Speed%20Sensor%20overview%20for%20SVDO%20-%20Elkhart

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Allegros ATS642 Hall Effect Sensor A great way
to measure the speed and position of a rotating
objectMichael DoogueDirector of Strategic
Marketing Allegro MicroSystems in Manchester, NH
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What is a Hall Effect Sensor?

• Hall effect sensors are silicon chips that
  convert magnetic fields into electrical voltages
• 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
• 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!

Hall Sensor
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Automotive Applications for Allegro Hall
SensorsI told you that you could do a lot with
Hall sensors!
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
Electronic Power Steering Position Sensing Motor
Commutation Current Sensing
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
Safety Systems Airbag Diagnostics Control
Occupant Sensing ABS/Traction - Wheel Speed
Sensing
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ATS642 A Sensor and Magnet in a Single Package
Grey denotes any material that a magnet will
stick to
High B
Low B
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Getting Started What is this thing?
16 mA
Quick review so that you understand what you are
dealing with when using this sensor!
8 mA
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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
• 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

t1
t2
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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 lt 4 V across the
  sensor leads (or else the device will not
  operate)

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DOs and DONTs When Using This Sensor
Do Dont

• 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
  doogue_at_alum.dartmouth.org
• Look at the ATS642 datasheet at
  http//www.allegromicro.com/datafile/0642.pdf

• 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!

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Good luck and Thank You for Your TimeCome see
me after this talk for a demo of a Hall sensor at
work!Michael DoogueDirector of Strategic
MarketingAllegro MicroSystems, Inc.Manchester,
NHdoogue_at_alum.dartmouth.org