Tag Archives: Measurement

How do Sensors Measure Gear Tooth Speed and Direction?

Measuring speed of gears is an important factor in various industries, especially in pharmaceutical, tobacco, printing, woodworking, paper, textile, food and others where rotational machinery predominates. Gear speed measurements also necessary in pumps, blowers, mixers, exhaust and ventilation fans, wheel-slip measurement on autos and locomotives, flow measurement on turbine meters and many more.

The most common gear tooth sensors detect a change in the magnetic field for determining the speed and direction. Usually, these are of three types – the Hall Effect, magneto-resistive and the Variable Reluctance. There are optical types of sensors as well, detecting a change in light levels as the gear rotates past the sensor.

Sensors using magnetic properties are good for measuring speed and direction of gears made of ferrous metals. All these sensors are non-contact type and sensitive to detect the presence of gear teeth passing in front of the sensor. As a gear tooth comes close to the magnetic sensor, its output flips and the electrical level at its output changes state. The output remains steady as long as the gear tooth is within the detectors sensing zone. As the tooth passes out of this zone, the output flips back. Therefore, a magnetic sensor placed in front of a rotating gear, the output from the sensor will be a series of electrical pulses.

There are several advantages when using magnetic sensors. Apart from the sensors being non-contact type, they are robust, hermetically sealed and can withstand unregulated power supply. Most manufacturers make then RoHS and IP67 compliant. That means no lead or other toxic materials are used for manufacturing these sensors and dust or liquid will not enter their enclosure. That makes such sensors suitable for use in food processing industries.

For measuring the speed of gears made of non-magnetic material, engineers often use optical sensors. The most common sensor of this type is the optical interrupters. Gear teeth interrupt a light beam from an LED source and the detector produces a corresponding electrical output. A continuously rotating gear in front of the sensor therefore, creates a similar series of electrical pulses as the output from magnetic sensors do.

The functioning of optical speed or proximity sensors is dependent of the dust and dirt level of the environment where they are used. Therefore, their range of applications is somewhat restricted as compared to magnetic sensors.

Measurement of direction involves a reference point, which means two sensors need to be used, with one of them being the reference sensor. An electronic circuit measures the time gap between the responses from each sensor. As the gear tooth passes in front of both sensors, one of them will change output before the other. If sensor A happens to trigger before sensor B does, the electronic circuit determines the gear is moving from A towards B. In case the output of sensor B switches before sensor A does, then the gear is moving from B towards A.

Usually, the sensors provide separate digital outputs for speed and direction. Their measuring capability may extend from detecting near zero speed up y 15 kHz.

How do you measure cable length?

Where miles of cable are involved, how do people determine where the fault lies and decide where to dig for initiating repairs? The method involves something very similar to how people determine the depth of a well, the distance of a cliff or the location of a thundercloud – by echolocation. If you know the speed of sound in air, you can find the distance of the sound source from the product of the amount of time sound is taking to travel the distance to its speed in air. For example, light travels much faster than sound; therefore, light from a thunderclap precedes its sound. By timing the gap between seeing the flash of light and hearing the thunder, it is easy to tell how far away the thunderclap occurred.

When an electrical pulse is directed into one end of the cable, it travels down the length until it meets a change in the cable’s impedance. This may be a fault in the cable or it may simply be its other open end. Whatever the situation, the change in impedance causes the pulse to turn back to its point of origin. The time gap between the original pulse and its return represents the length it has traveled. Therefore, if it has returned in, say 30ns, instead of the 100ns expected, the fault is at about 1/3rd the cable’s length from the end where the pulse was injected.

Engineers usually rig up an oscilloscope and a pulse generator for the purpose. Knowing the cable’s characteristics is necessary to set the pulse generator’s output impedance. The pulse generator needs to output a narrow pulse of about one to 100ns, with as small a duty cycle as possible – the two parameters depending on the length of the cable under test. The pulse voltage is not critical – 1V peak is enough.

The oscilloscope’s trigger level should be just under the peak voltage of the 1V pulse. The time base should be set just long enough to display one pair of the 1V pulses generated. That completes the setup.

As you launch the pulse into the cable, it triggers the oscilloscope sweep. The pulse now continues to the other end of the cable, until it encounters an open end. Since energy cannot be destroyed, the pulse is reflected back to the generator. When it passes the oscilloscope, it is displayed again. You can differentiate the reflected pulse from the original by the reduction of its amplitude and a difference in the rise/fall slopes. This happens because of attenuation when traveling within the cable and a loss of high-frequency harmonics. Although there may also be additional reflections caused by input capacitance of the oscilloscope, the echo of interest is only the first one after the original pulse was launched.

The round trip time is dependent on the cable length, which is usually known. For most cables, the pulse will travel at about 66% of the speed of light in vacuum (300m/µs). That makes its speed within the cable about 200m/µs. You may have to play with the time base and the pulse period until you can see both the launched and the reflected pulse.

Measuring Temperature Remotely

How to Measure Temperature Remotely

In hostile atmospheres like toxic zones, very high temperature areas or remote locations, where objects are not amenable to direct temperature measurements, remote measurement techniques are deployed. In such applications, remote temperature measuring techniques are resorted to, and devices used include Infrared or Laser Thermometers as described below.

Infrared Thermometers or Laser Thermometers

These devices sense the thermal radiation, also called Blackbody Radiation, emitted by all bodies, and the emission depends on the physical temperature of the object whose temperature is to be sensed. Laser Thermometers, Non-contact Thermometers or Temperature Guns are names of variants that use lasers to direct the thermometer towards the object.

In these devices, a lens helps the thermal energy converge onto a detector, which in turn, generates an electrical signal, and drives a display after temperature compensation. The devices produce fairly accurate results and have a fast response, unlike direct temperature sensing, which is difficult, slow to respond to or not accurate enough. Induction heating, firefighting applications, cloud detection, monitoring of ovens or heaters are some typical examples of remote measurement of temperature. Other examples from the industry include hot chambers for equipment calibration and control, monitoring of manufacturing processes, and so on.

These devices are commercially available in a wide range of configurations, such as those designed for use in fixed locations, portable or handheld applications. The specifications, among others, mention the range of temperatures that the specific design is intended for, together with the level of accuracy (say, measurement uncertainty of ± 2°C).

For such devices, the most important specification is the DISTANCE-TO-SPOT RATIO (D:S) where D is the object’s distance from the device, and S denotes the diameter of the area whose temperature is to be measured. This implies that a measurement by the device concerned provides the average temperature over an area having a diameter S with the object placed at a distance D away from the device.

Some thermometers are available with a settable emissivity to adapt to the type of surface whose temperature is being measured. These sensors can thus be used for measuring the temperature of shiny as well as dull surfaces. Even thermometers without settable emissivity can be used for shiny objects by fixing a dull tape on the surface, but the error would be larger.

Commercially Available Types of Thermometers:

• Spot Infrared Thermometer or Infrared Pyrometer, for measurement of temperature at a spot on the object’s body

• Infrared Scanning Systems, for scanning large areas. This functionality is often realized by using a spot thermometer that aims at a rotating mirror, such as piles of material along a conveyor belt, cloth or paper sheets, etc. However, this cannot be termed a thermometer in the true sense.

• Infrared Thermal Imaging Cameras or Infrared Cameras are the ones that generate a thermogram, or an image in two dimensions, by plotting the temperature at many points along a larger surface. The temperatures sensed at various points are converted to pixels, and an image is created. As opposed to the types described above, these are primarily dependent on processor- and software-for functioning. These devices find use in perimeter monitoring by military or security personnel, and monitoring for safety and efficiency.