What is an inductive encoder?

Rotary encoders based on inductive technology with sine/cosine (Sin/Cos) analogue output interface represent an advanced solution in the field of angular position sensing. These devices have been developed to overcome the limitations of fragility and sensitivity to external agents typical of optical encoders.

Operating principle

The operation of an inductive encoder is based on the physical principle of eddy currents.

• Structure and components: the encoder mainly consists of two elements: the stator (fixed, called sensor board), which contains the excitation electronics for the transmitting coil (implemented as a copper track on a printed circuit board (PCB)) and the receiving coils (also implemented as tracks on a PCB), and the rotor (mobile, called target board), which is a disc (often made of copper or aluminium) with a geometric subdivision depending on the resolution of the product. There are no mechanical contacts or delicate optical discs.
• Field generation and position detection: the transmitting coil generates an alternating electromagnetic field, which in turn induces secondary voltages in the two receiving coils. The amplitude and phase of these induced voltages vary proportionally to the position of the target above the coils. By demodulating and processing these secondary voltages, the electronics on board the stator acquire a precise signal representing the exact position of the target.

• Output signal: position information is used by the electronics in the stator to produce output signals that vary sinusoidally/cosinusoidally in relation to the angle.

Stator example (sensor board)	Rotor example (target board)

Analogue sine/cosine interface

The output of the inductive encoder consists of two phase-quadrature sinusoidal analogue signals: VSen and VCos.

• Quadrature: The two signals are phase-shifted by exactly 90° electrical degrees. This phase shift is essential because it ensures that, for every angle θ, one signal is at its maximum variation (slope), while the other is at zero or at its maximum value.

  VSen​∝A⋅sin(P⋅θ)

  VCos​∝A⋅cos(P⋅θ)

  Where A is the amplitude, P is the number of electrical periods per revolution (determined by the rotor pattern), and θ is the mechanical angle.

Sine/Cosine signals (differential)

The use of analogue sinusoidal signals is a prerequisite for electronic interpolation. The receiving circuit (often an external ASIC interpolator or one integrated into the drive) samples the Sin and Cos signals and uses the arctangent function to calculate the angle with high precision within a single electrical period:

• θelectric​=arctan(VSen​/VCos​)

This process allows you to obtain the equivalent of thousands or hundreds of thousands of digital pulses per revolution starting from a low-frequency analogue signal, simulating an encoder with a number of lines much higher than what can be achieved digitally at the same speed. Unlike high-resolution digital encoders, which generate signal frequencies in the MHz range at high speeds (risking signal integrity issues over long distances), the base frequency of Sin/Cos signals remains relatively low (usually in the kHz range), which facilitates signal transmission and processing.

Advantages and applications

Sin/Cos inductive technology is preferred for its robustness and reliability in harsh environments. The main advantages are:

• Environmental immunity: The operating principle is not affected by common contaminants such as dust, dirt, grease, moisture, oils and condensation. The sensor board can be further protected (e.g. with conforma coating) while the metal disc (rotor) is inherently robust, making these encoders ideal for applications in heavy robotics, machine tools (CNC), offshore installations and the wind power sector.
• Mounting tolerances: the coupling between the stator and rotor can have wider tolerances in terms of axial and radial misalignment than optical encoders, simplifying installation and reducing precision assembly costs.
• Feedback for servo motors: these are the standard choice for position and speed feedback in modern servo motors. The quality and smoothness of the Sin/Cos signals enable extremely precise torque and speed control, improving the dynamic performance of the motor.
• Continuous diagnostics: monitoring the trigonometric relationship VSen2 + VCos2 = constant (known as the Lissajous circle) enables real-time diagnostics of signal integrity, which is essential for safety applications.