New EV Inductive Resolver IC Claims to Be “Immune to Stray Magnetic Fields”

When designing electric vehicles (EVs), range is amongst the top priorities for engineers. While many factors affect range, a crucial factor is the efficiency (i.e., electrical to kinetic) that the vehicle is operating. 

 

Some basic factors that affect EV range. Image used courtesy of Charged Future

 

When it comes to the efficiency of an electric motor, positional sensing of the rotor relative to the stator can be of the utmost importance as it allows for synchronization of the stator supply current with the rotor position. There are different techniques to achieve this, one of which is the inductive resolver. 

Hoping to help improve resolver design, Melexis recently released its newest inductive resolver IC, which they claim is inherently immune to stray magnetic fields. 

In this article, let's talk about inductive resolver technology and then take a closer look at Melexis' newest release. 

 

What is an Inductive Resolver? 

An inductive resolver (IR) is a type of magnetic sensor used to determine the position of an electric motor's rotor at high speeds. 

This tool offers many advantages over rival position sensors, such as a variable reluctance resolver or a magnetic resolver.

 

Comparison of resolver technologies. Image used courtesy of Melexis

 

First off, IRs offer incredibly high linearity, allowing for highly accurate angle measurements. 

On top of this, the working mechanism of inductive resolvers results in them having extremely high immunity to stray magnetic fields and other noise. These two facts allow inductive resolvers to achieve positional accuracy up to 0.2 degrees. 

Beyond this, inductive resolvers are highly robust and safe, having achieved ASIL D classification. Considering that IRs are relatively cheap and small, it is clear why IRs could be an excellent choice in EVs. 

Now that what inductive resolvers have been briefly covered, let's get a general understanding of how they work.

 

How an Inductive Resolver Works 

In practice, inductive resolvers consist of four main components: 

• A coil system
• A target
• A signal processing sensor IC
• A PCB

 

The working components of an inductive resolver. Image used courtesy of Melexis

 

The system works by placing a metallic target attached to the motor's rotor in front of a set of inductive coils. 

The target is designed to have an equal amount of lobes as pole pairs of the motor. 

The coil set consists of transmission and receiving coils fixed to the motor stator and are generally ingrained in the physical PCB. The coils are then connected to the signal processing IC.

The IC will first drive the transmission coil to induce a magnetic field that causes the target to rotate together with the rotor of the motor. 

This excited energy from the transmission coils induces eddy currents in the target, which effectively reflects the excited magnetic field back to the receiving coils. 

The receiving coils then generate a 3-phase AC output signal. The IC then converts the signal into differential sinusoidal signals, which it sends to the system's electronic control unit to calculate the angle of the rotor relative to the stator. 

 

Melexis’ New IC: The MLX90510

Recently, Melexis introduced its first open market inductive sensor IC, the MLX90510. 

The MLX90510 was designed for high accuracy in high-speed sensing applications such as electric motors and is said to achieve an accuracy of <+/-0.36° at up to 240,000 RPM. 

The IC utilizes Melexi's patented loop technology to output differential sine and cosine signals with an average system propagation delay of 0 ns, +/-120 ns. 

 

Block diagram of the MLX90510. Image [PDF download] used courtesy of Melexis

 

Melexis also states that the use of decoupling between input and output and the inherent nature of IRs allows the MLX90510 to achieve complete immunity to stray magnetic fields. 

The device has also been proven to be very robust, as it has achieved an ASIL-C classification with support for ASIL-D system-level integration. 

 

Closing Thoughts 

With a need for high-efficiency electric vehicles, it is essential to have a highly accurate understanding of the electric motor's rotor relative to the stator. 

The MLX90510 shows promise to enable ASIL-D compliant, highly accurate inductive resolver systems, ultimately allowing for more efficient EVs.

All in all, as the push for EVs keeps ramping up, we're sure to see better and more efficient EV components.

 

Featured image used courtesy of Melexis