Download Speed Sensor overview for SVDO

September 2005
Allegro Confidential
Allegro’s ATS642 Hall Effect Sensor
“A great way to measure the speed and position of a rotating object”
Michael Doogue
Director of Strategic Marketing
Allegro MicroSystems in Manchester, NH
FIRST Presentation
January 2007
September 2005
Allegro Confidential
What is a Hall Effect Sensor?
• Hall effect sensors are silicon chips that convert
magnetic fields into electrical voltages
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–
These sensors can be used to detect the proximity
of a magnet or a ferromagnetic object
A ferromagnetic object is anything that a magnet
will stick to!
• The Hall effect sensor provided to the FIRST
teams is a special type of Hall effect sensor
• The sensor is used in anti-lock brake systems in
automobiles to sense the speed, position and/or
direction of a rotating gear
Hall
Sensor
• Sensor output is a digital signal that is
representative of the profile of the object
passing in front of the sensor
• You can do a lot with this little sensor!
FIRST Presentation
January 2007
September 2005
Allegro Confidential
Automotive Applications for Allegro Hall Sensors
I told you that you could do a lot with Hall sensors!
Powertrain Applications
Cam/Crank Sensors
Transmission Speed
Sensors
Throttle Position Sensors
EGR Valve Position
Sensors
Valve Position Sensors
Current Sensing
Suspension Systems
Position Sensing
Chassis height
FIRST Presentation
Electronic Power Steering
Position Sensing
Motor Commutation
Current Sensing
Convenience Systems
Door lock Position Sensing
Window Position/Speed Sensing
Window/Sunroof Direction
Sensing for anti-pinch
Seating Systems
Various Contactless Switches
Motor Controllers/Drivers
Wiper Systems
LCD screens
Safety Systems
Airbag Diagnostics & Control
Occupant Sensing
ABS/Traction - Wheel Speed Sensing
January 2007
September 2005
Allegro Confidential
ATS642: A Sensor and Magnet in a Single Package
Grey denotes any material that a magnet will stick to
High B
MAGNET
FIRST Presentation
Element
Hall IC
Low B
MAGNET
January 2007
September 2005
Allegro Confidential
Getting Started: What is this thing?
16 mA
Quick review so that you
understand what you are
dealing with when using
this sensor!
FIRST Presentation
8 mA
January 2007
September 2005
Allegro Confidential
ATS642: What Might You Do With It?
• The device is an edge detector – the output is not analog
• If you have a rotating shaft on a motor:
– If you use a gear with teeth in front of the sensor then the
sensor can also be used as a positional encoder that tells
you when the shaft has rotated by a certain number of degrees
• The number of degrees between sensor output transitions
depends on gear geometry and number of teeth on the gear
• Can also be used to measure the distance of travel for a wheel
by counting the number of digital transitions for a uniform gear
– Motor rotation speed information can be detected by measuring
the time between output edges with a microprocessor or timer circuit
t1
t2
• Summary of applications:
– Motor shaft or wheel speed/velocity measurement
– Motor shaft or wheel distance measurement via counting of edges.
Gear at right has 60 teeth so 60 rising edge output transitions
means that the gear has rotated one full revolution
– Can also use a “single tooth” ( maybe a screw) and
obtain one output pulse per revolution
– Advanced users: direction of rotation sensing
• Must use a gear with wide teeth and narrow valleys or vice versa
• Requires precise timer circuit to detect duty cycle of output signal
• Duty cycle is ratio of t1 and t2
FIRST Presentation
January 2007
September 2005
Allegro Confidential
Getting Started: Applications Circuit
•
There is a power pin and a ground pin for this electronic
device, but no output pin
•
The digital output is actually the electrical current
consumed by the device: 8 mA or 16 mA flowing into Vcc
pin and out of GND pin
•
A sense resistor and either an amplifier (comparator) or
microprocessor can be used to measure the voltage across the
sense resistor: for example VOUT(L) node at right
–
–
•
Consult your mentors (or me, via e-mail) regarding comparator circuits
that can be used to convert the current output of the ATS642 into a
voltage (I am not sure what you guys have in your device kits)
You may also use a low side resistor and a single microprocessor input
to process the signal
The voltage across the sense resistor is proportional to the current
passing through the resistor (ohms law)
Voltage drop across resistor = 16 mA * RSENSE
•
Be careful that the voltage drop across the device sense resistor
does not reduce the voltage across the sensor to a level < 4 V
across the sensor leads (or else the device will not operate)
FIRST Presentation
January 2007
September 2005
Allegro Confidential
DOs and DON’Ts When Using This Sensor
Do
•
•
•
•
•
•
•
Be certain that the branded face of the
sensor faces the object to be sensed
Decrease the gap between the sensor face
and the object to be sensed as much as
possible
Make sure that a magnet (like the one in
the 642!) will stick to the object to be
sensed
Use deep valleys between any teeth that
you are trying to sense on a gear (shoot for
at least 5 mm tall teeth)
Align the sensor as shown in this drawing
Feel free to e-mail me with questions at
[email protected]
Look at the ATS642 datasheet at
Don’t
•
•
•
•
Place the sensor more than 3 mm away
from the object to be sensed (if possible)
Allow the voltage across the sensor leads to
fall below 4 V (be careful to size your sense
resistor appropriately)
Surround the sensor with ferromagnetic
material that is closer to the sensor than
the object to be sensed
Rotate the device 90 degrees with respect
to this drawing. If the leads of the device
are in the same plane of a gear then the
device will not work!
http://www.allegromicro.com/datafile/0642.pdf
FIRST Presentation
January 2007
September 2005
Allegro Confidential
Good luck and Thank You for Your Time
Come see me after this talk for a demo of a Hall
sensor at work!
Michael Doogue
Director of Strategic Marketing
Allegro MicroSystems, Inc.
Manchester, NH
[email protected]
FIRST Presentation
January 2007