Topics:
- Oscilloscope introduction
- Picoscope introduction
- Picoscope: setting the voltage
- Picoscope: setting the time per division
- Picoscope: setting the trigger
- Picoscope: scale and offset
- Fluke: introduction
- Fluke: setting the null line
- Fluke: setting the voltage and time per division
- Fluke: setting the trigger
- Fluke: enabling or disabling the smooth function
- Fluke: enabling channel B
- Fluke: measuring with the current clamp
Oscilloscope introduction:
An oscilloscope (abbreviated as “scope”) is a graphical voltmeter that displays voltage as a function of time. The oscilloscope is highly accurate and can set the time scale so small that signals can be accurately analyzed. An oscilloscope cannot measure current like a multimeter, which can measure up to 400 mA or 10 Amps. To measure current, a current clamp is used to measure the magnetic field strength around a wire.
Picoscope introduction:
An oscilloscope is indispensable for diagnosing complex issues. There are different variants of oscilloscopes: integrated into diagnostic equipment (e.g., Snap-on or dealership diagnostic tools), a “handheld” oscilloscope (Fluke, also described on this page), and those that can be connected to a computer/laptop.
The latter applies to the Picoscope. The hardware of this scope is housed in a box that can be connected to a computer with Windows or Macintosh OS using a USB 2.0 or 3.0 (printer) cable.
On the computer, we use the Picoscope software. The scope’s hardware unlocks various software functions; a more extensive (and expensive) scope can thus do more than an entry-level version. The image shows the Automotive (4000-series) scope.
The basic settings for measurements with the Picoscope are described in the following paragraphs.
There are different models of the Picoscope. In the automotive field, the following types may be of interest:
Picoscope 2204a:
The Picoscope 2204A is available from €130 and is suitable for most automotive applications. The purchase price is attractive, but keep the following in mind:
- The grounds of channel A and B are internally connected. During a dual-channel measurement, there’s no issue if the ground cables are connected to a vehicle ground point. However, if during a dual-channel measurement there’s a voltage on one of the ground cables, the oscilloscope will short-circuit, damaging the hardware;
- The measurement range is from -20 to 20 volts. This is ample for measuring a PWM signal or CAN bus, but for higher voltages, for example, the inductive spike of an injector, an extra accessory called an “attenuator” must be used. This reduces the read and measured voltage by a factor of 20. If 60 volts is measured, it will display as 3 volts;
- Personally, I don’t like the provided cable set. I prefer to use alternative BNC measurement cables;
- Probe needles and crocodile clips make it easy to measure in connectors or connect to positive or ground points.0a
Tips for purchasing these accessories:
- Picoscope 2204 option 1 or option 2 without standard measurement cables
- Hantek HT201 attenuator 20:1 (Dutch webshop) or via Aliexpress
- Measurement cables with BNC connectors0aoption 1 with banana plugs, option 2, option 3 or option 4 with small crocodile clips
- Crocodile clips, probes or a set with clips and (extended) probes
An equivalent attenuator is fine. The Hantek is attractively priced, and the original Picoscope attenuator is considerably more expensive but works the same.
When choosing alternative cable sets, it is advisable to select 2.5 mm banana connections, as these are compatible with the probes and crocodile clips.
A laptop less than 10 years old with (preferably) Windows 10 is sufficient to run the software well. The Picoscope software is available for free download from Picotech.com. The 2204a is not an automotive scope, so when downloading, select the 2000-series software. The automotive software does not accept this scope.
Picoscope: setting the voltage:
One of the initial settings to start measuring is setting the maximum voltage we expect to measure. After opening the program, the scale is set to “automatic.” This setting can work against us if the voltage level changes significantly. For automotive applications, a scale of 20 volts is usually sufficient. To set this, click the “20 V” button below the red arrow. The menu that opens shows various options, ranging from 50 mV to 200 V. In this measurement, 20 V is selected. The maximum measurable voltage is displayed on the left Y-axis, indicated by the green arrow.
In this example, we measure a stable battery voltage of 12 volts.
When the measured voltage exceeds the set voltage of (in this case) 20 volts, the message “channel overrange” will appear at the top of the screen. The voltage scale must then be increased. The arrows on the left and right of the menu button allow you to increase and decrease the voltage step by step without opening the menu.
Picoscope: setting the time per division:
After setting the voltage to a maximum of 20 volts, the time per division can be set.0aTo set this time, click the button for the time setting (next to the red arrow). In the menu that appears, choose the desired time per division. In the image, 5 ms/div is circled.
After selecting 20 ms/div, you’ll see the time on the X-axis increase in each division, starting from 0.0 to 200.0 ms. The times 0, 20, and 40 ms are circled in green in this example.
The time setting depends on which component, system, or process we want to measure;0a
- battery voltage during starting or a relative compression test: 1 second per division;
- sensor and actuator signals: 10 to 100 ms/div.
During the measurement, the time base can be adjusted to display a correct signal on the screen.
Picoscope: setting the trigger:
Constant voltages, such as the onboard voltage in previous examples, can also be measured with a standard multimeter. Non-constant voltages, such as a highly variable signal voltage from a sensor or PWM control, cannot be displayed well or at all by a voltmeter. In the case of a PWM or duty cycle, a voltmeter will show an average value. We measure such voltages with the oscilloscope. The scoop image below is the PWM control of an interior fan. Without trigger setting, the image would keep shifting across the screen.
The block voltage constantly shifts across the screen. A change in pulse width is not clearly visible. To freeze the voltage on the screen while still real-time measuring (freezing prevents visible changes), we use the trigger. In the Picoscope software, the Trigger function can be found as a button among the settings at the top. The button is indicated with a red arrow in the image below. By default, it’s set to “None,” meaning no trigger is used.
The following image shows the display with the trigger enabled. The mode “Repeat” is selected. A yellow dot appears on the screen, indicating the triggered point. In this case, the green arrow points to the trigger. The mouse can be used to move this point to any other position within the voltage range.
After setting the trigger, changes in the PWM signal can be observed as the conditions change: the sensor transmits changes in the signal, or the actuator is activated more or less by the ECU. In the image below, a PWM signal is shown where the ground control widens and narrows.
When measuring the signal, it can also be desirable to trigger on the negative edge; for example, when measuring the voltage pattern of an injector because control begins at that point. The setup is as follows: click on the “advanced triggers” button (red arrow in the image). A new window opens where you can change the direction under “simple edge” from “rising” to “falling” (blue arrow). The trigger point in the signal is then placed on the negative edge (green arrow).
The following example shows the voltage pattern of an injector. Just like with the PWM control voltage of the interior fan in the previous example, this signal would shift across the screen without a trigger. In the example, the trigger is set on the falling edge of channel B. In the “Trigger” button, B is colored red with a Z symbol indicating the negative edge.
After setting the trigger point, the signal on the screen is fixed (see image below). The signal has a fixed starting point; where the injector is switched to ground, the control begins. When accelerating, enrichment occurs, and the injector opens for longer to inject more fuel. The ECU switches the injector over a longer period to ground in such cases.
During deceleration, fuel injection stops: in that case, the injector is not switched to ground. The voltage remains constant (around 14 volts). Because we set the trigger on the falling edge for this measurement, deceleration is not clearly visible. Only after turning off the trigger do we see the voltage remain at 14 volts, but as soon as the injection resumes, the image will shift across the screen again.
Picoscope: scale and offset:
The block signal from an ABS sensor (Hall) has a small voltage difference. The scoop image below shows the image measured directly on the ABS sensor. The ABS control unit contains a circuit that amplifies the voltage difference. For diagnosing the ABS sensor, this scoop image is not clear enough. By changing the scale and offset, the signal can be magnified.
In the measurement below, channel B is connected to the same wire as channel A. The measurement is identical, but due to different settings, the signal is better visualized. The green arrow indicates one of the places where you can change the scale and offset.
- The scale zooms in on the signal: we are now measuring within 12 and 14 volts.
- The offset can be adjusted to get the signal at the correct height on the screen. With an offset of 0%, the voltage on the Y-axis is visible between 0 and 2 volts.
Fluke introduction:
The image next to this text shows a Fluke 123 handheld oscilloscope, used in car workshops, test and development rooms, and in education. Although there are different brands, they often closely resemble each other and operate almost identically. On top of the oscilloscope are red and gray connections, referred to as channels A and B, respectively. In the middle is the ground connection.
The oscilloscope can display two measurements simultaneously on one screen (A and B separately), as also shown in the image. Measurement A is displayed at the top and measurement B at the bottom, allowing easy comparison of signals from two different sensors. For a single measurement, channel A is used by default.
The oscilloscope can measure both direct current and alternating current. In the automotive industry, we use the DC mode to measure direct voltages.
The image shows the battery voltage being measured. Between the null line (the black dash at the bottom left) and the measured voltage (the thick line above “A”), there are seven boxes visible. Each box is called a division.
Unlike the Picoscope, the total voltage range is not set, but the voltage per division. The voltage setting per division is set to 2 V/d (bottom left on the screen). This means each box represents 2 volts. With seven boxes between the null line and the signal, the voltage can be easily calculated: 7 × 2 = 14 volts. The average voltage is also displayed on the screen (14.02 volts).
Press the green button at the bottom left to turn on the oscilloscope. For a measurement, place the red test lead in channel A and the black test lead in the COM connection.
To measure a signal, connect the red test lead (channel A, plus) to the sensor’s signal connection or on the correct spot in the breakout box. Connect the black test lead (COM) to a good ground point on the body or to the battery’s ground. For a single voltage measurement, only channel A and the COM connection are needed.
When two voltage signals need to be compared, use channel B. In that case, place the test lead in connection B and enable channel B on the oscilloscope.
The oscilloscope has an “AUTO” button. This function automatically selects the best settings for the input signal. A disadvantage is that the signal isn’t always displayed correctly; the oscilloscope can continually adjust settings with a signal of variable amplitude (height) and frequency (width). Comparing voltage signals with different time settings can then become challenging. Therefore, it’s better to manually set the oscilloscope and perform multiple measurements with the same settings. The following paragraphs explain how to do this.
Fluke: setting the null line:
After turning on the Fluke oscilloscope, the null line is often automatically placed in the middle of the screen. With a setting of 1 volt per division, the range is only 4 volts above and 4 volts below the null line. This means that a voltage higher than 4 volts won’t fit entirely on the screen and will fall off the display-frame. To show the complete voltage range, the null line must be moved down. In the image, the null line is set at the lowest line of the screen.
With the null line at the bottom and the setting at 1 V/d, the oscilloscope can display a maximum voltage of 8 volts (8 × 1 = 8 V). This is suitable for measuring the supply voltage or signals from active sensors (maximum 5 volts), but insufficient for higher voltages like the battery voltage or the voltage across a lamp. For that, 2 V/d (for passenger cars) or 5 V/d (for commercial vehicles) are suitable settings.
Fluke: setting voltage and time per division:
As previously mentioned, the number of volts per division must be set correctly to ensure the voltage signal fits within the screen. Also, the correct setting of the time per division is essential. This paragraph describes how to adjust both settings.
If the number of volts per division is too low, the measurement falls off the screen. If the number of volts per division is too high, only a small signal is visible. For optimal measurement, the signal must utilize the entire screen.
In the image, the number of volts per division is adjusted with the button labeled “mV” and “V”. By pressing “mV” (red arrow), the time per division decreases, and by pressing “V” (blue arrow), it increases.
Setting the time per division allows adjustment of the measurement duration. With a setting of 1 second per division (1 s/d), the line moves one box every second. This is also visible in the voltage graph, where the line moves one division from left to right every second. Depending on the type of measurement, it may be desirable to adjust the time setting. When measuring the voltage pattern of an injector, the time setting should be lower than when measuring a duty cycle.
The time setting can be increased by pressing the “s” on the left side of the “TIME” button and decreased with the “ms” button. The time setting applies to both channel A and B; different time settings for channels A and B are not possible.
Fluke: setting the trigger:
When measuring voltages such as battery voltage, a trigger isn’t necessary. The battery voltage (as described in the “General” section) appears as a straight line, with divisions counted between the null line and the signal. This line remains constant unless the battery is charging or a user is activated, causing the voltage to drop over time.
When measuring a sensor signal, the voltage line will not be constant and shifts in height across the screen. Although the HOLD button can be used to temporarily freeze the image for further examination, this is not ideal. The button must be pressed at exactly the right moment, and once frozen, the screen shows no further changes in the signal. The trigger function provides a solution: by setting the trigger, the voltage signal is held at a specific point on the screen. The measurement continues so that signal changes remain visible as conditions change, such as fluctuations in engine speed or temperature.
The symbols for the trigger are as follows:
Trigger for the rising edge. This trigger function holds the voltage pattern at a point where it rises.
Trigger for the falling edge. This is the reverse symbol of the rising edge. This trigger function holds the voltage pattern when it first goes down.
Press the F3 button (see image) to move the trigger. Move the trigger up and down with the arrow keys. Change the trigger from rising to falling edge with the left and right arrows.
In the two lower images, the same voltage pattern is shown with two different trigger methods.
Trigger on the rising edge:
In the image, the trigger is set on the rising edge of the signal. As a result, the oscilloscope will hold the image steady as long as the sensor signal is being measured. Without setting the trigger, this signal would continuously shift across the screen.
Trigger on the falling edge:
Here, the trigger is set on the falling edge for the same measurement. The image clearly shows that the pattern is the same, but the signal has moved slightly to the left. This trigger function holds the image at the point where it goes down.
Of course, the trigger is not a way to pause the display. As soon as the measured object is turned off or the signal changes, the pattern on the screen will change with it.
This is evident in the image; the trigger remains at the same point, but the horizontal voltage line is now more than twice as long. The voltage of 1.5 volts (1500mV) is now active for 1104s (microseconds) instead of 454s in the previous measurement.
Fluke: enabling or disabling the smooth function:
Because the oscilloscope is very precise, some noise may always be visible on the screen. This can be distracting, especially when an accurate assessment of the voltage signal is required. To enhance the signal and reduce noise, the “smooth” function can be enabled.
The following measurement is taken at the signal output of the fuel pressure sensor, located on the fuel rail of the injectors of a common-rail diesel engine (indicated by the red arrow in the image here).
In the screenshots below, a clear difference can be seen between the images with Smooth disabled and enabled.0a
The Smooth function can be set by performing the following three steps via the settings menu that appears after pressing the “Scope Menu” button:
Fluke: enabling channel B:
When measuring signals, it’s often desirable to measure two signals relative to each other, such as the camshaft signal and the crankshaft signal in relation to time. The voltage pattern of both sensors is displayed neatly under each other, allowing conclusions to be drawn about the timing of the distribution.
To enable channel B, press the yellow button on the right side of the oscilloscope. Once the menu appears on the screen, use the arrow keys to select the desired option. Confirm the choice with the F4 button, which has “ENTER” indicated above it. Channel B can also be disabled in the same way.
In the images below, the menu appears after pressing the yellow button. In the left menu, “OFF” is selected under B. Use the arrow keys to set it to “ON.” Then, “Vdc” (direct current) must be selected, as shown in the right image. After confirming each option with “ENTER,” the menu disappears and channel B can be used for measurements.
After enabling channel B, the null line, just like channel A, must be set to the desired position. The time settings per division are for both channels simultaneously: it is not possible to make the voltage pattern of channel A and B different from each other.
Fluke: measuring with the current clamp:
With the oscilloscope, only voltages can be measured. Even when measuring with a current clamp, the oscilloscope will receive a voltage from the current clamp. This paragraph explains how to measure with the current clamp. To understand it better, here is an example using the multimeter.
The current clamp can also be used with a multimeter. Inside the current clamp is a Hall sensor which measures the magnetic field running through the jaws of the current clamp. This magnetic field is converted into a voltage of up to 5 volts.
Where the multimeter’s internal fuse can fail at currents above 10 Amps, the current clamp can measure currents of hundreds of Amps. The voltage the current clamp transmits is 100 times less than the actual current due to the conversion factor of 10 mV/A, which is also noted on the current clamp. Ensure the current clamp is in the first position and not on 1 mV/A (conversion factor 1000).
When the current clamp is connected to the multimeter’s volt input and the current clamp is switched on and calibrated to the multimeter reads 0 volts, the current clamp can be placed around the sensor or actuator cable. The measured value on the multimeter must be considered with the conversion factor: every millivolt indicated by the multimeter represents 1 Amp.
It’s easy to remember that the read value should be multiplied by 100. For instance, if 0.25 volts is displayed, the actual current is (0.25 * 100) = 25 Amps. If another measurement shows 1.70 volts on the display, the actual current strength is also 100 times higher, thus 170 Amps. In short, the decimal point moves two places to the right.
The previous example was measuring with the multimeter, to make recognizing it with the scope slightly easier. The same current clamp can also be connected to the oscilloscope. The red and black cables of the current clamp should be inserted into channel A (or B) and the COM connection of the clamp.
The oscilloscope is now set to Amps. First, calibrate the current clamp by turning the calibration knob until the oscilloscope reads 0 A.
When the current clamp passes a voltage of 0.050 volts, the oscilloscope automatically multiplies this by the factor of 100 since every 10 mV is actually 1 Amp. The oscilloscope display will then show 5 Amps.
The current clamp is very fast and allows for measuring even the current path of an injector. With the oscilloscope’s two-channel function, channel A can measure the voltage path and channel B can measure the current path. The voltage and current paths are neatly displayed one below the other.