Automotive Technology. Objectives. Types of Oscilloscopes

Transcription

1 Automotive Technology FIFTH EDITION CHAPTER 43 Oscilloscopes and Graphing Multimeters Objectives Compare the different types of oscilloscopes and explain how to setup and adjust oscilloscopes. Discuss DC and AC coupling, pulse trains, channels and triggers. Explain how to use a scope and discuss graphing multimeters and graphing scan tools. Types of Oscilloscopes Terminology Oscilloscope (usually called a scope): visual voltmeter with a timer that shows voltage changes Analog scopes: use a cathode ray tube (CRT) to display voltage patterns Digital scopes (DSO) commonly use a liquid crystal display (LCD) and take samples of the signals that can be stopped or stored 1

2 Types of Oscilloscopes Terminology A scope has been called a voltmeter with a clock because it captures and displays changing voltage levels and displays changes within a specific time period Oscilloscope Display Grid Scope faces: usually have eight or ten vertical grids and ten horizontal grids Types of Oscilloscopes Oscilloscope Display Grid Graticule: transparent scale (grid) used for reference measurements commonly with 8 x 10 or 10 x 10 divisions Figure 43-1 A scope display allows technicians to take measurements of voltage patterns. In this example, each vertical division is 1 volt and each horizontal division is set to represent 50 milliseconds. 2

3 Scope Setup and Adjustments Setting the Time Base Setting how much time will be displayed in each block (division) Time base should be set to an amount of time that allows two to four events to be displayed Milliseconds (0.001 sec.) are commonly used in scopes when adjusting the time base Scope Setup and Adjustments Setting the Time Base Time per division settings can vary greatly, including: MAP/MAF sensors: 2 ms/div (20 ms total) Network (CAN) communications network: 2 ms/div (20 ms total) Scope Setup and Adjustments Throttle position (TP) sensor: 100 ms per division (1 sec. total) Fuel injector: 2 ms/div (20 ms total) Oxygen sensor: 1 sec. per division (10 sec. total) 3

4 Scope Setup and Adjustments Setting the Time Base Time per division settings can vary greatly including: Primary ignition: 10 ms/div (100 ms total) Secondary ignition: 10 ms/div (100 ms total) Voltage measurements: 5 ms/div (50 ms total) Scope Setup and Adjustments Volts Per Division Abbreviated V/div Should be set so the entire anticipated waveform can be viewed Scope Setup and Adjustments Examples include: Throttle position (TP) sensor: 1 V/div (8 V total) Battery, starting and charging: 2 V/div (16 V total) Oxygen sensor: 200 mv/div (1.6 V total) 4

5 Scope Setup and Adjustments Volts Per Division Total voltage to be displayed exceeds the voltage range of the component being tested DC and AC Coupling DC Coupling Most used position on a scope which allows both alternating current (AC) voltage signals and direct current (DC) voltage signals to be displayed The AC part of the signal will ride on top of the DC component DC and AC Coupling AC Coupling A capacitor is placed into the meter lead circuit, effectively blocking all DC voltage signals but allowing the AC signal be displayed Can be used to show output signal waveforms from sensors such as: Distributor pickup coils Magnetic wheel speed sensors 5

6 DC and AC Coupling AC Coupling Can be used to show output signal waveforms from sensors such as: Magnetic crankshaft position sensors Magnetic camshaft position sensors Magnetic vehicle speed sensors The AC ripple from an alternator Pulse Trains Definition Pulse train: voltage that turns on and off in a series of pulses Pulse trains differ from AC signals because they do not go below zero Frequency Number of cycles per second measured in hertz Pulse Trains Duty Cycle Refers to the percentage of on-time of the signal during one complete cycle Also called pulse-width modulation (PWM) Can be measured in degrees Pulse Width A measure of the actual on-time measured in milliseconds 6

7 Figure 43-5 (a) A scope representation of a complete cycle showing both on-time and off-time. (b) A meter display indicating the on-time duty cycle in a percentage (%). Note the trigger and negative (-) symbol. This indicates that the meter started to record the percentage of on-time when the voltage dropped (start of on-time). Number of Channels Definition The number of events, which require leads for each, is called a channel A channel is an input to a scope Number of Channels Commonly available scopes include: Single channel: capable of displaying only one sensor signal waveform at a time Two channel: can display the waveform from two separate sensors or components at the same time Four channel: allows the technician to view up to four different sensors or actuators on one display 7

8 Triggers External Trigger An external trigger is when the waveform starts when a signal is received from another external source rather than from the signal pickup lead Trigger Level Trigger level is the voltage that must be detected by the scope before the pattern will be displayed Triggers Trigger Slope The trigger slope is the voltage direction that a waveform must have in order to start the display The scope display indicates both a positive and a negative slope symbol Using a Scope Using Scope Leads Leads usually attach to the scope through a BNC connector (miniature standard coaxial cable connector) Be sure to connect one lead to a good clean, metal engine ground The probe attaches to the circuit or component being tested 8

9 Using a Scope Measuring Battery Voltage With a Scope A lower voltage can be observed as the engine is started and a higher voltage should be displayed after the engine starts Graphing Multimeter A graphing multimeter (abbreviated GMM) is a cross between a digital meter and a digital storage oscilloscope Voltage is displayed at two places: On a display screen In a digital readout Graphing Scan Tools Many scan tools are capable of displaying the voltage levels captured by the scan tool through the data link connector (DLC) Read and follow the instructions for the scan tool being used 9

10 Summary Time base should be set to an amount of time that allows two to four events to be displayed. Trigger level is the voltage that must be detected by the scope before the pattern will be displayed. Summary Many scan tools are capable of displaying the voltage levels captured by the scan tool through the data link connector (DLC). 10