Fundamentals of Electronic Circuitry
for Electrolytic Tilt Sensors
Engineering Application Note
Document #9723
WARNING:
Use
of DC current can permanently polarize the electrolyte and irreversibly damage
the sensor.
DISCLAIMER:
The information and electronic circuitry in this document are provided for
the purpose of evaluation and testing of electrolytic tilt sensors produced by
Advanced Orientation Systems, Inc
( AOSI ). AOSI
shall not assume any warranty or responsibility for any claim of any kind in the
event of use or application of the information described in this document.
A task to design electronic
circuitry to properly drive electrolytic tilt sensors could become an
unpredictable challenge even to the most experienced hardware design engineer.
The design engineer should not only concentrate on a task of precise
signal timing, but also to control and eliminate the DC current from flowing
through the tilt sensor. This unwanted current could be a “KISS
of DEATH” to the whole angle
measurement stage. The other concern is control of the charge that is being
delivered to the sensor. It is not enough to generate a periodic waveform and
drive the sensor. The waveform should be maintained at 50% duty cycle at all
times. In the event that the waveform is not symmetrical, the sensor may
accumulate charge and output some electrical value which could be mistaken for
actual tilt. The following electronic diagram ( FIG 1 ) demonstrates a simple
way to excite the single axis tilt sensor by using a standard compactor as an
oscillator. Diodes perform the function of a demodulator. The DC voltage output
proportional to tilt allows users easy low cost testing and utilization of all
types of single axis tilt sensors. C1 should be a tantalum capacitor.
When using Polymer dual axis sensors
with the following circuit (FIG-1) please note the following:
1.
Select diagonal 3 pins ( out of two diagonals )
2.
Number them sequentially ( 1=left, 2=center, 3=right )
3.
Connect as shown on the schematic below (FIG 1) instead of TX1.
4.
Hold the sensor perpendicular and tilt in the plane of the selected
diagonal.
FIG-1
When starting to design a circuit to
utilize dual axis electrolytic tilt sensors it is practical to start from the
alternating axis excitation technique. In
this method each sensing axis is excited for a fraction of a second just long
enough to produce a stable output for
the currently excited axis. The circuit in FIG-1 could be used as a global
driver while having the two external pins of each diagonal switched in and out
periodically. It is very important to synchronize the select signal with the
proper axis being excited at the same time. This is crucial for proper axis
separation.
The block diagram
( FIG-2 ) shown below is a general proposed method for the excitation of
any dual axis tilt sensor. As stated above, this method is based on alternating
excitation of both axis by the same waveform generator or clock. Precise
synchronization with the output stage will allow proper separation of outputs
for both axis. The speed of switching between tilt axis will define the update
rate of the output signal. Suggested
switching rate 10 - 100 Hz. Two SX
(single axis) sensors, when center terminals are joined, could be excited using
the same principle. In this case sensors could be of different types and provide
different range and scale factor characteristics for each axis. In order to
maintain stable output, the Vcc should be well regulated. In the event of an
unstable power source, the user may experience drift at null and unpredictable
change of scale factor ( Vdc/arcdeg ). When using Polymer dual axis sensors and
the excitation technique as per FIG-2, the tilt direction for each axis will
correspond to the plane of each diagonal crossing at the center pin. For
applications requiring temperature compensation, a simple feedback loop can be
added at the last output stage for
each axis giving independent control for thermal compensation in case two
different single axis sensors are used. If a Polymer dual axis sensor is
selected for that application, the temperature correction stage should be added
at the first signal processing stage before axis separation occurs. Please note
that the scale factor thermal dependency of AOSI electrolytic tilt sensors is
about (neg0.08% / deg C). Standard
LM-50 type temperature sensors are perfectly suited to be used in the full
thermal operational range.
FIG-2
It is important to note that
when using alternating axis excitation technique the designer should
precisely control the number of positive and negative pulses delivered to each
axis. In case the number of pulses are not equal, the sensor will accumulate
electrical charge which may result in permanent damage to the sensor and
inaccurate tilt measurement.
This method is considered reliable
for applications requiring low to medium accuracy tilt measurements. For
applications where high accuracy is required other avenues of signal conversion
are available, and are contingent on individual applications.
Some methods suggest exciting
the sensor with multiple oscillators of common base, thus creating timing zones
where voltage levels on various pins make the dual axis sensor behave as a
single axis sensor. This method, if properly developed, will result in a very
precise angle conversion module for any dual axis electrolytic tilt sensor based
on a common pool of electrolyte.
The following block diagram FIG-3
demonstrates a custom SINGLE IC
design for a dual axis conversion module. Not only that this module
excites both axis of tilt, but it also outputs data in a serial format
compatible with RS-232 format in 8 bit resolution. The unit operates at 9600 or
19200 baud rate.
FIG-3
All AOSI electronic modules do
not utilize diagonal tilting of the sensor, but operate when the sensor is
tilted in a plane 45o off the diagonal as indicated by the sketch below:
Stars represent pins, and X
/ Y represents the direction of tilt.
*
*
*
a axis
*
*
i
Y axis
For any additional technical
information please contact Advanced Orientation Systems Inc by phone, fax or
E-Mail.
To see the on-line catalog please click on the following link:
"click here: ---> " http://www.aositilt.com
 |
ADVANCED ORIENTATION SYSTEMS, Inc. 2525 Brunswick Ave, Suite 205 LINDEN, NJ 07036, USA. Phone: 908-474-9595 Fax: 908-474-9090 sales@aositilt.com http://www.aositilt.com |