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The Hall-Effect Sensor and the Rise of Electric Power-Assisted Steering

April 20, 2020 by Steve Arar

What can Melexis' new hall-effect sensor teach us about non-contact torque sensors and electric power-assisted steering in general?

The MLX91377 is a recently-released linear hall-effect sensor IC from Melexis.

In this article, we’ll look at one of the main use cases of the MLX91377—steering torque measurement. We’ll see that by using a reliable hall-effect sensor, you can take accurate and robust torque measurement—of paramount importance in electric power-assisted steering (EPAS) systems. Finally, we’ll briefly look at some of the important features of the MLX91377. 

 

A Brief History on Hydraulic Power-Assisted Steering

When you drive a fully manual steering system, you might struggle to turn the steering wheel at times, especially when you're driving a large vehicle at a low speed. To address this issue, car manufacturers introduced the first hydraulic power-assisted steering (HPAS) system in the 1950s. The basic idea of this system is shown below. 

 

HPAS system

Figure 1. Simplified architecture of an HPAS system. Image (modified) used courtesy of Global Auto Solutions

 

This system uses high-pressure fluid, provided by the power steering pump, to assist the driver with steering. The fluid can be conducted to either the right side or the left side of the hydraulic piston through the fluid lines.

When the driver rotates the steering wheel clockwise or counterclockwise, the pressurized fluid gets applied to the appropriate side of the hydraulic piston. In this way, the piston (and consequently) the rack is moved in the desired direction without excessive physical effort from the driver.

Although the hydraulic power-assisted steering allows the driver to easily steer the vehicle, it has several shortcomings. A major problem with this system is its low power inefficiency since the engine has to continuously drive the power steering pump even when the steering system is not used. To overcome the limitations of the HPAS, automotive engineers decided to redesign the system to use the power from an electric motor. 

 

The Rise of Electric Power-Assisted Steering

Electric power-assisted steering (EPAS) uses an electric motor rather than a hydraulic system to assist the driver with rotating the steering wheel. The basic idea is illustrated here. 

 

Simplified architecture of an EPAS system. 

Figure 2. Simplified architecture of an EPAS system. Image used courtesy of DCE Motorsport

 

Since the electric motor will be turned on only when the steering system is used, an EPAS system can be more power-efficient than an HPAS system. To control the motor, we should be able to calculate the required assisting power. An important parameter that determines the required assisting power is the driver input—that is, the torque that the driver applies to the steering wheel.

 

Non-Contact Torque Sensor

Here, you'll see a contactless torque-sensing mechanism based on a hall-effect sensor.

 

A contactless torque-sensing mechanism based on a hall effect sensor

Figure 3. A contactless torque-sensing mechanism based on a hall-effect sensor. Image used courtesy of W.J. Fleming, Semantic Scholar

 

It consists of:

  • Two co-rotating stator rings made of ferromagnetic material. The stator rings are fixed to the input end of a torsion bar that is connected to the steering wheel column. We’ll explain in a minute what a torsion bar is.
  • An encoder ring with alternating magnetic north and south poles. This encoder ring is connected to the output end of the torsion bar.
  • One or two (for redundancy) hall-effect sensors fixed to the stator rings.  

Before explaining the operation of this torque sensor, we need to learn about the operation of a torsion bar.

A torsion bar is basically a metal bar with spring-like behavior. If we fix one end of the torsion bar and apply a torque to the other end, it will twist by an angle θ, which is a function of the bar's length and stiffness. The torsion bar wants to resist the twisting effect and will return to its normal state when the torque is removed. The following figure shows how a torsion bar twists in response to the applied torque.   

 

how a torsion bar twists in response to the applied torque

Figure 4. How a torsion bar twists in response to the applied torque

 

Now, let’s see how this torque sensor above works. The key point is that the stator rings are connected to one end of the torsion bar and the encoder ring is connected to the other end. As the driver rotates the steering wheel, the torsion bar twists slightly and the encoder ring angularly moves with respect to the stator rings.

As the position of the encoder ring changes with respect to the stator rings, the magnetic flux sensed by the hall-effect sensor changes accordingly. This allows us to generate an electrical signal that is related to the torque that the driver applies.

For more information about this torque-sensing mechanism, please refer to a research paper out of Moving Magnet Technologies S.A. titled “Development of a Contactless Hall Effect Torque Sensor for Electric Power Steering.”

 

Melexis' New Linear Hall-Effect Sensor IC

The  MLX91377 consists of a hall-effect magnetic frontend, an analog-to-digital converter, and some built-in digital signal processing (DSP) blocks. The device measures the magnetic flux density that is perpendicular to the IC and outputs the information in different modes, namely analog, short PWM code (SPC), and single-edge nibble transmission (SENT) modes.

 

Block diagram of MLX91377

Figure 5. Block diagram of MLX91377. Image used courtesy of Melexis

 

The built-in signal processing resources allow the device to provide high linearity and thermal stability. The new device supports a wide operating temperature up to 160°C. It employs a dual-die, fully-redundant design that makes it suitable for safety-critical automotive applications.

The MLX91377 can be used to measure the magnetic flux variations in the above torque sensing mechanism. Now that we are familiar with the basics of using a hall-effect sensor to build a non-contact torque sensor, let’s take a look at some of the important characteristics of the MLX91377.

 

DSP Chain of the MLX91377

The figure below depicts the DSP chain of the new sensor. 

 

DSP process chain from ADC to MLX91377's output.

Figure 6. DSP process chain from ADC to MLX91377's output. Image used courtesy of Melexis

 

Several parameters of these blocks are programmable. This allows the user to adjust the transfer function of the sensor and compensate for the errors caused by the magnetic and mechanical construction of the design. 

For example, the user can adjust the “offset” value to place the quiescent output value of the sensor at any point on the transfer function. 

 

Default OFFSET positioning

Figure 7. Default OFFSET positioning. Image used courtesy of Melexis

 

To compensate for the non-idealities you may encounter in practice, the MLX91377 allows the user to adjust the transfer function from the node Bz_DSP in Figure 6 to the sensor output. This can significantly increase the linearity of the sensor for a given application. The programmable parameters for the four-point linearization of the MLX91377 are shown below.

 

Description of the 4-pts linearization parameters.

Figure 8. Description of the 4-point linearization parameters. Image used courtesy of Melexis

 

As you can see, seven segments of the above transfer curve can be programmed by the user. The first and the last segments are necessarily flat. For a more demanding application, we can use an 8-point, 17-point, or a 33-point transfer function. The 33-point transfer function is shown below.

 

Description of the 33-point linearization parameters

Figure 9. Description of the 33-point linearization parameters. Image used courtesy of Melexis

 

The features discussed above are only some of the programmable parameters of the MLX91377. The user can adjust several other aspects of the design. For example, the user can trim the sensitivity drift of the system to achieve higher thermal stability. 

 

Summary

In this article, we examined one of the main use cases of the MLX91377: the torque measurement for an EPAS system. We also briefly looked at some of the important features of this sensor.

The MLX91377 can implement a programmable transfer function to increase the linearity of the system. This allows the designer to compensate for the non-idealities that can arise from the magnetic and mechanical construction of the design. Moreover, the device can be programmed to correct the thermal drift of the sensor gain. 

 

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