Introduction
Modern automobiles are packed with sensors to monitor the various systems and provide critical signals to the engine control unit (ECU). But the raw sensor outputs cannot be directly used by the ECU and need proper signal conditioning to make them usable for control and diagnostics. Signal conditioners play a vital role in interfacing the wide variety of sensors to the ECU in the harsh electrical and environmental conditions seen in automotive applications.
This article provides an overview of the different types of sensor signal conditioning circuits used in automobiles and their importance in sensor interfacing. Key design considerations and implementation methods are also discussed.
Automotive Sensors Overview
Some major sensors used in automobiles along with sensed parameter and typical output:
| Sensor | Parameter Measured | Typical Output |
|---|---|---|
| Mass Airflow | Intake Air Flow | 0-5 V |
| Oxygen (UEGO) | Exhaust Oxygen Content | 0-5 V |
| Manifold Absolute Pressure | Intake Pressure | 0-5 V |
| Coolant Temperature | Engine Temperature | Resistance |
| Throttle Position | Throttle Plate Angle | 0-5 V |
| Cam/Crankshaft | Rotation Speed/Position | Digital Pulses |
| Wheel Speed | Wheel Rotation Speed | Digital Pulses |
| Accelerometer | Lateral/Longitudinal Acceleration | 0-5 V |
This demonstrates the wide variety of sensor signals the ECU has to process – analog voltages, digital pulses, variable resistance. The signals need to be conditioned before they can be digitized by ECU analog to digital converters (ADCs) and used in control algorithms.
Need for Signal Conditioning
The key functions of sensor signal conditioners are:
Gain – Boost weak sensor outputs to improve signal to noise ratio and match ADC input range.
Filtering – Remove out-of-band noise that can cause errors. Anti-aliasing filter for ADCs.
Linearization – Convert non-linear sensor responses to linear format for simplicity.
Impedance Conversion – Alter sensor output impedance to prevent loading effects.
Isolation – Protect ECU from transients and abnormal sensor voltages.
Excitation – Provide stable voltage/current to passive sensors like thermistors.
Compensation – Counteract sensor inaccuracies like shift over temperature.
Standardization – Present sensor data in normalized formats like 0-5V irrespective of sensor type.
Proper signal conditioning is vital for the ECU to get clean, accurate data from the sensors in the harsh, noisy on-vehicle environment. It acts as the interface between sensors and ECU ADC.
Sensor Signal Conditioner Architectures
Sensor signal conditioners can be implemented in different ways:
Discrete Conditioners – Use op-amps, discrete passives on PCBs. High flexibility but large size.
Integrated Circuits – Special ICs tailored for common functions like amplification, filtering. Compact but limited configurability.
FPAAs – Field Programmable Analog Arrays allow reconfiguration of signal chain. Good tradeoff between size and flexibility.
Module Based – Complete sensor interfacing on a module or board including ADC. Medium flexibility and size.
SoC Based – Sensors, signal chain and ADC integrated on a single chip. Highest integration but custom development needed.
Selection depends on size constraints, development cost and customization needs. Module based conditioning provides a good balance and reduces development effort.
Common Conditioning Circuits
Some typical conditioning circuits used with major automotive sensor types are discussed next:
Bridge Sensors
Load cells, strain gauges use a Wheatstone bridge structure. A basic bridge circuit completes the bridge and amplifies the differential output voltage:
The differential gain rejects common mode noises. Adjustable potentiometers are provided for calibration. The amplified output represents the sensed parameter.
Thermistors
NTC thermistors exhibit large resistance changes with temperature. A potential divider topology can convert this to a voltage:
The voltage varies non-linearly with temperature. Linearization using the Steinhart-Hart equation embedded in the ECU firmware gives accurate temperature.
Digital Hall Sensors
Hall effect position sensors like throttle position sensors have a digital PWM output whose duty cycle varies with position. An integrating filter converts this to an analog voltage:
The RC filter integrates the PWM signal to analog. The diode clamps negative cycles. Result is a clean 0-5V varying with position.
Piezoresistive Pressure Sensors
Sensors like the manifold absolute pressure (MAP) sensor use a Wheatstone bridge piezoresistive structure to detect intake pressure. Similar to bridge sensors, a differential amplifier conditions the output:
Differential gain boosts small mV level signals to 0-5V range. Adjustable potentiometers used for calibration.
Capacitive Position Sensors
Non-contacting capacitive position sensors have a variable capacitance output depending on shaft position. It forms part of an RC oscillator:
The oscillator frequency varies with capacitance change, which is demodulated to a analog voltage representing position by using a PLL, counter or ADC frequency measurement.
Magnetic Wheel Speed Sensors
Active wheel speed sensors produce a square wave frequency directly proportional to the wheel speed:
Signal is buffered via a comparator to clean it up before sending to ECU counter input. No analog conditioning required since sensor output is digital pulse train.
Current Loop Sensors
Some sensors like MAF output a current proportional to intake air mass flow rate and require a simple resistor to convert to voltage:
A low value sense resistor converts the 4-20 mA current to a 0-5V voltage for the ECU ADC. Care taken to ensure voltage burden does not affect sensor performance.
Design Considerations
Some key points considered during design of sensor signal conditioning circuits:
- Sensor output characteristics – magnitude, impedance, linearity, frequency response, etc.
- Noise and interference – EMI, crosstalk, engine electrical noise, etc.
- Tolerance to environmental stresses – temperature, vibration, humidity
- Fail safe provisions – defaults to known state upon failure
- Effect on sensor function – biasing, loading, source impedance, feedback etc.
- Diagnostics capability – able to detect open/short sensor faults
- Protection – prevent damage to ECU from overvoltage and transients
- Performance over supply voltage and temperature range
- Cost, size and design effort constraints
Simulations, prototyping and testing ensures the conditioning circuits provide clean, accurate, normalized sensor data to the ECU under all on-vehicle conditions.
Implementation Methods
There are different approaches to implement the sensor signal conditioning circuits:
Discrete – Using separate opamps, discrete resistors, capacitors
Allows precision conditioning but large size, assembly effort
Integrated Circuits – Dedicated sensor interface, amplifier ICs
Small size but limited configuration flexibility
FPAAs – Field Programmable Analog Arrays
Reconfigurable signal chain blocks for decent flexibility
Module Based – Complete circuit on a dedicated PCB module
Self-contained, quick to integrate but moderate flexibility
SoC – Integrated sensor, signal chain and ADC in a single IC
Maximum integration but fully custom mixed-signal IC development needed
Software Based – Digitize raw sensor output and use software algorithms
Configurable but latency, noise can affect control performance
A module-based approach provides a good tradeoff – easy integration with conditioning tailored for each sensor for automotive production use.
Testing and Calibration
Thorough testing of sensor conditioning electronics is needed to ensure proper operation under all conditions:
Functionality Testing – Validates circuit operation over temperature and voltage ranges with known simulated sensor inputs.
Noise Testing – Quantifies noise and distortion levels introduced by the conditioning circuits.
Error Budgeting – Calculates overall system error by considering all component tolerances, drifts and nonlinearities.
Fault Testing – Verifies fail safe behaviors upon open, short or out of range sensor inputs.
Calibration – Potentiometers, digital trims are adjusted based on calibration with sensor reference standards to minimize errors.
Lifetime Testing – Assesses performance degradation due to thermal cycling, vibration, humidity and aging effects. Confirms adequate service lifetime.
The signal conditioning circuits are a vital link between sensors and ECU. Proper design and testing ensures the ECU gets accurate, noise-free data from the wide variety of sensors in the harsh on-vehicle environment over the vehicle’s lifetime. This enables advanced engine management, fuel efficiency, diagnostics and safety features.
Conclusion
An overview of the common signal conditioning methods used with major automotive sensor types has been presented. Discrete circuits based on opamps, integrated amplifier ICs, FPAAs and module based approaches provide flexible solutions for the varying needs of different sensors while meeting challenges like noise, nonlinearities etc. When designed keeping in mind sensor characteristics, environmental conditions, ECU interface requirements and performance constraints, the sensor conditioners reliably acquire and process raw sensor signals into the standardized, accurate data needed by ECUs for precise engine control. Advancements in programmable mixed-signal ICs and miniaturization will enable higher levels of integration and intelligence in sensor interfaces, moving towards more accurate and responsive engine control systems.
Automotive Sensor Conditioners – FAQs
Q1. How does signal conditioning help the ECU analyze sensor data?
Signal conditioning transforms the raw sensor output into a clean, standard format required by ECU ADC and algorithms – amplifying, linearizing, protecting from transients/noise, converting impedance/format etc. This enables accurate measurement.
Q2. What are some important specifications for automotive sensor signal conditioners?
Key parameters are bandwidth, linearity, stability, drift, noise performance, fault tolerance, protection rating, size/weight, reliability, EMI/EMC compliance, temperature range, input/output impedances and flexibility.
Q3. Which type of sensor interface circuit is most suitable for wheel speed sensors?
Wheel speed sensors output a digital pulse train whose frequency is proportional to speed. Only buffering is needed so a basic comparator circuit provides the required conditioning to clean up pulses before input to ECU counter.
Q4. How can capacitive type position sensors be interfaced to an ECU?
The capacitance versus position characteristic can be converted to a frequency using a capacitance-to-frequency converter circuit. The frequency can then be measured digitally by the ECU using a timer input to determine position.
Q5. What are some methods used for linearizing thermistor response vs temperature?
Using microcontroller algorithms to implement mathematical linearization models like Steinhart–Hart model or look-up tables. Analog linearization circuits using resistor networks or diodes to counteract the thermistor nonlinearity.
