To measure the rotational speed of a rotating piece of machinery, a tachometer or encoder device is often used. These devices measure the time between pulses from multiple stripes or ridges evenly laid around the circumference of a rotating shaft. The timing between the pulses is used to calculate the rotational speed of the shaft in revolutions per minute or rpm.
Problems can arise when even a single stripe or ridge is missing. This causes the rpm calculation to not be correct as shown in
Figure 1: A rpm versus time trace shows periodic downward spikes in rpm due to one missing pulse per revolution on an encoder signal.
In this article, the
Simcenter Testlab missing pulse correction algorithm and its settings will be discussed.
1. Background 2. Magnetic Sensor Example 3. Simcenter Testlab: Post-Acquisition Correction 4. Missing Pulse Correction Settings 4.1 Number of Pulses 4.2 Number of Missing Teeth 4.3 Cross level 4.4 Slope 4.5 Missing Pulse Correction 4.6 Pulse Correction Factor 5. Simcenter Testlab: Correction During Acquisition
Measuring rotating speed can be done using a tachometer sensor with varying levels of detail or resolution depending on the insight needed for a given rotating shaft. When more pulses are measured per revolution, more detail and insight is given into causes of potential speed variation of the given shaft (
Figure 2: Multiple pulses per revolution of a shaft (top, red) are converted to rpm (bottom, green) which shows rpm variations over time.
Often, for torsional vibration or angle domain measurements, more pulses per revolution are required to get a detailed analysis the of the rotational speed variation under different operating conditions. These multiple pulses per revolution need to be evenly spaced in order to not introduce artificial speed changes that could be misinterpreted as a
imbalance issue, or other operational defect.
2. Magnetic Sensor Example
There are many types of rotational speed sensors. A common sensor is a magnetic tachometer sensor (
Figure 3. Examples of Magnetic Tachometer Sensors.
Magnetic tachometer sensors output a voltage proportional to the distance to a metal surface. This can be particularly useful when mounted aligned with the teeth on a metal gear as shown in
Figure 4: Magnetic pickup sensor used with gear teeth.
As the gear teeth pass the magnetic tachometer sensor, a pulse is registered to the data acquisition system. Some major benefits of this setup are that multiple pulses per revolution are measured based on the number of gear teeth on the referenced gear as shown in
Figure 5: Magnetic pickup sensor used with gear teeth (left) produces a series of voltage pulses (yellow, right).
If multiple pulses are measured, the subtle variations of rotational speed within one revolution of the shaft can be measured and analyzed.
However, when collecting speed information, if there is a missing tooth the rotational speed will not be correct. The missing tooth creates an extra gap, or larger spacing, between measured pulses as illustrated in
This gap causes the calculated rpm to appear to momentarily slow down. An example showing the effect of the missing tooth on the calculated rpm is shown in
Figure 6: Incorrect calculation of rpm due to missing pulse gap over five rotations. Should be around a constant 3000 rpm. Significant decreases measured in rpm speed based on longer gap between measured pulses occurs once per revolution.
This calculated decrease or variation in speed due to the missing pulse can be corrected using Simcenter Testlab. This correction can be done through post-processing methods or during live data acquisition as described in the next sections.
3. Simcenter Testlab: Post-Acquisition Correction
In order to correct for a missing pulse in a tachometer signal offline, use the
Time Signal Calculator. From the Simcenter Testlab main menu, select Tools -> Add-ins to turn it on (
Figure 7: Time Signal Calculator Add-In in Siemens Simcenter Testlab.
token licensing, 26 tokens will be occupied while the add-in is active. Click on the Time Data Selection workbook to use it.
Once this add-in is turned on, the user can access the TACHO_PULSE_TO_RPM function in the Time Data Selection Worksheet as shown in
Figure 8: TACHO_PULSE_TO_RPM function located in Time Signal Calculator.
After selecting this function, the TACHO_PULSE_TO_RPM menu appears. It is shown in
Figure 9. The TACHO_PULSE_TO_RPM menu with parameters for missing pulse correction highlighted in yellow.
The parameters in this menu are used to convert a series of pulses to a rpm. The parameters highlighted in yellow in
Figure 9 are the ones needed for missing pulse correction.
The highlighted parameters effect how a missing pulse is corrected for in the resulting rpm calculation. They are described in the next section.
4. Missing Pulse Correction Settings
As an example, a tachometer pulse signal from a gear with 36 teeth and one missing tooth has been measured using a magnetic pickup. The voltage signal from the sensor is shown in
Figure 10: Raw pulse signal measured for 36 tooth gear with one missing tooth (i.e. 36 pulses with one missing pulse) on a gear. This is the voltage output of a magnetic pickup.
The settings to calculate the rpm while accounting for the missing tooth will be illustrated using this signal.
4.1 Pulses per Revolution
The pulses_per_rev parameter tells the software how many pulses to count to determine one revolution. If a tooth is missing, this number needs to be entered as the of teeth (or pulses) that would have been present if there was no missing pulse (or tooth). In this example, that parameter is 36 , meaning there are 35 physical teeth and one missing tooth.
4.2 Number of Missing Teeth
The number of missing teeth (or pulses) also needs to be identified. The space between teeth on the reference gear can be equal to one, two, or possibly more missing teeth (or pulses). The Simcenter Testlab software correct for up to four missing pulses. For this example, that number is 1 because there is one missing tooth.
4.3 Cross level
The signal must cross the specified value to create a pulse count. In this signal from the magnetic pickup, the signal is centered around zero volts. A crossing level of zero is used. Other types of signals, for example a TTL signal, ranges from 0 to 5 volts. For such a TTL, a crossing level of 2.5 volts would be appropriate.
When the pulse signal crosses the detection level, it can be with either an upward (positive) or downward (negative) slope. For a gear, this would correspond to the leading edge or trailing edge of the gear tooth. For this data, setting the slope parameter to negative versus positive shows a difference (
Figure 11: For this example signal (red, top) from a magnetic pickup on a metal gear, a negative slope (blue) setting provides a better estimate of rpm than the positive slope setting (green). A uncorrected estimate of the rpm (red, bottom) is also shown.
For this signal, a negative slope yields a more reasonable estimate of the rpm than a positive slope.
4.4 Missing Pulse Correction
It is common that the pulse immediately following the missing pulse can also be distorted as shown in
Figure 12. Both the missing pulse and the pulse immediately after both need correction.
Figure 12: The pulses described by the voltage output of a magnetic sensor due to a missing tooth. Not only is one pulse missing, the pulse immediately following is distorted in amplitude and timing.
The missing_pulse_correction setting is used to either correct the missing pulse only, or correct the adjacent pulses. With this example, the following settings were tried:
10 : 1 missing pulse corrected
12 : 1 missing pulse, pulse corrected
The results are shown in
Figure 13 below:
Figure 13: Signal (red, top) from a magnetic pickup on a metal gear with missing tooth. Correcting for just the missing pulse (green) does not fully correct the signal. A better correction is achieve by also correcting the pulse after the missing pulse (blue). The uncorrected rpm (red, bottom) is also shown.
In this example, without also performing a correction for the pulse after the missing pulse, the rpm would not have been fully corrected.
The Simcenter Testlab software can correct for more than just the missing pulse and the pulse after. Other options include the pulse just before, and considering multiple missing pulses.
4.5 Pulse Correction Factor
The pulse_correction_factor setting influences the sensitivity of the detected missing pulse. Possible settings are integer numbers 1, 2, 3, and 4.
The chart in
Figure 14 explains these settings.
Figure 14: Chart showing possible 'Pulse Correction Factor' settings.
The 'Pulse Correction Factor' tries to determine if a pulse is missing by looking at the time between pulses (T
n-1) from the previous crossing set. It compares it to the time (T
n) between crossings of the current crossing pair.
If a pulse is missing, than T
n will be greater than T
n-1. However, this may be due to the rotational speed of the shaft actually slowing down, or could be due to a missing pulse. If no pulse occurs within the percentage of the previous crossing time as specified in
Figure 14, than a pulse is considered to be missing and a correction needs to be applied.
5. Simcenter Testlab: Correction During Acquisition
Simcenter Testlab Signature Acquisition, within the Tracking Setup workbook, the user can click on the More icon located next to the pulses per revolution setting to access the menu shown in
Figure 15 below.
Figure 15: Tacho Settings in Simcenter Testlab Signature Acquisition for missing pulses.
In the Tracking Setup workbook, the user can define the number of pulses per revolution and add a missing pulse correction if necessary. Here the parameters already discussed, such as the pulse correction factor, number of missing pulses, slope, smoothing factor, and other settings can be entered also.
This feature allows for the user to correct for a missing pulse in the RPM measurement during live data acquisition online. Keep in mind that if the corrections made here are not correct, the raw tachometer pulse train acquired can be saved and evaluated offline as shown previously in this article.