Sensor types and signals

Subjects:

  • Preface
  • Passive sensors
  • Active sensors
  • Intelligent sensors
  • Applications in automotive technology
  • Measuring on sensors
  • Signal transmission from sensor to ECU
  • Power supply and signal processing

Preface:
Sensors measure physical quantities and convert them into electrical voltages. These voltages are processed in the microcontroller (ECU) and read as a “signal”. The signal can be judged by the magnitude of the voltage, or the frequency with which a signal changes.

Passive sensors:
A passive sensor detects and measures a physical quantity and converts it into another physical quantity. An example of this is converting a temperature into a resistance value. A passive sensor does not generate any voltage itself, but responds to a reference voltage from the ECU. A passive sensor does not require a power supply to function.

Passive sensors usually have two or three connections:

  • reference or signal wire (blue);
  • ground wire (brown);
  • shielded wire (black).

Sometimes a passive sensor contains only one wire: in that case the housing of the sensor serves as ground. A third wire can serve as a shield. The sheath is grounded via the ECU. The shielded wire is used especially for interference-sensitive signals such as from the crankshaft position sensor and the knock sensor.

Active sensors:
Active sensors contain an electrical circuit in the housing to convert a physical quantity into a voltage value. The electrical circuit often needs a stabilized supply voltage in order to work.  

In most cases, this type of sensor has three connections:

  • plus (usually 5,0 volts);
  • pasta;
  • signal.

The stabilized 5 volt supply is supplied by the control unit and used by the sensor to form an analog signal (between 0 and 5 volts). Often the plus and ground wires from the ECU are connected to multiple sensors. This can be recognized by the nodes to which more than two wires are connected.

The analog signal is converted into a digital signal in the ECU. 
In the section “spanning supply and signal processing” we will discuss this in more detail.

Intelligent sensors:
Intelligent sensors usually have three connections. As with the active sensors, there is a power wire (12 volts from the ECU or directly through a fuse) and a ground wire (through the ECU or an external ground point. An intelligent sensor sends a digital (LIN bus) message to the ECU and the other sensors. There is then a master-slave principle. 

Internally in the sensor, an A/D converter converts an analog to a digital signal.

  • Analog: 0 – 5 volts;
  • Digital: 0 or 1.

In the LIN bus signal in recessive state (12 volts) there is a 1, and in dominant state (0 volts) a 0.

Applications in automotive technology:
In automotive technology we can make the following classification of the different types of sensors:

Passive sensors:

  • knock sensor;
  • crankshaft position sensor;
  • Temperature sensor (NTC/PTC);
  • Oxygen sensor (jump sensor / zirconium);
  • Inductive height sensor;
  • Switch (on/off)

Active sensors:

  • Crankshaft/Camshaft Position Sensor (Hall);
  • air mass meter;
  • Wideband oxygen sensor;
  • Pressure sensor (charge pressure / boost pressure sensor);
  • ABS sensor (Hall / MRE);
  • Acceleration/deceleration sensor (YAW);
  • Radar / LIDAR sensor;
  • Ultrasonic sensor (PDC / alarm);
  • Position sensor (throttle valve / EGR / heater valve).

Intelligent sensors:

  • Rain/light sensor;
  • cameras;
  • pressure sensor;
  • Steering angle sensor;
  • battery sensor

Measuring on sensors:
If a sensor malfunctions, the driver of the car will notice that something is wrong. Failure of a sensor in the engine compartment could result in loss of power and a lit MIL (Engine Malfunction Light).

When read out, a related fault code is displayed. However, not in all cases the fault code leads directly to the cause. The fact that the sensor in question does not work can be because it is defective, but a problem in the wiring and / or plug connections is not excluded.

The following image shows a reading from an active sensor. The voltage difference on the plus and minus terminals of the sensor is checked with a digital multimeter. This is OK.

Signal voltages can be measured with a voltmeter or an oscilloscope. It depends on the signal type which meter is suitable:

  • voltmeter: analog signals that are nearly constant;
  • oscilloscope: analog signals and digital signals (duty-cycle / PWM).

With one or more measurements we can show that the sensor is not working properly (the signal delivered is implausible or the sensor does not give a signal), or that there is a problem in the wiring.
With passive sensors, in most cases a resistance measurement can be performed to check if there is an internal defect in the sensor.

Possible sensor wiring problems may include:

  • interruption in the positive ground or signal wire;
  • short circuit between wires or the body;
  • contact resistance in one or more wires;
  • bad plug connections.

On the page: troubleshooting sensor wiring we delve into seven possible faults that can occur in the wiring of sensors.

Signal transmission from sensor to ECU:
There are several methods to transfer signals from the sensor to the ECU. In automotive technology we can deal with the following signal types:

  • Amplitude Modulation (AM); the height of the voltage provides information;
  • Frequency Modulation (FM); the frequency of the signal provides information;
  • Pulse Width Modulation (PWM); the time variation in the square-wave voltage (duty-cycle) provides information.

The following three examples show scope signals of the different signal types.

Amplitude Modulation:
With an AM signal, the level of the voltage transmits the information. The figure shows two voltages from the throttle position sensors. To guarantee reliability, the voltage curves must be mirrored relative to each other. 

Tensions at rest:

  • Blue: 700mV;
  • Red: 4,3 volts.

From approximately 0,25 seconds after the start of the measurement, the accelerator pedal is slowly pressed and the throttle opens to 75%.
At 2,0 sec. the accelerator pedal is released and at 3,0 sec. is given full throttle.

Full throttle voltages:

  • Blue: 4,3 volts;
  • Red: 700 mV.

Frequency Modulation:
For sensors that transmit an FM signal, the amplitude (height) of the signal does not change. The width of the square-wave voltage transmits the information. The following image shows the signal from an ABS sensor (Hall). The wheel has turned during the measurement. With a higher rotational speed, the frequency of the signal becomes higher.

The voltage difference is caused by the change in the magnetic field in the magnetic ring, which is incorporated in the wheel bearing. The difference in height (low: magnetic field, high: no magnetic field) is only 300 mV. When setting the scope incorrectly (voltage range from 0 to 20 volts), the block signal is barely visible. For that reason, the scale has been adjusted in such a way that the block signal becomes visible, with the result that the signal is less pure.

Pulse Width Modulation:
With a PWM signal, the ratio between high and low voltage changes, but the period time remains the same. This should not be confused with a square-wave voltage in an FM signal: the frequency changes and therefore also the period time.

The next two images show PWM signals from a high pressure sensor in an air conditioner duct. This sensor measures the refrigerant pressure in the air conditioning system.

Situation during the measurement:

  • Ignition switched on (sensor receives a supply voltage);
  • Air conditioning turned off;
  • Coolant pressure read with diagnostic equipment: 5 bar.

In the next scope image we see that the period time has remained the same, but the duty cycle has changed.

Situation during the measurement:

  • Air conditioning turned on;
  • High pressure has risen to 20 bar;
  • Duty cycle is now 70%

Power supply and signal processing:
In the first paragraphs, we talked about whether or not a power supply voltage was used. In this section we discuss the main components in the ECU that are responsible for the power supply and signal processing of the respective sensor. The pin numbers of the in-depth diagrams are the same as in the previous paragraphs: pins 35 and 36 of the ECU are connected to pins 1 and 2 of the passive sensor, etc.

In the first image we see a NTC temperature sensor. The voltage (Uref) from pin 35 of the ECU is obtained from the voltage stabilizer 78L05. The voltage stabilizer supplies a voltage of 5 volts at an on-board voltage of 6 to 16 volts.
The resistor R (fixed resistance value) and RNTC (temperature dependent resistance) together form a series connection and also a voltage divider. The Analog-to-Digital Converter (ADC) measures the voltage between the two resistors (analog), converts it into a digital signal and sends it to the microprocessor (µP).

With a multimeter you can measure the voltage on pin 35 of the ECU or pin 1 of the sensor.

On the page about the temperature sensor In addition to some measurements with good signal transmission, the measurement techniques in the event of a fault in the wiring are shown.

The second image shows the circuit of an active MAP sensor to display.
The stabilized supply voltage of 5 volts reaches the so-called “Wheatstone Bridge”, in which a number of fixed (R1, R2, R3) and a variable resistor (Rp) are included.
The resistance value of Rp depends on the pressure in the intake manifold. Here too we are dealing with a voltage divider. The resistance change causes a voltage change, so that the bridge is no longer in balance. The voltage difference created in Wheatstone's bridge is converted in the amplifier/filter into a voltage with a value between 0,5 and 4,5 volts. Digitization of the analog signal takes place in the analog-to-digital converter (ADC). The ADC sends the digital signal to the microprocessor.

The resolution of the ADC is in most cases 10 bits, divided over 1024 possible values. At a voltage of 5 volts, each step is approximately 5 mV.

The internal circuit of the ECU contains one or more passive and active sensors resistors included in the power and signal circuits. The resistance in the NTC circuit is also referred to as the “bias resistor” and serves for the voltage divider. The resistors R1 and R2 in the ECU circuit of the MAP sensor have the purpose of allowing a small current to flow from the plus to ground.

Without these resistors, a so-called “floating measurement” would occur if the signal wire was interrupted or if the plug of the sensor was removed. In those cases, the circuit with resistors causes the voltage on the ADC input to be pulled up to about 5 volts (minus the voltage across resistor R1). The ADC converts the analog voltage into the digital value 255 (decimal), i.e. FF (hexadecimal) and sends this to the microprocessor.

A very small current flows through resistor R1 (low Ohm). There is a small voltage drop of between 10 and 100 mV. It can happen that the applied voltage is a few tenths higher than 5 volts; A low-ohmic resistor is included between the ground connection of the voltage stabilizer 78L05 and the ground of the ECU (brown wire in the above diagram). The voltage drop across this resistor can be, for example, 0,1 volts. The voltage stabilizer sees its ground connection as actual 0 volts, so it raises the output voltage (the red wire) by 0,1 volts. In that case, the voltage sent to the plus of the sensor is not 5,0 but 5,1 volts.

The intelligent sensor receives a voltage of 12 volts from the ECU. As with the active sensor, the intelligent sensor incorporates a Wheatstone bridge and an amplifier/filter. The analog voltage from the amplifier is sent to the LIN interface (LIN-IC).

The LIN interface generates a digital LIN bus signal. The signal varies between 12 volts (recessive) and about 0 volts (dominant). With this LIN bus signal, the sensor communicates with the other slaves (usually the sensors and actuators) and the master (the control unit).
There are branches to the master and other slaves on the wire between pin 3 of the sensor and pin 64 of the ECU.

For more information, see the page LIN bus.

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