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CS4172

Single Air-Core Gauge Driver

ON Semiconductor 

Applications Information

Theory of Operation

The SACD is for interfacing between a microcontroller or

microprocessor and air-core meter movements commonly

used in automotive vehicles for speedometers and

tachometers. These movements are built using 2 coils

placed at a 90° orientation to each other. A magnetized

disc floats in the middle of the coils and responds to the

magnetic field generated by each coil. The disc has a shaft

attached to it that protrudes out of the assembly. A point-

er indicator is attached to this shaft and in conjunction

with a separate printed scale displays the vehicle’s speed

or the engine’s speed.

The disc (and pointer) respond to the vector sum of the

voltages applied to the coils. Ideally, this relationship fol-

sine

lows a cosine equation. Since this is a transcendental and

non-linear function, devices of this type use an approxi-

mation for this relationship. The SACD uses a tangential

algorithm as shown in Figure 1. Only 1 output varies in

any 45 degree range.

Quadrant II

[ ] θ = 180° – Tan−1 VSIN+ – VSIN−

VCOS+ – VCOS−

For θ = 90.176° to 134.824°:

VSIN = 0.748 × VBB

VCOS = −Tan(θ – 90°) × 0.748 × VBB

For θ = 135.176° to 179.824°:

VSIN = Tan(180° – θ) × 0.748 × VBB

VCOS = −0.748 × VBB

Quadrant III

[ ] θ = 180° + Tan−1 VSIN+ – VSIN−

VCOS+ – VCOS−

For θ = 180.176° to 224.824°:

VSIN = −Tan(θ – 180°) × 0.748 × VBB

VCOS = −0.748 × VBB

SIN+

Output

0°

Max (128)

Min (0)

SIN–

Output

Max (128)

Min (0)

Degrees of Rotation

45°

90°

135°

180° 225° 270°

315°

360°

COS+

Output

Max (128)

Min (0)

COS–

Output

Max (128)

Min (0)

000

001

010

011

100 101

110

111

000

MUX bits (D9 – D7)

Figure 1. Major gauge outputs.

Quadrant I

[ ] θ = Tan−1

VSIN+ – VSIN−

VCOS+ – VCOS−

For θ = 0.176° to 44.824°:

VSIN = Tanθ × 0.748 × VBB

VCOS = 0.748 × VBB

For θ = 45.176° to 89.824°:

VSIN = 0.748 × VBB

VCOS = Tan(90° – θ) × 0.748 × VBB

For θ = 225.176° – 269.824°:

VSIN = −0.748 × VBB

VCOS = −Tan(270° – θ) × 0.748 × VBB

Quadrant IV

[ ] θ = 360° – Tan−1 VSIN+ – VSIN−

VCOS+ – VCOS−

For θ = 270.176° to 314.824°:

VSIN = −0.748 × VBB

VCOS = Tan(θ – 270°) × 0.748 × VBB

For θ = 315.176° – 359.824°:

VSIN = −Tan(360° – θ) × 0.748 × VBB

VCOS = 0.748 × VBB

270°

VSIN–

VCOS+

360/0°

0.748VBB

θ

IV

I

0.748VBB

0.748VBB

III

II

0.748VBB

180°

VCOS−

Graph 1. Major gauge response.

4

90°

VSIN+

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