Using Reference Curves for Time and Frequency Signal Comparison in DewesoftX

Course overview

This course dives into creating and leveraging Reference Curves—an essential DewesoftX tool for comparing current measurements against predefined limits in multiple domains. You’ll explore:

1. Time Reference Curves: Learn to define curve points (time vs value), set start conditions (e.g., vehicle speed > 2 km/h), and visualize them on recorders. Use additional Boolean output channels to flag out-of-bound conditions.

2. XY Reference Curves: Define polygonal or custom shapes in XY data space (e.g., shaft X vs Y probes). Dewesoft outputs a status signal indicating if the signal stays within set boundaries, ideal for real-time monitoring or fault detection.

3. Frequency-Domain (FFT) Reference Curves: Use FFT-based curves to define amplitude limits across frequencies—handy for tuning fork verification or spectral limit checks. Signals breaching limits can trigger warnings or alarms.

4. Vector Reference Curves: Perfect for array data (such as cylinder pressure vs crank angle), supporting upper/lower bounds, interpolation, multi-band checks, and generating boundary-crossing outputs.

Hands-on modules will guide you to import reference curves from recorded sessions or external spreadsheets, use copy-paste workflows, and integrate real-time limiting logic into dashboards or automated systems. The boundary-check channels can be used in formulas, event counting, triggering, or logging alarms.

By the course’s end, you’ll be equipped to define multi-domain reference limits, flag deviations automatically, and integrate signal compliance monitoring into your measurement workflows.

Video overview

The reference curve is defined by the user. Reference curves allow us to compare different types of signals (time domain, frequency domain) to the reference signal and determine whether the signal is above or below the reference limit.

The available reference curves in DewesoftX are:

• Time reference curve

• XY reference curve

• Frequency-domain reference curve

• Vector reference curve

With a Time Reference Curve, the reference curve is defined in the time domain and displayed on the recorder. It is used for monitoring time-domain data. The Time Reference Curve can be added under math functions: Add Math → Time Reference Curve.

Time based Reference curve

The Time-Based Reference Curve is useful for defining a curve in the time domain as a reference during a specific test that must follow a defined protocol. For example, consider a test where the vehicle must accelerate to 100 km/h in 10 seconds, drive at a constant speed for 10 seconds, and then decelerate to 0 km/h in 10 seconds.

Each curve requires starting criteria. If the Start Condition channel is not defined, the curve will begin at the start of the measurement. A common approach is to define the start of the test on a channel that measures vehicle velocity. In this case, a limit must also be defined—the value above which the measurement will begin. (This field appears only when a Start Condition channel is selected; the default is empty.) In our example, we could set this limit to 2 km/h.

The next step is to define the Number of Points and specify the points in the list. In this example, we would have four points, so this value must be entered in the field.

Single value based Reference curve

A Single-Value-Based Reference Curve can be understood as a form of non-linear scaling. To define it, you must specify a First Reference Channel, the Number of Points, and a table containing the values of the reference and the output channel.

For example, in Image 4, if the input channel value is 3, the output value will be 5. Values between points are interpolated. Thus, if the input is 2.5, the output will be 3.5. If the value is below the lowest point or above the highest point, the data will be extrapolated using the two lowest or highest points. For instance, if the input is 0, the output will also be 0; if the input is 5, the output will be 11.

In the upper-right corner, a preview diagram of your inputs is displayed.

Dual value based reference curve

A Dual-Value-Based Reference Curve has two inputs: the First Reference Channel and the Second Reference Channel. It can be visualized as a three-dimensional surface, where the X and Y axes represent the two reference channels, and the Z axis represents the defined points. First, the channels and the Number of Points for each channel must be specified.

Next, enter the reference values in the list. The display in the upper-right part of the Reference Curve Setup window will now show the curve.

A good example of using a Dual Reference Curve is defining the oil pressure limit referenced to both RPM and oil temperature.

From data file reference curve

A Time Reference Curve can also be imported from another data file.

Let’s say we have a test where the vehicle must accelerate to 100 km/h in 10 seconds, drive at a constant speed for 10 seconds, and then decelerate to 0 km/h in 10 seconds.

In this example, we can set the start condition to 2 km/h. The table below shows how the vehicle should behave.

The next step is to define the number of points and the points themselves. In our case, we have four points.

We can display the time reference curve on the recorder. Then, by using the Formula Math channel, we can check whether the signal is above or below the reference.

1if('Vehicle_Acc'>'Reference5',1,0)

This formula returns a value of '1' if the signal from the vehicle is above the reference curve, and '0' if the signal is below the limit.

The XY Reference Curve math module provides the reference curve for the XY recorder. It can be added in the same way as the Time Reference Curve, under Add Math → XY Reference Curve.

When you open the setup for the XY Reference Curve, the window shown in Image 9 will appear.

After selecting the input channels for the X and Y axes, the Number of Points for the XY Reference Curve must be defined. The reference points can either be entered manually, value by value, or transferred using the copy-paste function. The Copy button copies all points to the clipboard in a tab-delimited format, making it easy to paste them into any spreadsheet software. Values can also be copied from other spreadsheets using the Paste command, provided the data is tab-delimited and each line is terminated with a carriage return/line feed character.

For example, in the following MS Excel screenshot, we selected a data range and used Copy. The data can then be easily pasted into DewesoftX.

Another advantage of the reference curve is that the output channel (in our case, Status) returns a value of 1 when the XY curve crosses the reference curve. This can be used as a trigger criterion or for event counting (using the ECNT function in the Formula Editor).

In the example below, two Eddy current proximity probes are used to measure displacement. While positioning the probes around the shaft, the shaft’s movement can be observed. In the Reference Curve setup, a rectangle was defined to represent the maximum allowed movement.

With the XY Reference Curve displayed in the XY recorder, we can monitor the movement of the shaft and check whether it remains within the allowed boundaries.

The FFT Reference Curve math module provides the reference curve for the FFT display. It can be used to display the reference on the FFT screen and to provide a math output channel that goes high when the defined levels are exceeded.

The FFT Reference Curve can be added under the Math section: Add Math → Frequency-Domain Reference Curve.

In the Channel Setup, you can define the maximum number of lines, the window type, and the overlap.

These values are used for the calculation of the FFT spectrum. Next, the limits (limit table) must be defined. They can be entered manually, value by value, or taken from the current data.

Get Current Data means that the current FFT (in the setup screen) will be taken as the reference. An offset can then be applied to that reference using the Up/Down and Left/Right arrow buttons. Similar to the FFT trigger, this will raise or widen the current transfer curve.

The FFT Reference Curve can be used in the FFT display to show reference lines, while the output channel (named Status in the screenshots) indicates whether the current data is above or below the defined reference.

Let’s take the example of a tuning fork. With the reference curve, we can check whether the tuning fork is working correctly. The frequency of a standard tuning fork is approximately 440 Hz. A reference curve can be created with the allowed amplitude and frequency range.

Add an FFT display, select the channel (Tuning Fork), and display the FFT Reference Curve. The reference curve can be obtained during the measurement using the Get Current Data option.

During the measurement, the time signal can also be observed on the recorder, along with the FFT of the signal, as shown in Image 18. If the FFT signal from the tuning fork (green) is below the FFT Reference Curve (red), we can conclude that the tuning fork is working properly.

In a Vector Reference Curve, the reference curves are defined for the input channel, which is usually an array (e.g., FFT data).

The Reference Curve Type can be selected from:

• Lower – defines the lower limit

• Upper – defines the upper limit

• Lower & Upper – defines both the lower and upper limits

With linear interpolation, the program draws a straight line between two neighboring points, providing a smoother transition between points on the curve..

Check Signal Bounds creates an additional output channel that outputs '1' when the input is higher than the reference curve and '0' when the input is below the reference curve.

Axes Settings can be selected from:

• Linear

• Logarithmic

• 0 dB – the dB reference is set from the range of the first channel

• Noise dB – the reference value is 20 µPa

Next, we define the points in the limit table. The points can be entered manually or copied and pasted from an external program.

Channels can be renamed by clicking the icon in the top-right corner and selecting the Change Axis Name/Unit option.

When pasting data from an external program (e.g., Excel), ensure that the correct axis names are used.

Create a Reference Curve math function with Add Math → Vector Reference Curve.

Open the settings for the Vector Reference Curve and select the channel to be used for level checking—for example, CylPressure.

Linear Interpolation provides a smoother transition between points on the curve. Check Signal Bounds creates an additional output channel that outputs 1 when the input is higher than the reference curve and 0 when the input is below it.

The reference curve is defined using a table. The first column represents the crank angle (X-axis), and the second column represents the amplitude of the reference (Y-axis). In this example, only two points have been defined, each with a value of 41 bar.

We added a CA scope and selected the cylinder pressure signal along with the reference curve for display. An indicator lamp was also added, and the output channel from the Vector Reference Curve was assigned to it. When the pressure exceeds the reference curve, the indicator lamp turns red; otherwise, it remains green. The output from the channel switches from 0 to 1 when the reference axis is crossed.

A simple reference curve can be modified, and it is also very easy to create a reference curve from a recorded pressure signal. For this, a data file containing the pressure signal is required, which will then be used as the reference.

First, add Basic Statistics in the Math section.

Open the Basic Statistics setup and select the pressure channel from the Combustion Analysis module (the Combustion Analysis module must be added to work in the crank-angle domain instead of the time domain). Select Maximum to obtain the highest value across all cycles, and under Calculation Type, choose Time-Based and Single Value.

When this new channel is added to the CA scope, it displays the maximum pressure value across all cycles.

Now we need to create an offset curve that will be used as a trigger. For example, we can create a curve that triggers the Vector Reference Curve output channel when the maximum pressure exceeds 110%.

To achieve this, create a new Formula in the Math module, where the previous Basic Statistics channel is multiplied by 1.1.

1'ABS MAX Pressure'*1.1

The next step is to display only the most recently created formula on the CEA scope and copy the data to the clipboard.

Reopen the Vector Reference Curve setup and simply paste the values

It is also possible to reduce the number of points for the reference curve in Excel.

Copy the data to Excel and use the formula =OFFSET(Sheet2!A$1,(ROW()-1)*100,0) to output only every 100th line. This makes modifying the reference curve much easier, since only 37 points will remain instead of 3600.