Introduction

Modern vehicles rely on Electric Power Steering (EPS) to deliver responsive, efficient, and comfortable steering. Unlike traditional hydraulic systems, EPS uses an electric motor controlled by electronic signals, which introduces both opportunities and challenges. One critical challenge is electrical noise—unwanted signals that can corrupt the precise control required for safe steering. Active filters have emerged as an essential technology to manage this noise, ensuring that EPS systems operate reliably under real-world conditions. As vehicles become more electrified and automated, the role of active filters in EPS continues to expand, making them a key enabler of next-generation steering performance.

How Electric Power Steering Systems Work

Core Components and Control Loop

An EPS system consists of a torque sensor, an electronic control unit (ECU), an electric motor, and a reduction gearbox. When the driver turns the steering wheel, the torque sensor measures the applied force and sends a signal to the ECU. The ECU calculates the required assist torque and commands the motor to deliver it. This closed-loop control relies on accurate sensor signals and clean PWM (pulse-width modulation) drive to the motor. Any noise injected into the sensor or control lines can cause the ECU to misinterpret driver intent, leading to torque ripple, oscillation, or even system instability.

Noise Sources in EPS

Three primary sources of electrical noise affect EPS: PWM switching from the motor drive, commutation from the brushless DC motor itself, and electromagnetic interference (EMI) from nearby high-current devices like the alternator or inverters. Additionally, the torque sensor's analog output is susceptible to conducted noise from the vehicle's 12V bus. Active filters are strategically placed to suppress these unwanted signals at critical points in the signal chain.

The Problem of Electrical Noise in EPS

Impact on Performance

Electrical noise degrades EPS performance in several measurable ways. Torque ripple causes a gritty or uneven feel at the steering wheel, reducing driver comfort. Position error leads to misalignment between driver input and actual wheel angle, especially noticeable during highway driving. In severe cases, noise can cause the controller to go unstable, resulting in oscillations or loss of assist. Automotive manufacturers must meet strict standards for noise immunity, such as those defined in SAE J551 and ISO 11452, making active filters a regulatory requirement as well as a performance enhancement.

Active Filters vs. Passive Filters

Both active and passive filters can suppress noise, but active filters offer distinct advantages in EPS applications. Passive filters use inductors, capacitors, and resistors; they are simple but bulky, and inductor cores can saturate at high currents, introducing nonlinearity. Active filters, built around operational amplifiers (op-amps) or digital signal processors, provide higher Q (quality factor), precise frequency selection, and tunability without large magnetic components. In the space-constrained environment of an EPS ECU, active filters are often the only viable solution for achieving high attenuation at specific frequencies while maintaining signal integrity. Check this application note from Texas Instruments for a comparison of filter types in automotive systems.

Types of Active Filters and Their Application in EPS

Low-Pass Filters

Low-pass active filters are the most common in EPS. They remove high-frequency PWM ripple (typically 20 kHz–50 kHz) from motor current sense signals and from the torque sensor output. A second-order Sallen-Key topology is widely used because it offers a good balance between roll-off steepness (40 dB/decade) and component tolerance sensitivity. Cutoff frequencies are usually set between 1 kHz and 10 kHz, depending on the sensor bandwidth and the expected noise spectrum.

Band-Pass Filters

Band-pass filters are employed when only a specific frequency band carries useful information. For example, some EPS torque sensors modulate a high-frequency carrier (e.g., 5 kHz) with the applied torque; a band-pass filter centered on the carrier extracts the signal while rejecting DC offset and high-frequency interference. Multiple-feedback (MFB) topologies are preferred here for their stable gain and narrow bandwidth.

Notch Filters

Notch (band-stop) filters target specific nuisance frequencies, such as the commutation harmonics of the motor (e.g., 12th harmonic at 1.2 kHz for a 6-pole motor running at 10,000 RPM). A twin-T notch filter can be implemented with a single op-amp to provide deep attenuation (40 dB or more) at a single frequency without affecting other signals. This is critical for preventing resonance in the mechanical steering column.

Higher-Order Filters and State-Variable Designs

For EPS systems that must meet stringent EMI standards like CISPR 25, fourth-order or sixth-order filters may be needed. State-variable filters can simultaneously provide low-pass, high-pass, and band-pass outputs from the same circuit, allowing the ECU to switch between modes depending on driving conditions. These advanced topologies are now being integrated into motor driver ICs from suppliers like ROHM and STMicroelectronics.

Design Considerations for Active Filters in EPS

Order and Cutoff Frequency Selection

The filter order determines the roll-off steepness. A second-order filter is a baseline for most sensor signals, but higher orders may be needed to attenuate strong noise from the 12V power bus. Cutoff frequency must be set high enough to pass the steering dynamics (typically below 100 Hz for human control) but low enough to block PWM switching noise. Trade-offs between phase shift and attenuation must be carefully evaluated—excessive phase lag can destabilize the control loop.

Component Tolerances and Temperature Stability

Automotive temperature ranges (-40°C to +125°C) demand components with low drift. Metal film resistors and COG/NP0 capacitors are essential for maintaining filter characteristics. Active filters using op-amps with high slew rate and low offset voltage (e.g., automotive-grade LT6013) are recommended. Simulation tools like SPICE can model worst-case shifts, ensuring the filter remains effective over the entire operating range.

Layout and PCB Design

Even the best active filter design can be ruined by poor PCB layout. Traces carrying noisy PWM signals must be kept separate from filter inputs. Ground planes and ferrite beads are often added to prevent conducted EMI from coupling into the filter's pass band. Reference designs from Analog Devices provide layout guidelines specific to automotive environments.

Benefits of Active Filters in EPS

Improved Signal Integrity and Control Accuracy

By removing noise, active filters allow the ECU to read torque and motor position with higher resolution. This directly translates to smoother assist and better road feel. Test data show that EPS systems with well-designed active filters reduce torque ripple by 60–80% compared to unfiltered systems, as reported in SAE Technical Paper 2020-01-0504.

Enhanced System Reliability and Lifetime

Active filters protect sensitive electronics from voltage spikes and high-frequency noise that could otherwise degrade components over time. Reduced electrical stress on the motor drive ICs and microcontrollers improves mean time between failures (MTBF). This is especially important for autonomous driving systems where EPS availability is safety-critical.

Better Driver Experience

Noise suppression leads to a quieter, vibration-free steering wheel. Active filters help eliminate the "gritty" sensation that some electric steering systems exhibit, particularly at low speeds. The result is a more refined, premium feel that automakers can leverage for brand differentiation.

EMI Compliance and Ease of Certification

Automakers must pass radiated and conducted emissions tests. Active filters, when placed at the power input and at sensor lines, dramatically reduce the EMI footprint of the EPS module. This simplifies the overall vehicle EMC certification process and reduces the need for bulky shielding.

Digital Active Filters and Adaptive Systems

As EPS controllers move to higher-performance microcontrollers (e.g., ARM Cortex-R series), digital active filters implemented in firmware are becoming viable. These filters can adapt their cutoff frequencies in real time—switching to a lower cutoff during cruise to eliminate noise, and a higher cutoff during parking for fast response. Digital filters also eliminate component drift and allow easy updates via software.

Integration in Steer-by-Wire

Steer-by-wire systems, which remove the mechanical link between steering wheel and wheels, rely entirely on electronic control. Active filters will be even more critical in these systems to ensure fail-safe operation and fault-tolerant signal processing. Filter redundancy (dual active filters on parallel channels) is an emerging design pattern.

Wide Bandgap Semiconductors and New Noise Profiles

The adoption of SiC and GaN power devices in electric vehicles introduces higher switching frequencies (100 kHz–1 MHz) with sharper edges, creating new noise harmonics. Active filters must evolve to handle these higher frequencies while maintaining low insertion loss. Ongoing research at universities (e.g., an IEEE paper on GaN-based EPS filter design) points to novel topologies using inductors on-chip and op-amps with 500 MHz+ gain bandwidth.

Conclusion

Active filters have transitioned from a specialized component to a cornerstone of electric power steering design. They solve the fundamental challenge of electrical noise that plagues all EPS systems, enabling accurate control, high reliability, and a superior driving feel. As vehicle electrical architectures become more complex—with higher voltages, faster switching, and more stringent EMI standards—the importance of active filters will only grow. Engineers designing next-generation EPS must prioritize active filter selection and layout early in the development cycle to ensure safe, smooth, and compliant steering performance.