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
0.5
DISTANCE TRAVELLED (Inches)
12115-016
ERROR (Inches)
0.005
–0.005
0.25” DOWN
0.25” UP
0.5” DOWN
0.5” UP
0”
0.025
Figure 16. Magnetic Field Position Error: Rotational Misalignment
0.015
Figure 17 displays one last common error source, sensor-to-magnet
distance. The ideal distance between the sensor and magnet is
half of the magnet length. Increasing or decreasing this distance
introduces errors into the measurement. Figure 17 shows the bench
test setup where the magnet and sensor are too close together.
0.010
0.005
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
0.5
DISTANCE TRAVELLED (INCHES)
12115-014
0
–0.005
–0.5
RIGHT
STRAIGHT
LEFT
0.010
Figure 14. Magnetic Field Position Error: Vertical Misalignment
By modifying the gain correction factor, these large errors can
be improved but not removed completely. Increasing the
distance from the magnet negatively influences the magnetic
field strength and orientation of the magnetic field lines making
some of the data unrecoverable.
12115-017
Figure 15 shows a second common error source, rotational
misalignment. While the sensor and magnet are positioned
ideally with respect to the vertical, the sensor is not parallel to
the face of the magnet.
Figure 17. Photo of Bench Test Setup: In-Plane Distance Variation
The distance between the magnet and sensor is set to 0.1 inch,
0.5 inch and 1 inch, and then data sets are collected. Figure 18
shows the errors associated with the different configurations.
0.020
0.1”
1”
0.5”
ERROR (Inches)
Figure 15. Photo of Bench Test Setup: Rotational Misalignment
Figure 16 shows the readings associated with this error source.
The green line shows the errors recorded for a parallel
configuration while the red and blue lines show the additional
errors introduced by rotating the sensor left or right with
respect to the face of the magnet.
Rev. 0 | Page 5 of 8
0.010
0.005
0
–0.005
–0.010
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
0.5
DISTANCE TRAVELLED (Inches)
Figure 18. Magnetic Field Position Error: In-Plane Distance Variation
12115-018
12115-015
0.015
CN-0341
Circuit Note
By modifying the gain correction factor, these large errors can
be improved but not removed completely. Increasing or
decreasing the distance from the magnet negatively influences
the magnetic field strength and orientation of the magnetic field
lines making some of the data unrecoverable.
12115-020
A screen shot of the LabVIEW® evaluation software used for all
readings and calculations is shown in Figure 19.
Figure 20. Photo of the EVAL-CN0341-SDPZ PCB
12115-019
COMMON VARIATIONS
Figure 19. Screenshot of the CN-0341 Evaluation Software
The calibration tab determines the maximum and minimum
voltage (VMAX and VMIN) output of each Wheatstone bridge.
Knowing these values allows a more precise mapping of voltage
to digital code. The user can manually enter the values to
minimize calculation errors.
Two changes are required to create a linear position measurement
system from the angular position system described in the CN-0323
Circuit Note. First, replace the AA747 sensor with the AA745. This
sensor specifically senses linear movement and has identical
electrical characteristics as the AA747. Second, replace the magnet
with a multi-pole bar magnet consisting of a series of alternating
north and south poles as shown in Figure 21.
PCB
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to consider
the power supply and ground return layout on the board. The
PCB should isolate the digital and analog sections as much as
possible. The PCB for this system was constructed in a 4-layer
stack up with large area ground plane layers and power plane
polygons. See the MT-031 Tutorial for more discussion on
layout and grounding, and the MT-101 Tutorial for information
on decoupling techniques.
Decouple the power supply to all ICs with 1 μF and 0.1 μF
capacitors to properly suppress noise and reduce ripple. Place
the capacitors as close to the device as possible. Ceramic
capacitors are advised for all high frequency decoupling.
Power supply lines should have as large a trace width as possible to
provide low impedance paths and reduce glitch effects on the supply
line. Shield clocks and other fast switching digital signals from
other parts of the board by digital ground. Figure 20 is a photo
of the PCB.
D=
SENSOR
1
POLE LENGTH
2
P
P = POLE LENGTH
12115-021
D
Figure 21. Linear Position Measurement Magnet, PCB, and Sensor
The AA745 comes in a horizontal package that mounts flush
against the edge of the PCB. This allows optimization of the
distance between the magnet and sensor, the ideal distance being
one-half the pole length of the magnet.
As the sensor moves parallel to the magnet it detects the magnetic
field which rotates 180° for every pole length travelled. The pole
length of the magnet (P) and the angular accuracy of the sensor
(ΔØ = 0.05°) determine the theoretical accuracy (Δx).
Δx = P × ΔØ/180°
A complete design support package for this circuit note is at
www.analog.com/CN0341-DesignSupport.
Rev. 0 | Page 6 of 8
Circuit Note
CN-0341
This provides an absolute measurement system for only one
pole length. If the magnet has more than one pole, counting the
number of poles passed provides a more accurate reading.
Additional electronics are required to implement this
functionality, and traditionally a second magnet with different
pole length provides a reference point for an additional sensor.
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-SDP-CB1Z System Demonstration
Platform (SDP) evaluation board and the EVAL-CN0341-SDPZ
circuit board. The two boards have 120-pin mating connectors,
allowing for the quick setup and evaluation of the performance
of the circuit.
The EVAL-CN0341-SDPZ contains the circuit to be evaluated, as
described in this note. The EVAL-SDP-CB1Z is used with the
CN-0341 evaluation software to capture the data from the
EVAL-CN0341-SDPZ evaluation board.
Setup
Connect the 120-pin connector on the EVAL-CN0341-SDPZ to
the connector on the EVAL-SDP-CB1Z. Use nylon hardware to
firmly secure the two boards, using the holes provided at the
ends of the 120-pin connectors.
With power to the supply off, connect a 6.0 V DC barrel jack to
connector J4. Connect the USB cable supplied with the EVALSDP-CB1Z to the USB port on the PC. Note: Do not connect
the USB cable to the mini-USB connector on the SDP board at
this time.
Place the neodymium magnet directly on top of the IC or in
some fixture designed to spin the magnet, which minimizes the
distance between the IC and magnet itself.
It is important to keep other sources of magnetic fields away
from the IC as any stray magnetic field can cause errors in the
output voltage of the sensor.
Equipment Needed
Test
The following equipment is needed:
Apply power to the DC barrel jack, connector J4. Launch the
CN-0341 evaluation software and connect the USB cable from
the PC to the mini-USB connector on the EVAL-SDP-CB1Z.
• PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows® 7 (32-bit)
• EVAL-CN0341-SDPZ evaluation board
• EVAL-SDP-CB1Z evaluation board
• 6 V power supply or wall wart
• CN-0341 evaluation software
• Neodymium magnet with a minimum magnetic field
strength of 25 kA/m at the package of the sensor.
Once USB communications are established, the EVAL-SDP-CB1Z
can now be used to send, receive, and capture serial data from
the EVAL-CN0341-SDPZ.
Information regarding the EVAL-SDP-CB1Z can be found in
the SDP User Guide.
Getting Started
Load the evaluation software by placing the CN-0341
evaluation software CD into the PC. Using My Computer,
locate the drive that contains the evaluation software CD and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
Information and details regarding test setup and calibration, and
how to use the evaluation software for data capture can be found in
the CN-0341 Software User Guide at: www.analog.com/CN0341UserGuide .
Functional Block Diagram
Figure 22 shows the functional block diagram of the test setup.
6V
POWER
SUPPLY
PC
J4
EVAL-SDP-CB1Z
J1
120-PIN
CONNECTOR
12115-022
EVAL-CN0341-SDPZ
Figure 22. Test Setup Block Diagram
Rev. 0 | Page 7 of 8
CN-0341
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0341 Design Support Package:
http://www.analog.com/CN0341-DesignSupport
CN-0341 Circuit Evaluation Board (EVAL-CN0341-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
AD7866 Data Sheet
AD8227 Data Sheet
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
AD8615 Data Sheet
AN-688 Application Note, Phase and Frequency Response of
iMEMS Accelerometers and Gyros, Analog Devices
REVISION HISTORY
AA700 Application Note, AMR Freepitch Sensors for Angle and
Length Measurement, Sensitec
1/14—Rev. 0: Initial Version
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©2014 Analog Devices, Inc. All rights reserved. Trademarks and
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CN12115-0-1/14(0)
Rev. 0 | Page 8 of 8
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