A Design Analysis of Pairing Inductive Encoders With Flat Brushless Dc Motors

Various encoder solutions, including inductive, magnetic, and optical designs, are utilized in electric motors for optimal servo control. The diameter and length of an encoder are determined by the technology used, encoder resolution, and package style; typically, an encoder adds length to the motor while maintaining a diameter similar to or smaller than the actual motor diameter.

For flat or pancake-style BLDC motors, the encoder length may negatively impact the overall length of the motor-encoder assembly. For example, a standard encoder adds more overall length to a flat BLDC motor, as shown in Figure 1. To optimize the overall length of a flat brushless DC motor, an alternative technology like the inductive encoder is explored. This paper examines the principles of inductive technology and reviews the conceptual design of a flat motor with an inductive encoder.

WORKING PRINCIPLES OF INDUCTIVE ENCODERS

Inductive encoders are comprised of two major components: a stator (sensor) that includes the primary and secondary coils; and a rotor or target (code wheel).

The primary coil generates a high-frequency magnetic field, typically from 1 to 2 MHz, which then induces a voltage in the secondary coil according to Faraday’s Law. The amount of voltage generated in the secondary coil depends on how quickly the current in the primary coil changes, which is illustrated by the following equation: Vsec = dI(prim)/dt

This high operating frequency range also reduces system susceptibility to interference from external magnetic fields and electromagnetic sources operating within the lower frequency range (kHz), thereby enhancing the encoder’s reliability and performance.

To regulate the secondary coil signal, the rotor (or code wheel), which is made from a ferromagnetic material, interacts with the magnetic field from the primary coil to affect the current in the secondary coil, opposing the generated magnetic field. Depending on the position of the rotating target in the primary coil, the induced signal in the secondary coil varies:

•	When the target is placed at the center, the magnetic field experiences changes while retaining its symmetry, resulting in zero voltage in the secondary coil.
•	Positioning the target to the left or right disrupts the symmetry of the magnetic field, inducing a non-zero voltage in the secondary coil.

This signal can then be used as feedback from the flat BLDC motor and interpreted by the servo drive as velocity and position.

Two sets of differential coils are arranged to ensure the resulting sine and cosine waves are phase-shifted by 90 degrees, as shown in Figure 3. One set generates the sine wave for channel A, while the other set generates the cosine wave for channel B.

The sine and cosine signals are then converted into digital signals by an analog-to-digital converter (A to D). This conversion creates an interpolated digital signal output, providing more detailed position information beyond the physical resolution of the target code wheel. The A to D converter transforms the sinusoidal signals into digital signals for both channels A and B, maintaining a 90-degree phase shift. To enhance precision, interpolation algorithms analyze the frequency and amplitude of these signals to accurately determine positions between the physical increments of the code wheel.

The encoder resolution, indicated by pulses per revolution (PPR), depends on the code wheel's design. The interpolation technique enables the creation of very high encoder resolutions.

NOVEL CODE WHEEL EXPLAINED

The overall performance of an encoder is influenced not only by the sensor element, but by the design of the code wheel, which is a crucial component of inductive encoders. An electrically-conductive object that is essential for the encoder to produce an output, it typically consists of a circular ring-type printed circuit board (PCB) embedded with copper strips. These strips are precisely patterned to meet specific resolution requirements and accommodate the motor size. The exact positioning of these copper strips is critical, as it directly affects the accuracy and precision of the feedback information provided by the encoder.

We have designed three types of code wheels: straight, angular, and curved. The curved code wheel is optimal due to its increased and uniform copper surface area on each strip, which enhances the inductive encoder’s performance.

PROOF OF CONCEPT: 45ECF SLOTTED FLAT MOTOR AND INDUCTIVE ENCODER

A conceptual design of Portescap’s 45ECF slotted flat BLDC motor, paired with an inductive encoder, was developed to showcase the benefits of this pairing. The motor, driven by a standard BLDC driver using a six-step trapezoidal technique, featured Hall sensors spaced 120° apart, ensuring a consistent 50% duty cycle. Trapezoidal current waveforms for all phases confirmed smooth operation in both forward and reverse directions, meeting specified speed and current requirements.

When the inductive encoder was connected to a separate 5V source, the captured output signals demonstrate a 90° phase shift between Channel A and Channel B outputs, with a consistent 50% duty cycle across both channels. The mechanical error of the inductive encoder was within standard magnetic encoder specifications (less than 1.5°) and proved to be more accurate. Motor speed, calculated by the encoder output frequency, matched the motor specifications and remained consistent across different voltages and resolutions.

Given the above results, the study indicated that the inductive encoder is an optimal fit for a flat BLDC motor, providing improved accuracy, maintaining overall motor length, and enhancing performance.

BENEFITS OF INDUCTIVE ENCODERS FOR FLAT BLDC MOTORS

The primary benefit of an inductive encoder design is that both the encoder elements and the commutation hall sensor are situated on the same printed circuit board (PCB). The addition of the flat target code wheel with the encoder/Hall sensor PCB maintains the overall motor length compared to a standard encoder setup. Additionally, this configuration is suitable for robust environments, as it is completely integrated within the motor housing. Specific application examples benefiting from this pairing include medical devices and robotics, where flat BLDC motors paired with inductive encoders are ideal in maintaining a compact size and high accuracy. Other benefits include:

•	Resistance to Electromagnetic Interference: Inductive encoders are less affected by electromagnetic interference and external magnetic fields, leading to stable performance in electrically noisy environments.
•	Robustness: They are highly durable and resistant to environmental factors such as dust, dirt, and moisture, which makes them suitable for harsh industrial conditions.
•	Long Lifespan: The absence of physical contact in inductive encoding mechanisms results in minimal wear and tear, extending the operational life of the encoder.

CONSIDERATIONS FOR PAIRING INDUCTIVE ENCODERS WITH FLAT BLDC MOTORS

When pairing inductive encoders with flat BLDC motors, design engineers should consider several factors:

•	Encoder Resolution: Ensure that the code wheel design meets the required resolution for the application, as it depends on the diameter of the flat motor.
•	Environmental Conditions: Assess the operating environment for factors such as temperature, humidity, and potential electromagnetic interference.
•	Integration with Motor Design: Verify that the encoder and hall sensors can be seamlessly integrated onto the same PCB without affecting the overall flat motor dimensions.
•	Target Material and Design: Use appropriate ferromagnetic materials and design for the target to ensure optimal interaction with the magnetic field.

CONCLUSION

Integrating inductive encoders with flat brushless DC motors presents significant advantages, particularly in applications where size constraints and precision are critical. The combination of these technologies not only maintains a compact motor-encoder assembly but also enhances overall performance by reducing mechanical errors and providing highresolution feedback. The inductive encoder’s integration on the same PCB as the commutation hall sensors ensures a streamlined design, ideal for robust environments and demanding applications such as medical devices and robotics. The successful proof of concept demonstrates that this pairing optimizes motor length while delivering precise control, making it a compelling choice for advanced engineering solutions.

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