Sensors and Digital Electronics

1. Sensors and Digital Electronics

2. Sensors • Convert pressure, temperature, position, or other physical condition into an electrical signal. • Three basic types • Voltage Producing • Variable Resistance • Voltage Dividing

3. Voltage Producing • Most common type is variable reluctance • Also called magnetic pulse generator • Mini AC voltage generator • Uses a coil of wire wrapped around a permanent magnet • Tone wheel is attached to rotating component • As tone wheel rotates voltage is created • Used as ABS wheel speed sensor • Also used for other applications to measure speed and/or position • Not very accurate at low shaft speeds

4. Cut Away View of Variable Reluctance Sensor PG 326 in Text • As tone wheel rotates voltage is created, as tone wheel rotates faster more voltage is created. • Can be used to measure position by leaving off a tooth, this changes the voltage signal when missing tooth passes sensor.

5. As tone wheel rotates one full tooth, one full AC voltage wave is created • Amount of voltage produced is controlled by speed of tone wheel • Also by air gap between sensor and tone wheel • Air gap must be correct for sensor to operate correctly

6. Hall Effect • Generates AC voltage • Uses a magnetic field and a tone wheel • As tone wheel rotates past magnetic field voltage is generated, with missing tooth can measure position • Accurate at low and high speeds • Used most often for Crank and Cam speed/position sensors. • Most use three wires, • Supply in, can be battery voltage or 5v reference • Ground • Signal voltage back to electronic control module

7. As magnet moves past semiconductor material voltage is created, also known as Hall Voltage.

8. Thermocouples • Voltage producing sensor • Made by joining two insulated metals • The junction of the two metals produces very small amounts of voltage (less than 200mV) when heated • This signal is then sent to the pyrometer where the voltage is amplified and exhaust temperature is displayed

9. Variable Resistance Sensors2 wire sensors that change resistance • There are several types • Thermistors • Used to measure temperature • Most have high resistance at low temps, low resistance at high temps

10. Rheostats • One terminal connected to output • Other terminal connected to moveable wiper arm • Used primarily in fuel level senders, radio volume control, light dimming

11. Variable Voltage Sensors • Sensors that divide voltage • Connected to electronic module with 3 wires • Voltage, typically +5V • Ground • Voltage signal back to controller • Output voltage will vary between 0-5V, typically output voltage will be .5V to 4.5V • ECM sets code if sent 0V or 5V • ECM thinks open or shorted circuit if 0V or 5V is sent back from sensor

12. Potentiometer • Used to measure position • Examples of where potentiometers are used • Throttle position on nearly all electronic engines • Three point hitch on ag tractors • Boom or blade position on back hoes and dozers • Header height position on combines • Used on tillage tools for automatic depth control

13. Other Electronic Sensors • Other variable voltage sensors • Supplied with reference volts (+5V), Ground, and Signal back to ECM. • Sensor measures pressure, or temperature • As pressure changes or temperature rises output signal voltage changes • Common applications, engine oil pressure, boost pressure, exhaust back pressure, hydraulic and lube pressures

14. Digital Electronics • Everything we have discussed so far has been analog (hardwired voltage in, voltage out) • Digital electronics use SMART switches or sensors • The switches or sensors send binary code back to the ECM for operating accessories, or when machine is operating • Binary code uses only two numbers 0 and 1 • Combination of 0s and 1s gives all other numbers • This reduces amount of wiring, and components

15. Digital Electronics • For a switch or sensor to send a binary signal we only need two voltage levels, low voltage is seen as a 0, while higher voltage is seen as a 1 • ECM receives sensor input as digital form instead of 3v for example

16. Logic Gates • All controlled machine functions are viewed as requests • A request may be engine cranking, fuel delivery, transmission gear selection • AND Gates, multiple events must happen for the requested output (switches in series)

17. More Logic Gates • OR Gates, only one event, out of several must happen for the requested output (switches in parallel)

18. Microprocessors • All use the integrated circuit (IC) • Circuit in which transistors, diodes, and resistors, are manufactured into a single device known as a “CHIP” • Logic gates are typically transistors • The transistors then control the requests from the operator, or control module • Microprocessors operate off of inputs, outputs and memory.

19. Inputs, Outputs • Microprocessors can only use digital information, some sensors and components use analog inputs or outputs • Need analog to digital converter (A/D)

20. Inputs, Outputs • Microprocessors use very small amounts of current for inputs and outputs • Output signals such as sensor 5v circuits use a resistor to get battery voltage to 5v, but also use current limiting resistors in case the sensor circuit gets grounded to protect the ECM • Only a DMM should be used on sensor circuits, NEVER a test light, the testlight will draw enough amperage to destroy an ECM

21. Memory • Volatile- memory that is lost when power is disrupted, RAM (random access memory), information is stored in RAM until saved • Nonvolatile- memory that is NOT lost when power is disrupted, ROM (read only memory), information that is permanent, can’t be deleted or changed, this memory contains all operating instructions for a control module

22. Memory • EEPROM (electrically erasable programmable read only memory)- memory that stores changing information, such as engine hours, or mileage will also store information such as engine overheated, low or no engine oil pressure with engine running, no braking before an accident, usually only manufacturer can erase this type of memory • Manufacturers then use this information for deciding warranty claims, or for deciding who caused an accident

23. Memory • Flash Memory- nonvolatile memory that stores most or all of module programming, can be erased and rewritten by “flashing or reflashing” • This is the type of memory that gets software updates • Updates performed through scan tool, or laptop, and either internet connection or hard disk