Magnetic Apogee
Detection Sensor
 

One of the most noteworthy concepts to show up in
model rocketry in many years.  This article will show
you how to build, test, and fly a Magnetic Apogee Sensor.

Written by Scott Aleckson

References:

Sport Rocketry, Sep/Oct 1999, pp 6-9
Sport Rocketry, Mar/Apr 2000, pg 18
To get Sport Rocketry, join the NAR

Honeywell, Inc.
Honeywell Magnetic Sensor Products

Usenet: rec.models.rockets
Free web-based access at RemarQ

Original Article by Robert Galejs
Archived by
Essence's Model Rocketry Reviews
 

This concept was first introduced by Robert Galejs
in Sport Rocketry Magazine.  As it turned out,
there were a couple errors in the published
schematics and that design didn't work.
Since I had the Honeywell HMC1001 chip in
my possession, I began doing some breadboard
tests and came up with a very simple circuit
that performed flawlessly.  Robert recently
updated the original schematics to correct
the previously published errors.

Get your HMC1001 from Newark Electronics.
Search for Newark part #91F4713 from their homepage.

The theory behind the Magnetic Apogee Sensor is to use a small
magnetoresistive (MR) microcircuit which contains a Wheatstone Bridge.
This tiny chip is sensitive to changes in a magnetic field.
The Honeywell HMC1001 has only one axis of detection,
which runs from the side with the pins toward the beveled top edge.
Changes in the orientation of the magnetic field, or changes in the
HMC1001's position within a magnetic field, will cause internal
changes in resistance.  By applying a supply voltage to the chip, we can
compare the two output voltages.  When the chip is aligned with a
magnetic field, the outputs will be equal.  When the chip is perpendicular
to a magnetic field there will be a change in resistance, which will cause
an offset in the outputs.  This offset can be measured with an
Operational Amplifier, such as a 741 Op Amp, and when these
offset voltages change, the op amp will amplify the difference
between them.  Look at the diagram below.

When the HMC1001 is aligned with a magnetic field,
the outputs will be equal and there will be no output
from the op amp.  A change in the magnetic field
orientation will cause the outputs to change and a
signal will be amplified by the op amp.
 

Wheatstone Bridge coupled to an Op Amp
 
 

Now we need to make that signal useful.

By connecting that output to a switching transistor,
known as a Field Effect Transistor, we can use that "switch" to
activate an ejection charge circuit to fire a flashbulb or match.
In this case we are using an N-MOSFET, which means
an N-Channel Metal Oxide Semiconductor Field Effect
Transistor.  Since that's a mouthful, we'll just call it a FET.
One of the best qualities of a FET is that it has a very high
resistance in the "off" state which isolates the Source and Drain
sides very much like a mechanical switch.  There is also
isolation between the Gate side and the Source and Drain.
It only takes a very small voltage at the Gate to allow
current to flow between the Source and Drain on a FET.
Note that FETs are very sensitive to static electricity
and should be handled with great care.
The diagram below shows how the FET is
connected to the output of the op amp.

The presence of a voltage at the Gate will allow current
to flow between the Source and Drain pins on the FET.
With zero volts at the Gate, the resistance between the
Source and Drain is nearly infinite--  an excellent switch.
 
 

Op Amp coupled to an N-MOSFET
 
 
 

Calibrating the sensor.

As is, this circuit may work correctly in some parts of the world.
The trick here, is going to be calibrating your sensor to match the
area where you are going to launch your rocket.  The Earth is a big
magnet, and we are going to use this sensor to detect the Earth's
magnetic field.  Think back to science class.  Remember that
experiment where you put iron powder on a piece of paper
and held a magnet under it?  A magnet has field lines that
start near the south pole and end near the north pole.
Here is a simplified diagram showing a typical magnetic field.

Magnetic field of a typical bar magnet

 The Earth's magnetic field is very similar.  If you are near the
north or south magnetic poles, the magnetic field lines would be
close to vertical.  If you are near the equator, then the lines would
be close to horizontal.  For the purposes of our sensor project,
the closer you are to either of the poles, the more accurate
your sensor will be.  You should mount the sensor so that it's
sensing axis is vertical to match the field lines.

A problem exists for the areas in between.  As you move from one
of the poles towards the equator, the angle of the field lines to the
Earth's surface will gradually fall from vertical towards horizontal.
In order to use a magnetic reference in these areas, you would
have to mount the sensor parallel with the field lines and have a
guidance mechanism onboard the rocket to keep it rotationally
oriented in the correct direction.  This is beyond the scope of
model rockets, so we'll just say that you shouldn't use this sensor
in those areas.  So, for everyone below the Mason-Dixon line,
sorry, but you'll have to stick with timers and altimeters.

Finally, there will be some differences between individual MR chips
as well as op amps and other components in your circuit.  For this reason
it is important to calibrate every sensor before using it.  To calibrate, we
are going to put a resistor between the MR's positive output and ground.
We will adjust the value of that resistor until the unit triggers our FET
at the appropriate angles.  It should trip near the horizontal position
when tilted to the East or West.  In the Northern Hemisphere, the sensor
will activate at a higher angle to the North than to the South.
Try to calibrate it to where it trips just above horizontal to the North
and just below horizontal to the South.  The Earth's magnetic
inclination angles are somewhere around 70 degrees at
my location.  These are the results of my bench tests, with
positive angles measured above horizontal and negative
angles measured below horizontal:

Calibration resistor values with bench test triggering
angles measured referenced to horizontal.
Positive angles are above horizontal and
negative angles are below horizontal.


RESISTANCE NORTH SOUTH
Infinite 60°
25°
10 Mohms
55°
20°
1 Mohm
50°
470 Kohms
45°
220 Kohms
20°
-20°
200 Kohms
10°
-30°
147 Kohms
-45°
100 Kohms
-5°
-60°
<80 Kohms
No Activation
No Activation

I chose to use 200 K as it is the closest to a perfectly horizontal
activation of the sensor.  You should run your own tests to confirm.

As an additional safety, I also put a large resistor between the
op amp output and the FET gate.  I used a 10 Mohm resistor here
to keep any extremely small amounts of voltage leaking through
the op amp from activating the FET.  I want the FET to know when
it's time to work and not have any accidental firings.  This resistor
also increases the vertical "window" angle in which the sensor
will not fire the ejection charge.  This will prevent an early recovery
system deployment in the case of severe weather cocking.
 

Calibrate the circuit by placing a resistor between the
positive output to op amp lead and ground.
The filter resistor helps to stabilize the circuit and
prevent accidental firings while widening the
angle of operation from side to side.

Calibration of the sensor circuit
 
 

Other circuit issues.

The HMC1001 sensor must be "reset" prior to use.
This is to ensure that the Permalloy film that is sensitive
to magnetic fields has it's sensitive axis set in the correct
direction.  Exposure of the sensor to any strong magnetic
fields may upset the natural orientation of this material.
The MR chip has a built in circuit to expose the sensor to
a magnetic field of correct orientation to ensure that the
sensor will be set correctly prior to use.
This small circuit consists of a momentary push-button
switch with a capacitor that is kept charged by the battery
through a resistor.  Pushing the button discharges the voltage
stored in the capacitor into the MR chip's reset circuit.

It is always desirable to have some type of arming switch
in any pyrotechnic electronics used in rocketry.  The first level
of safety should be having a master power switch or removal
of the battery while installing and connecting the flashbulb or
electric match and ejection charge.  The last step prior to
taking the rocket to the launch pad would be to install the
battery or turn on the power switch.  By having an external
switch that will keep the flashbulb isolated from the rest of the
circuit you can safely test the sensor operation, place the rocket
on the launch pad, and connect the engine ignitor leads prior
to arming the ejection charge.  To do this job, I like the miniature
headphone jacks that have a built in switch.  These are readily
available at Radio Shack and when a plug is inserted into the
jack, a small switch inside is opened (disconnected).  The
type of switch we want here is the Normally Closed variety.
In this switch, the circuit is armed only when the plug is removed.
To make the plug even more useful, it is possible to mount an LED
to a plug and run your test circuit so that the LED will indicate
that your circuit is working properly.
 

A picture of the HMC1001 connected to a length
of computer ribbon cable and a 3.5mm plug with
an LED.  The cover is removed for clarity.


 
 
 
STILL UNDER CONSTRUCTION!!!
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OLD PAGE FOR A FULL SCHEMATIC!
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construction report soon!
 
Magnetic sensor schematic
 
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Last updated June 30, 2000