Working principle

The inductive position sensor is in fact a differential transformer, in which the coupling (mutual inductance) between primary and secondary coils is modulated by a target (e.g. the teeth of a gear). The figure below shows an oscillator followed by an amplifier that injects a high-frequency current into the primary coil. The current injected in the primary winding creates a time-varying magnetic field, which is coupled into the secondary coils. The coupling between primary and secondary coils is locally perturbed by the presence of a metallic object, like the tooth of a gear, the opening in a metal disk etc.

The coupling between the primary and secondary coils is illustrated by means of the FEM simulations in the figure below. The red lines represent the magnetic field lines, the target is made of ferromagnetic material and therefore attracts the magnetic fieldlines. Without target, the magnetic field through the two detection coils is symmetrical and the differential signal is zero. In position 1, the left tooth draws the magnetic field to the left and creates a positive signal. In position 2, the right tooth draws the magnetic field to the right and creates a negative signal (same amplitude, but opposite sign compared to Position 1).

The differential signal measured with the detection coils is modulated with the carrier frequency of the oscillator. After amplification, the detection coil signal is synchronously demodulated using the carrier signal of the oscillator. After low-pass filtering, the analog signal is available at the output. When the analog signal is passed through a comparator with hysteresis, an A quad B signal is obtained, which is often used for tooth counting applications. Although not drawn in the schematic above, the analog sine and cosine signals can be interpolated in order to output a high-resolution angle signal. This can be in the form of an A quad B signal, a serial interface (SPI), a PWM signal or an analog ratiometric signal.

The POSIC differential inductive sensor is extremely insensitive to perturbations by external sources. This is because of two reasons:

Firstly the differential operation within one very small chip. It is nearly impossible to create a magnetic, electromagnetic, thermal or other perturbation between the two detection coils. The electromagnetic waves at the carrier frequency have a wavelength that is much longer than the distance between the two coils, so no perturbation in the relevant frequency range can be created.

Secondly the modulation frequency. The carrier frequency supplied to the primary coil and measured back via the secondary detection coils is synchronously demodulated. Mechatronic systems often have many sources of noise and interference, typically in the range of zero to several kHz. As the carrier frequency of the inductive sensor is around 1 MHz, the perturbations are not in the same range as the carrier frequency and any introduced perturbations are filtered out during demodulation.

The most important advantages of the differential inductive sensing mechanism are:

• Non-contact sensing mechanism
• No bias magnet (iron dust/particles not attracted)
• Very thin sensor down to 0.6 mm (mounted on a flex carrier)
• Very thin target disk 0.2 mm
• Insensitive to magnetic disturbances
• Wide temperature range (-40 to +125°C)
• Robust against fluids, particles, dust, oil, grease etc.
• Linear and rotational applications
• Simple low-cost target (stamped)
• Wide variety of target materials (steel, aluminum, brass, copper)
• High frequency range up to 40 kHz
• True zero-speed operation

Targets

The overall performance of a position sensor system depends not only on the sensor element, but also on the target that is sensed by the sensor. A target consists of a material that has a certain form and dimensions. These three factors material, form and dimensions have an important influence on the sensor performance in terms of resolution, sensor-target distance, linearity, repeatability, speed etc.

Fabrication

POSIC’s sensors are very compact and suitable for high-volume production because the sensor coils and the associated electronics are all integrated in a single silicon chip. The structure of a sensor can be observed in the figure to the right: the large round structures are the primary coils and the smaller round structures inside the primary coils are the detection coils. These have multiple colors because of optical interference (the pitch of the windings is in the same range as the wavelength of visible light)

Customization of sensor and target

POSIC’s sensors are applied in many different mechatronic applications in a variety of market segments. Each application has its own technical and commercial requirements and no standard sensor exists that fulfills all application requirements. As we are confronted with a plurality of requirements for different applications, we offer our customers the possibility to customize sensor and/or target to their specific application. Examples:

• Sensor assembly support: PCB of 0.4 – 1.6 mm thickness, flex material or ceramic
• Specific interfaces: A quad B and I, BLDC motor commutation, SPI, PWM, analog ratiometric, analog sin/cos
• Mechanical alignment: holes for alignment pins, visual alignment patterns, positioning against one or two sensor-sides, front- or backside reference for target distance
• Rotational targets: radial and axial readout of gears, cogwheels, slotted disks, end-of-shaft targets, etc
• Linear targets: slotted metal bands fabricated by etching or stamping

POSIC offers the following services for sensor/target customization:

• FEM simulation of magnetic interaction between sensor and target, taking into account sensor settings, target material, target form/dimensions
• Material identification and mechanical design of target
• Mechanical and electrical design of sensor assembly on PCB, flex or ceramic
• Prototype fabrication of sensor and target
• Qualification, industrialization and volume production of custom-developed sensors and targets