Written by Hans-Ulrich Meyer
Sylvac inductive system
● Requirements
● Incremental sensors
● Inductive vs. capacitive
● Low power inductive sensor
● Zigzag coils and ladder scale
● Scanning measurement
● Printed circuit coil layout
● Integrated circuit
Requirements
Must meet the following requirements:
● Low power consumption (coin cells)
● Low cost (essential for digital calipers)
● Reliability (no wear, insensitive to contaminants)
● 20 μm accuracy over a 150 mm range (digital caliper) or 2 μm over 25 mm (dial indicator /micrometer),only an incremental sensor, subdivided into smaller, easier to measure increments, meets these requirements.
“Generic” incremental sensor
To precisely find the scale position within a pitch (T), detector signals as a function of x should be uniform, evenly spaced and preferably sines (or triangles) of period T. Displacement exceeding T is measured by tracking the detector signals.
Incremental sensor technologies currently used in small tools:
| technologies | main + | main - |
| Optical | accurate (<1μm) | high battery drain |
| Magnetic | writable scale | mediocre linearity |
| Capacitive | linear (0.2% of T) | influenced by water |
| Inductive | linear (0.2% of T) | capacitive transients |
Capacitive and inductive sensors: simple, cheap and very linear
Only conductors needed to generate and detect fields : coupling along x can be tailored by their shape to achieve a high linearity. For a larger gap (h), coupling (Δv/ΔV) decreases, but becomes more sinusoidal and may further improve linearity within a pitch T.
Sylvac capacitive and inductive circuits for digital calipers
<= Capacitive circuit (1983)
0.2% linear over scale pitch (T=5.08 mm,not shown) Long electrode pattern for more accuracy: 6x8 driving electrodes (0.635mm pitch) and one sensing electrode.
<= Inductive circuit (2003)
0.3% linear over scale pitch (T=1.524 mm, not shown) Long flat zigzag coils for more accuracy: 3 interlaced driving coils and 3 interlaced sensing coils on two layers (coil-to-coil pitch 0.508mm).
Inductive sensor wins by K.O.
● Capacitive:
Simplest design, lowest power consumption.
Achilles' heel: extremely sensitive to water, which strongly distorts electrical fields (Єr=80)
● Inductive:
More complex (2 layers for coils, avoid capacitive coupling).
Slightly higher power due to coil current.
But: not affected by water and most other contaminants, which do not distort magnetic fields (μr=1).