The DS-TACHO4 sensor is a threshold sensor. This is important especially for the proximity detection mode, the most commonly used for rotating: working distance could change with the albedo and/or the form and distance of the target, also, contrast appears as an important parameter: teeth-no teeth, black and white marks. The recommended distance for encoding application is a few millimetres: put the probe closed to the target to avoid an incorrect reading resulting from rocking and wagging of the turning part (Descartes optical law); on the other hand, the reflective tape allows for much more than 100 mm. It is highly recommended that you use the adhesives encoders for optimal results.
A few phenomena may affect the detection function, such as a drop of liquid on top of the probe, excessive dust covering the top, more generally, a non-transparent environment for our light source such as: diesel engine sump film ( i.e. carbon is not transparent for the near I.R.). Patented concept implemented in the sensors strongly simplifies mounting and set-ups. Prior to measurement, it is recommended that a detection test is performed, even at low speed, to ensure detection feasibility and determine detection distance required for the sensor.
If impossible to perform a test due to technical reason or mounting specifics, a theoretical method would be to fix the probe at a distance equivalent to the width of the black and width strips to detect- in any event, without exceeding 4 mm.
Fixing and support of the probe will influence the acquisition of the reading. Please be careful regarding vibration. We recommend that you design your supports including appropriate vibration orders studies. The further the probe will be away from the target, the more the TTL amplitude signal will decrease.
Mounting the probe
- Ensure that you have all items required at your disposal, i.e. the sensor, the probe, and the two hand-pieces for optical fixation
- Put the two hand-pieces down if they are on the optical head of the sensor
- Insert the two optical fibres with their respective rivets
- Screw the first hand-piece on and tighten moderately; a little gap between the rivet head and the optical head is normal
- Remove the two fibres in order to allow for mounting of the second hand-piece
- Make sure that the two fibres and their rivets are assembled correctly
- Hold both probe and sensor simultaneously when inserting the rubber sleeve to avoid damaging the two optical fibres on the level of the rivets.
Adjusting the probes
The operational mode of the sensor can be seen at the end of the optical fibre by a light beam (not dangerous), which is emitted when the sensor is in “1” mode and not emitted when the sensor is in “0” mode. The sensor keeps its wavelength in near Infra -Red to ensure power and immunity of the detection function. This also gives an indication of the condition of the optical fibre.
The sensor should be placed about 2 to 5mm above the tape. A sensitivity potentiometer is available to adjust the trigger level for reliable pulse output.
First turn the potentiometer in mid position. Bring the probe closer to the target until the indicator at the head lights up, targeting the white mark. Shift the probe, and repeat this operation in order to detect the triggering limits on the black marks of the target. Set up the probe in an average position (length), review this operation to confirm the accurate detection: the set up is finished.
Automatic gap detection
When applying the black/white tape to the rotating shaft there will be an irregular rasterization at the transition point. This can be used as the zero pulse to indicate a defined start position. On the other hand, this would result in an rpm drop or spike in our rpm measurement.
A software procedure automatically measures the pulses per revolution and also detects the exact gap length to enable robust and high-quality measurement.
The zero pulse must be at least 3 pulses long!
Power supply must be perfectly rectified, filtered, and constantly deliver more than 120mA /12V.This is not an “open collector” output sensor, but PNP output. 152 G7 can support reverse tension, this tension modifys signal’s Amplitude. 152 G7 TTL Voltage output is 5 Vcc , 152 G7 Voltage output is nominal voltage input -1.5Vcc. If the sensor is connected to the acquisition system the use of dedicated measurement connectors and matching cables is recommended. Please refrain from extending the cable. Otherwise, the sensor’s operation may be affected. To confirm that the sensor is live, check if a faint red LED glows on the small light channel in front of the sensor optical head; You can also use a digital camera to see the I.R. Light. The brightness of this small red light is independent of the position of the potentiometer.
- V rating: 12/24 Vcc
- V Minima: 10 Vcc
- V Maxima: 30 Vcc
- I: 120 mA/12Vcc
Connector type: L1B7f
We have to detect the frequency drop, so that gap is seen, and software can calculate start and stop of the angle(0 to 360deg.) So in case the sampling rate is lower than input frequency of TACHO probe the gap could be missed.
Lets assume we have about 64 pulses/rev the machine is running with 1000 RPM. 1000rpm/60= 16rps = 16Hz.
Per one second, we would get: 16Hz * 64pulses/rev= 1024Hz input frequency.
In the example below the sampling rate was set to 1kHz, so we could see that the gap was not recognized at every revolution. In this case the sampling rate must be at least 2 times higher. 1024Hz *2 is about 2kHz, because speed of the machine could go up we also have to consider that. We should set it at least to 10kHz.
Sampling rate > Maximum input frequency * 10
For measuring RPMs and angle of rotating machines, we need angle sensors. RPM and angle measurement is important in balancing, order tracking and rotational and torsional vibration.
We need to choose a rpm sensor that is convenient for our measurement. Not all of the sensors can be installed in our rotating system and sometimes it takes a lot of effort to install then. Also, we have to choose the sensor that has good resolution for our purpose (e.g.: sensor with one pulse per revolution is not appropriate for measuring precise angle).
Tape sensor is an optical sensor for measuring speed and angle. It uses black and white tape that is attached to the rotating part of a machine.
The sensor is made of optic fibers and should be placed about 5 mm (or less) above the tape. We have to use a sensitivity potentiometer to adjust the trigger level to such a level which gives us steady pulses. The reflection is then converted with an electronic circuit into a TTL signal. The sensor in connected directly to an LEMO counter input.
Tape sensor can be used in many applications: RPM measurement, angle measurement, order tracking, rotor balancing, rotational and torsional vibration.
Tape sensor setup
First we glue the tape (with black and white stripes) onto our rotating part. If both ends of the tape would come perfectly together we would have no zero pulses per revolution, which are an indication of the start position. If we don't have the information about start position, the angle would be different at every start of a measurement.
In the picture above we can see the transition point of a tape - we use that as the ZERO pulse. This is an indication of a new revolution so the angle will start all the time at this position - angle information related to shaft will be the same.
In the picture below we can see the drop in frequency where we have the zero pulses. The drop is seen nicely so we could use that to detect the ZERO pulse. Angle will always start at that position. For the software to clearly see this drop or peak, the length of the gap must be more than 3 pulses. So the software will have no problem detecting ZERO pulse because the frequency will drop for 70%.
We have to adjust the trigger levels to get reliable pulses from the optical sensor. Trigger level has to be set after the sensor is mounted because it depends on a distance to the tape.
Sample rate must be high enough to detect the frequency drop and that the gap is seen so the software can calculate a start and stop of the angle.
Example: We have 64 pulses per revolution and machine is running at 1000 rpm - 1000rpm/60 = 16 Hz.
Input frequency is: 16 Hz * 64 pulses/revolution = 1024 Hz. If the sample rate would be set to 1 kHz, the gap would not be recognized at every revolution.
Sampling rate must be at least ten times higher than the maximum input frequency.
Defining sensor type
When we do a RPM measurement we have to select Sensor mode in Counter setup in Dewesoft X.
When Sensor mode is selected, we select our sensor from the Counter sensor database, where different types of sensors and their setting are already stored.
If we are using a sensor that is not yet in Counter sensor database we have to define it.
We go to Settings -> Counter sensor editor or just click on to enter Counter sensor editor:
In Counter sensor editor, we add a Tape sensor as sensor type. I renamed it Tape_sensor. When we click Save&Exit the sensor is added to Counter sensor database and is ready to be used.
We created a tape sensor that can now be selected from the dropdown menu in counter channel setup:
For a precise measurement, we have to know how many pulses per revolution we get from tape sensor and how many pulses in the gap wide. We shouldn't count that manually, there is a function called Detect gap - it will automatically measure the pulses per revolution and detect the gap length.
For the gap length calculation the rpm should be as stable as possible. So try to operate the machine in a stable area, so that rotational vibration (rpm deviation) is as small as possible.
The algorithm will average the speed of the machine a few samples before and after the gap, so the average speed around the gap is extracted, and from that we can calculate the missing pulses.
Please be aware that we are in setup, so Dewesoft X is running with the setup sampling rate, and if that is not high enough, like described in 3.4 gap and teeth detection will not work.
Output channels of a tape sensors are angle and frequency channels. Angle runs from 0° to 360°, frequency channel can be seen in RPMs of in Hz.
On recorder we can see angle in range from 0° to 360° (when tape is rotating angle values increases, when ZERO pulse is passed, the angle value returns to 0) and frequency channel in rpm. The rpm channel (green curve) is not a straight line because our rotor was not balanced. So we can use the tape sensor for balancing rotary parts.