This article explains the different options for measuring RPM with a tachometer with the Simcenter SCADAS RS hardware.
In this article, a tachometer is a rotational speed sensor. There are many types of tachometers including magnetic pickups, optical sensors, and lasers. Sometimes these speed sensors are also called encoders, especially if they have a high number of pulses per revolution.
An example of tachometer setup is shown in Figure 1 below:
Figure 1: Example of a magnetic pickup used with a gear to produce pulses proportional to the rotational speed of the gear.
The tachometer is pointed at a rotating shaft. In the case of the magnetic pickup, the metal gear teeth pass in front of the magnet and creates a series of voltage pulses spaced apart in time. The spacing of the pulses are proportional to the speed of the rotating shaft.
The tachometer device outputs a series of pulses that are fed into the Simcenter SCADAS RS hardware. The Simcenter SCADAS RS is programmed to interpret the timing between pulses to calculate the RPM of the rotating shaft.
Article contents include: 1. Analog versus Digital Tachometer 2. SCADAS RS Digital Tachometer Setup 2.1 Digital Tachometer on REC Unit 2.2 Digital Tachometer Recorder App Setup 2.3 Pulses to Skip 3. SCADAS RS Analog Tachometer Setup 3.1 Analog Tachometer on the U12 Conditioning Unit 3.2 Analog Tachometer Recorder App Setup 3.3 Pulse Threshold Detection 4. Viewing the RPM 5. Warnings: Too Slow
1. Analog versus Digital Tachometer
There are two types of pulses that a tachometer device can produce: analog and “digital" as shown in Figure 2.
Figure 2: Digital tachometer signal (left) versus analog tachometer signal (right)
The difference between a digital tachometer signal and analog tachometer signal are:
Digital: An analog voltage signal that only has two analog values to describe the pulses. Because there are only two values, the signal is described as “digital”. Ideally the two values are usually 0 and 5 volts, but the SCADAS RS has some tolerance for other voltages. This signal looks like a square wave and is sometimes referred to as a TTL signal.
Analog: The pulses can be of any amplitude and do not need to be square. The amplitude of the pulses can vary. A voltage threshold will be entered to detect the pulses whenever it is crossed.
Depending on which type of tachometer sensor is used, different SCADAS RS hardware is used:
Digital Tachometer: The SCADAS RS REC unit has nine digital tachometer inputs.
Analog Tachometer: Use the 12th channel of the SCADAS RS conditioning unit.
2. SCADAS RS Digital Tachometer Setup
This section covers the hardware and software settings needed to measure RPM from a tachometer that produces a digital series of pulses.
2.1 Digital Tachometer on REC Conditioning Unit
The REC conditioning unit of the SCADAS RS supports nine digital tachometers on three connections. Figure 3 below shows all the connections, the digital tachometers are labelled 1P, 2P, 3P in the illustration.
Figure 3: The SCADAS RS REC unit has three dedicated inputs (labelled DI1, DI2, DI3) for measuring “digital” pulses. Each input can support three separate tachometer devices or one incremental encode.
There is a open wire cable available from Siemens to use with the Digital Inputs (DI) on the SCADAS RS REC Unit. This wire is shown in Figure 4 below.
Figure 4: The digital input wire for the SCADAS RS REC unit (part number REC-DI-WIRE-3M).
The cable has several open wires that are all colored differently. The wires and their corresponding input pins are shown in Figure 5 below.
Figure 5: Pinout for the Digital Inputs of the SCADAS RS REC unit with corresponding wire colors.
For the three tachometers without any power supplied by the SCADAS RS, use the following cables:
Tachometer 1: For a single ended voltage use 7 (white) and 8 (brown). A- and A+
Tachometer 2: For a single ended voltage use 1 (green) and 2 (yellow). B- and B+
Tachometer 3: For a single ended voltage use 3 (grey) and 4 (pink). INDEX- and INDEX +
When using an incremental encoder (which has 2 high pulse per rev signals A and B and a INDEX signal) the first eight wires will all be used for a single encoder. The incremental encoder uses the A and B high pulse per revolution signals to determine the direction of rotation (clock wise or counter clockwise) of the shaft speed being measured. A regular tachometer cannot determine the direction of rotation.
2.1 Digital Tachometer Recorder App Setup
Simcenter SCADAS RS Recorder App (Figure 6) is used to enter the settings to convert the pulse signals from the tachometer into RPM.
Figure 6: Main menu of the Simcenter SCADAS RS Recorder App, the embedded software on the SCADAS RS hardware. Press the “Channels” icon to setup tachometer information.
Software settings include the input mode, number of encoder pulses, and the detection levels (for analog tachometer mode). To set up the input channels to measure a tachometer, start by clicking on the “Channels” icon in the main menu.
More information on getting started with the Simcenter SCADAS RS and the Recorder App in the knowledge articles:
After selecting “Channels” from the main menu, find the REC unit (REC1 – Digital Pulse) in the channel list tree as shown in Figure 7 below:
Figure 7: After turning on the first digital input on the REC Unit, change the “Conditioning” field to ‘Tacho (Digital)”.
Highlight the first Digital Input under “REC1 – Digital Pulse”.
Set the following:
“Measured Quantity” to “RotationalSpeed” (if not already set)
“Conditioning” to “Tacho (Digital)”
In the properties bar on the right of the screen, set “Encoder pulses per revolution” to the number of pulses corresponding to the encoder as shown in Figure 8.
Figure 8: In the “Pulse processing” section of the channel properties, set the “Encoder pulses per rev” to the number of pulses corresponding to the tachometer device.
Because this is a “Digital Tachometer” it is not necessary to set a voltage threshold for the detection of pulses.
To ensure the entered settings are correct, see the upcoming section about viewing RPM.
2.3 Pulses to Skip
Under the “Pulse Processing” setting in Properties, the “Skip Pulses” can be activated to smooth the RPM as shown in Figure 9.
Figure 9: Under "Pulse Processing" turn on "Skip Pulses" to be able to smooth the RPM. Set the "Pulse to Skip" to one less than the "Encoder pulses to rev".
By setting the “Pulses to Skip” to one less pulse the “Encoder Pulses per Rev”, the RPM estimated from the pulses is only updated once per revolution. Without “Skip Pulses”, the RPM is updated with each pulse.
When working with a high pulse per revolution encoder, using all pulses can result in a noisy rpm as shown in Figure 10.
Figure 10: RPM estimate from a high pulse per revolution encoder. The blue trace is based on estimating the rpm value for every pulse. The green trace estimates the rpm once a revolution ignore all pulses in between using the “Skip Pulses” feature.
Whether to use the “Skip Pulses” feature depends on the desired use of the high pulse per revolution encoder. If it is to simply measure the overall rpm, then using the “Skip Pulses” is useful. If understanding the torsional rpm fluctuations within each revolution important, do not use “Skip Pulses”.
The "Pulses to Skip" feature works with both the analog and digital tachometer signals.
3. SCADAS RS Analog Tachometer Setup
This section covers the hardware and software settings needed to measure RPM from a tachometer that produces a analog series of pulses.
3.1 Analog Tachometer on the U12 Conditioning Unit
The U12 conditioning module comes with one dedicated input (labelled 12P, item 2) for analog tachometer signals as shown in Figure 11 below.
Figure 11: The SCADAS RS U12 conditioning unit has a dedicated analog input tachometer called “12P”.
The cable has three open wires that are all colored differently. The wires and their corresponding input pins are shown in Figure 12 below.
Figure 12: Pinout for the 12P dedicated analog tachometer input on the SCADAS RS U12 conditioning module.
In the Simcenter Testlab Recorder App software, after switching channel 12 on the U12 to “Tacho (Analog)” mode, use the channel labelled "12P" rather than the number "12" only.
3.2 Analog Tachometer Recorder App Setup
To setup the analog tachometer on the SCADAS RS U12, log into Simcenter SCADAS RS Recorder App.
From the main menu, click on the “Channels” icon shown in Figure 13.
Figure 13: To setup an analog tachometer, choose the “Channels” icon from the main menu of the SCADAS RS Recorder App.
In the Channel list, under the U12 conditioning unit, select the twelfth (last) channel on the unit. Set the conditioning to “Tacho (Analog)” as shown in Figure 14:
Figure 14: Set the conditioning to “Tacho (Analog)” on the 12th channel of the U12 conditioning unit.
Under “Pulse processing” enter the “Encoder pulses per rev” as shown in Figure 15.
Figure 15: Enter the "Encoder pulse per rev" to match the number of pulses per revolution on tachometer device being used.
Under “Pulse detection” there are additional settings for analog tachometers as shown in Figure 16.
Figure 16:Additional settings for a analog tachometer include "Pulse Detection Level", "Pulse Hysteresis", and "Pulse hold-off".
Because the voltage pulses of a analog tachometer could be any level, the detection threshold can be set manually using the "Pulse Detection Level" under the "Pulse Detection" setting area.
The "Pulse hysteresis" and "Pulse hold-off" are used to help ignore spurious crossings in the tachometer pulse signal. The hysteresis gives the threshold a width to cross, while the hold-off ignores any crossings of the current time between two pulses within the percentage given of the previous crossing pair time interval.
4 Viewing the RPM
To verify all tachometer settings were entered correctly and the rpm is being read, go to the “Monitor” view of the Simcenter SCADAS RS Recorder App.
The “Monitor” icon can be found on the left side of the screen (Figure 17) or from the home screen.
Figure 17: RPM runup and rundown in the Monitor view of the SCADAS RS Recorder App.
After selecting “Monitor”, choose the “Strip” display to see the evolution of the RPM over time.
Under the “Custom” tab (Figure 18), select the type of display for viewing the rpm data.
Figure 18: In the “Custom” tab there are multiple displays to select from: Line, XY, Numerical, Analog, and Video.
Possible display choice include: Line, XY, Numerical, Analog, and Video.
Select “Analog” to view the RPM on a gauge as shown in Figure 19. Select the Digital Input RPM channel (or other any other desired channel) to view as a gauge.
Figure 19: The analog display for RPM in the Simcenter SCADAS RS Recorder App.
Clicking on the vertical “…” symbol in the lower right corner, the display properties can be adjusted to have different colors for different limits.
The vertical “…” can also switch the display from an analog gauge to a numerical read out (Figure 20).
Figure 20: Numerical display of RPM from tachometer.
After the measurement is finished, the resulting time file will have a rpm versus time channel. The original pulse train is not saved. If the a recording of the original pulse signal itself is desired, the signal should be split into a data channel and recorded in parallel.
5. Warnings:Too Slow
In the “Channel” or “Monitor” area of the SCADAS RS Recorder App, a “Too Slow” warning may appear on the tachometer channels as shown in Figure 21.
Figure 21: The “Too Slow” warning appears in the SCADAS RS Recorder App is no pulses are detected on the tachometer input channel.
“Too Slow” message means that no pulses are detected. Possible reasons are:
Cable may be broken or the wiring might not be connected properly.
The rotating machinery may not be running, hence no rpm can be read.
For the analog tachometer, the voltage threshold may not be set properly such that the signal crosses the threshold.
