This guide, authored by our expert Steve Smith, will show you how to use a simple math channel in PicoScope 7 Automotive software to calculate road speed directly from a vehicle’s ABS wheel speed sensor signal. This powerful technique provides a new level of diagnostic insight, allowing you to quickly troubleshoot ABS issues and verify sensor performance. We’ll use a real-world case study of a Subaru with an ABS warning light to demonstrate the process.
The Math Channel formula: How it works #
We’ll break down the formula required to convert the frequency of the wheel speed sensor signal into a road speed value. By inputting vehicle-specific information like tire diameter and gear ratio, you can create a custom math channel that displays a live graph of road speed in either miles per hour (mph) or kilometers per hour (km/h). This eliminates guesswork and provides a clear, visual representation of the sensor’s output.
Step-by-step: The Subaru ABS case study #
Follow our step-by-step walkthrough to see how this diagnostic method helped solve a tricky ABS problem. From setting up the PicoScope to applying the formula and analyzing the final waveform, you’ll learn a practical skill that can be applied to a wide range of vehicles.
Case study – Subaru with incorrect ABS operation
Using a math channel to calculate road speed from an ABS wheel speed signal could be very useful in the future for those faced with such a diagnosis or those who wish to qualify vehicle speeds against serial data.
Using a tyre size of 215 x 45 R17 and an ABS pole/tooth count of 48, we require:
- ·Number of teeth or pole count at the ABS pickup ring. (Physical count = 48)
- ·Tyre circumference. = Diameter (0.625 meters) x π (3.14)
- ·The number of wheel/tyre revolutions in one minute. 60/48 poles x signal frequency = RPM
The formula for wheel speed in miles per hour is:
- Wheel/Tire Speed (Frequency Hz x 60 = RPM) x Tyre Circumference (Meters) = Meters per minute
- Meters per minute x 60 = Meters per hour
- Meters per minute / 1000 = Kilometres per hour
- Kilometres per hour x 0.621371 = Miles per hour
Math channel formula required to calculate mph from the wheel speed signal on channel B:
60/48*freq(B)*(3.14 * 0.625)*60/1000*0.621371
To create an MPH math channel for the wheel speed signal on channel B, open the Math Channel Wizard by clicking on More… > Math Channels > Add > type in the formula above > Next. Follow the Math Channel Wizard to completion by selecting a color, measurement unit (mph) and range for your math channel. To help with the recognition of your created math channel, select a matching colour for the channel input to be graphed. (E.g. Channel A frequency math channel Blue, Channel B frequency math channel Red etc.)
The images below highlight the math channel formula entry procedure
Should you require km/h as your desired unit for road speed, assuming your ABS signal is on channel A, your ABS pick up has 48 teeth and a tyre size of 215 x 45 R17 the formula would read as follows:
freq(A/48)*(3.14*0.625)*60/1000
The .psdata file below has the maths channels for mph in red and blue, whilst km/h can be seen in the magenta channel
Using the .psdata file in this article, you can import the math channels into your own library for later use and edit accordingly.
Open the .psdata file and click on any of the Math channels. You should be able to save each Math channel, and then you can modify each to suit your specs.
Download the .psdata file here
Ready to take your diagnostics to the next level? Explore the full range of Pico Automotive oscilloscope kits here and discover how they can help you solve complex problems with confidence.