Important Terminology for Measurement System Selection
Mounting conditions / Mounting mode – to properly measure an object/target with the aid of a laser displacement sensor, you must reflect the light from the object to the receiver. Measurement systems that use a triangulation method for measurement, along with variable surface conditions of the object, have sensor heads that are installed at an angle so that light can be reflected and received in the proper manner.
- Opaque targets – both the sensor head and the target must be installed parallel to each other so that a portion of light hit the receiver.
- Transparent targets – the sensor is set up in a way that the reflection and incident angles are level with one another.
Reference distance – the reference distance refers to the default zero point for the sensor head. In most cases this is characterized as the distance from the bottom of the sensor head to the center of the measuring range.
Measurement range – as the name implies the measurement range is the distance that a sensor can perform a measurement. When writing down a measurement range it’s usually written out as thus: ±xx mm (this is based on the reference distance).
Light Source – A transmitter illuminates a target with light and the reflected light is picked up by the receiver. This happens only in non-contact measurement systems. There are a large number of light sources that can be used to illuminate a target. These include green LEDs, SLDs, white light, semiconductor lasers, blue semiconductor lasers, green LEDs, and red semiconductor lasers. The light source in question is determined by the measurement system (remember, that all measurement systems are NOT equal). The utilization of the right lens when receiving a light source will give you the most high accuracy measurement possible. Using the wrong light source with the wrong type off measurement system may lead to inaccurate results and low accuracy measurements.
Spot diameter – when it comes to non-contact measurement systems there are only two types of beam spots that are of consequence – circular and elliptical. Elliptical spots are used to measure an average height within a specific area. They are not as prone to be influenced by the roughness of target’s surface. But, as the size of the spot begins to grow it becomes less suitable for measuring smaller targets and profiling shapes. Because circular spots are smaller they are much more effective at performing inspections.
Repeatability – characterizes the primary variance in a measurement value taken at the same location on a target.
Linearity – an indicator of a measurement system’s capability. Where linearity is concerned the value signifies the maximum error value between an ideal value and the actual result of the measurement. Ultimately, measurement systems with smaller linearity are considered to be better. Those with a larger linearity are considered to be not as accurate.
Temperature Characteristics – the representation of the full measurement error value that transpires when the temperature of the sensor head changes by a single degree. The sensor head contains a number of vital components vital to its successful operation. Inside, you’ll find a lens and a CMOS sensor. You’ll also find jigs that secure these components in place. Temperature plays a big role because as the environment heats up or cool down the internal components can expand and contract which can affect the positioning of the imaging location on the CMOS and can lead to issues. In other words, your final result may be skewed due to changes in temperature. Typically, temperature characteristics are defined as ±○○% of F.S./℃. The measurement range is represented by F.S. It’s a general belief that measurement systems with a lower temperature characteristic are much better.
Ambient light – the maximum illumination intensity from an external light source (such as a lamp or candle), at which the measurement system can analyze and measure without affecting the operation of the system.
Ambient humidity – the humidity of the environment in which a measuring system is guaranteed to continue operating without problems.
Vibration resistance – an index that focuses on the effect vibrations might have on a measurement system. The evaluations on the vibrations are represented by displayed values. For example, a description might look a little like this: “12 to 60 84 Hz, 3.5 mm double amplitude, 1 hour each in X, Y, and Z directions”. This information would imply that a test has indeed been performed.
Sampling frequency / Sampling speed – a representation of the amount of data points the measurement system can measure per second. For example, a frequency of 100 Hz can execute 100 measurements in 1 second. Measuring system that possesses a faster sampling frequency are typically capable of achieving more precise target measurements with an inline measurement. Due to the fact that more than one averaging process can be performed all at the same time the measurements in question should be much more stable. Sampling frequency should not be confused with sensor frequency response. The frequency response of a sensor must be more than 2X the maximum sampling rate use. Analog output sensors will have a frequency response associated with the output. Digital system have a sampling rate.
Received-light wave pattern – a representation of the light received by the light-receiving element. Typically, a received-light wave pattern is represented on an axis. The horizontal axis displays the position of the light-receiving element. The vertical axis displays the strength of the light. To determine whether the measurement is being performed in the most accurate manner possible, you have to check the shape of the received-light wave pattern. There are five received-light wave patterns:
- Ideal received-light wave pattern – allows for the most stable measurement
- Short received-light wave pattern height – the amount of reflected light obtained is so nominal that the measurement cannot be performed
- Excessively tall received-light wave pattern height – the reflected light has become highly saturated thus variation in measured values will occur.
- The received-light wave pattern is not horizontally symmetrical – when measuring objects such as resin, the laser beam will be absorbed into the object and the received-light wave pattern becomes horizontally asymmetrical. To compensate for this, shifting is performed on the measured values.
- Multiple received-light wave patterns – when measuring a transparent object (like glass) more than one peak may occur. When measuring an object such as glass, two peaks may appear: the peak from the top of the surface and the peak from the back of the surface.
Optical axis, optical axis elastic region – the optical axis is the central point of light that shines from the measurement system’s transmitter. Due to the fact that light can’t reach the target or receive if a component such as a jig were to enter this area, a measurement would not be possible to take.
Power supply voltage – the voltage needed to operate a device.
Maximum current consumption – a representation of the amount of current that is consumed when operating the device. It’s essential to choose a device that has a power supply with a bigger capacity than the maximum current consumption.