Issue
132 July 2001
Inductive Sensors
by
George Novacek
Start
•
LVDT Revisited • The
Works • Electrical
Interfaces • Mechanical
Interfaces • The
Future • Sources &
PDF
THE
WORKS
The CSEM sensor,
shown in Figure 1, is deceptively simple. Packaged on
a small PCB, the sensor module is merely 0.35² × 1.18²
(9 mm × 30 mm) and weighs approximately 0.7 grams (0.025
oz). The sensor, which is encapsulated for protection,
consists of two individual silicon chips on a single carrier
(see Photos 1 and 2).
 |
| Figure
1—The
CSEM sensor diagram is simple. An oscillator drives
the excitation coil while the position signal is
picked up and decoded by four series-parallel sensor
coils. |
One chip holds
the micro-machined coil assembly with five coils and
the other holds the interface electronics. An AC current
excites the large octagonal coil. The excitation frequency
isn’t externally adjustable and, therefore, isn’t
specified. Judging by the overall operation of the sensor,
I estimate it to be in the 200- to 500-kHz range. Its
purpose is to generate a magnetic field, which is amplitude-modulated
by the movement of a structured metallic target, typically
a gear or slot. Four sensing coils are connected in
differential pairs to pick up the modulated field and
feed the resulting signal to a set of amplifiers and
amplitude demodulators in the second chip.
Because of the
use of differential signal pickup and processing, the
sensor is nearly independent of temperature and target
distance shifts. The amplitude demodulators, followed
by low-pass filters to attenuate the excitation frequency
remaining in the signal, produce sine and cosine outputs
representing the target’s teeth movement. The sinusoidal
signals are squared in the following voltage comparators
to generate two trains of pulses; the pulses are 90° phase
shifted because of the positioning of the pickup coils.
Their duty cycle is affected by the geometry of the target
teeth, but is usually kept to 50% by the teeth design.
Although the sensor does not achieve the admirable resolution
and precision of its variable differential transformer
cousin, it consistently delivers a respectable 12 bits,
adequate in most applications.
