Dieter Verstreken, Global Marketing Manager Position Sensing, Melexis
Over the past three to four decades, formerly mechanically actuated and regulated systems in the automotive world have been replaced with electrical and electronic equivalents that deliver numerous advantages.
In addition to saving weight and bill-of-materials costs, the electrical systems are typically smaller and have fewer moving parts which aids reliability. Also, with computerised control, they can be engineered to provide differing responses to driver inputs depending on a variety of parameters. A key trend has been the adoption of x-by-wire systems - which use electrical or electro-mechanical systems for performing vehicle functions that would have previously been achieved by mechanical linkages. Coordinated electronic control of x-by-wire systems (such as those associated with steering, braking, and acceleration) is a key enabler for higher safety, comfort and reliability.
Ultimately, electric drives are replacing the traditional powertrains based on internal combustion engines (ICEs). While powering the transition to clean mobility, this new technology also enables vehicle brands to deliver transformative experiences for customers and differentiate their products in an increasingly competitive marketplace.
Sensing the position of driver-actuated controls is a critical part of the human-to-vehicle interface in today’s highly electrified vehicles (as illustrated in Figure 1). The steering angle, and position of gear-selector, accelerator and brake pedals provide the main indication of the driver’s wishes at any time.
Continuously sensing the position of various mechanisms is also critical for closing the loop in many automatic control systems. These include the positions of valves and flaps. The information is needed to adjust and optimise settings and to confirm that requested actions have taken place. Additionally, electronic sensors offer diagnostics and reporting capabilities of the system’s operational status.
Historically, the potentiometer has been the go-to electrical sensor for position detection. However, there are some challenges. The basic operating principle depends on a wiper moving along a resistive path, such as a coil or carbon track. Its lifetime is limited by wear and there is the possibility for breakage leading to early failure. Potentiometers can also be vulnerable to electrical noise, as well as corrosion and contamination, which can impair correct operation.
Non-Contact magnetic position sensing
Contactless angular-position detection with magnetic sensors, leveraging the Hall effect, eliminates wear factors and is also largely immune to electrical noise, contamination, and corrosion. Standard Hall sensors are sensitive to magnetic flux in a single plane. Melexis Triaxis® sensors measure three components of magnetic flux using a single IC.
The Triaxis Hall sensors intended for automotive applications integrate important functionality like analogue signal conditioning, data conversion, digital signal processing (DSP) and signal driving. The MLX90421 devices (shown in Figure 2) and related sensors provide both analogue and digital outputs. Included in Melexis’ ASIL-ready portfolio, they are qualified to ISO26262 ASIL-B function safety - as a safety element out of context (SEooC).
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Figure 2: Melexis’ Triaxis Hall sensors integrate a complete signal chain for robust magnetic position sensing
The sensors can operate at temperatures of up to 160°C, allowing them to be deployed in harsh environments such as engine compartments and in start-stop applications where heat soak is frequently a challenge.
Stray Field Immunity
In addition to temperature and tolerances, magnetic stray fields are another phenomenon that can affect measurement accuracy with Hall sensors. Disturbances can be particularly strong close to electrified systems that contain motors. Hence, given the current momentum towards full electric drivetrains, it is important to find a solution. The areas around high-voltage cables that conduct heavy current between the battery, power inverter, and motor are critical locations.
The induced magnetic field around a wire reduces linearly with distance, according to the relation 1/r. If the cable carries a large current, say 400A, which is a realistic value for an electric traction system, the field strength can be as much as 0.3mT at a distance r = 30cm from the cable. This is enough to induce a significant measurement error in a conventional Hall sensor. By measuring in-plane magnetic flux density gradients, gradiometric sensors minimise the effects of uniform external magnetic fields. Melexis’ Triaxis Hall sensors can reject stray magnetic fields up to 5mT or 4000A/m in rotary and linear position-sensing applications.
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Figure 3. The ISO 26262 programmable solutions offer the flexibility to address both linear and rotary movements
High-speed position sensing with inductive technology
With the advent of electric power steering (EPS) systems and, more importantly, electric powertrain applications comprising e-brake booster, e-axle, and e-motor systems, there is an emerging need for high-speed angular position sensing to control motor drives. Brushless motors, which require forced commutation, are preferred for their reliability, quietness and absence of brush wear or arcing.
Forced commutation calls for measuring the angular position of the rotor to coordinate the delivery of current to each of the stator windings, which provides control over torque and speed. Proper coordination is also critical for optimum efficiency. Inductive resolver sensors are the preferred solution, due to their high accuracy and natural immunity to stray fields. The most common type is the variable reluctance (VR) resolver, which consists of a ferromagnetic rotor and a stator with several secondary coils. Although these have been used for many years in the automotive industry, they tend to be heavy and expensive. These are unwelcome characteristics in modern automobile design, where affordable electric vehicles (EVs) and greater driving range per battery charge are being expected. Lower cost, more lightweight solutions are needed.
Melexis has created a new type of inductive position sensor to address this need. It combines a 3-phase coil set composed of transmitting (Tx) and receiving (Rx) coils, a mixed-signal interface chip, and a metallic target attached to the rotor to be sensed. The 3-phase coil set is lightweight and eliminates harmonic effects that typically impair the accuracy of a conventional quadrature coil arrangement. The MLX90510 mixed-signal sensor interface IC contains the Tx coil oscillator, Rx analogue input filters, plus digital circuitry for control and signal processing. With its strong EMC capabilities, this interface IC meets the requirements of e-motor control, e-brake booster and e-power steering applications.
The MLX90510’s digital signal processing chain includes specially designed linearization to mitigate errors and generate differential sine/cosine output signals. These are communicated to the ECU, which simply calculates the rotor angle using an arctangent operation - thus minimizing the effort required to achieve high accuracy under extreme mechanical and electrical conditions. With diagnostics and protection features also included, requiring no external safety measures from the ECU, the MLX90510 is easy to use in systems that must be qualified up to ASIL-C (in accordance with the ISO 26262 automotive functional safety standard).
Magnetic sensors provide accurate and reliable angular position sensing for automotive applications, leveraging enhancements such as gradiometric sensing to withstand the challenges encountered in today’s highly electrified vehicles. Moving towards full electrification raises demand for angular sensing in brushless motor drives at high rotational speeds, to control large subsystems such as EPS and the main traction drive. While resolver sensors provide the most suitable solution here, the latest sensors that contain new and innovative technology are compact, lightweight, accurate, reliable, stable and easier to use.