Knowledge Base

  • What is the difference between measuring accuracy and repeatability?

    The measuring accuracy is specified with ±2σ (see Measuring accuracy). This accuracy includes also distance errors due to temperature changes or linearity errors.
    In contrast to this, the repeatability is only valid for stable measuring conditions like same distance, identical measuring target, etc. (see Repeatability).

    Is it possible to measure to a moving target?

    The Dimetix sensors are able to measure on moving targets. Thereby the maximal possible object speed depends on the following factors:

    • Measuring condition (light conditions, ambient light)
    • Object- / Measuring surface condition
    • Sensor specification measuring speed / rate (see Products)

    Generally with higher measuring rates the maximal object speed will be higher as well. Realize that the measured distance is averaged over the measuring time of one measurement.

    How is the measuring rate influenced?

    The measuring rate of the sensors is influenced by different factors. Essentially the signal level of the reflected laser light has an important influence on the measuring rate. On a bright measuring surface (e.g. white) with good reflectivity, a measurement takes less time than on a dark surface (eg. black) with low reflectivity.

    The following factors can influence the measuring rate / measuring speed in a positive manner:

    • Condition of the measuring surface (e.g. mat, smooth)
    • No / little background light (e.g. sunlight, spotlight)
    • Reduction of the measuring distances
    • Configuration of the sensor

    What is the measuring accuracy?

    What is the measuring accuracy?

    The measuring accuracy of the Dimetix sensors is specified with a statistical confidential level of 95.4% (corresponds to ISO 1938-2015). This is equivalent to ±2σ or 2 times the standard deviation σ (see the figure).

    The following distance errors are considered in this measuring accuracy as well:

    • Distance error due to temperature influence (Sensor temperature)
    • Linearity error

    Realize that the sensors do not compensate the humidity, the air pressure or the air temperature. If the environment conditions differ from 60% relative humidity, 953mbar air pressure or 20°C air temperature, the accuracy can be influenced if the measuring distances is longer than 150m. The influence of these environment conditions is described in H. Kahmen & W. Faig: “Surveying”, (1988).

    What is the repeatability?

    What is the repeatability?

    The repeatability is achieved by repeatedly approaching the same distance under the same measurement conditions during a short time interval.
    For example, stable measurement conditions include:

    • Same distance
    • Identical measurement target
    • Same temperature conditions
  • How long could be the RS-422 / RS-485 or SSI signal cable?

    RS-422 / RS-485 and SSI are differential serial data interfaces designed for long data cables. The cable length depends on the cable quality and data rates.
    See the following guidelines for the RS-422 / RS-485 interfaces:

    • 19’200 Baud → up to 1000m
    • 115’200 Baud → up to 500m

    See the following guidelines for SSI interfaces:

    • ≤ 100kBit/s → up to 1000m
    • ≤ 500kBit/s → up to 200m
    • ≤ 1000kBit/s → up to 100m

    Cable type and termination:

    • Use shielded twisted pair cables only
    • Termination according to characteristic impedance of the cable (typ. 100 … 150 Ω)

    The cable length is also influenced positively / negatively by other factors:

    • Quality of the cable (Shielding, cross-section, wire resistance, etc.)
    • Environmental conditions (Disturbance sources like motors, etc.)

    Further details in the Technical Reference Manual, see Downloads.

    What must be considered with the connecting cables?

    When selecting the sensor connection cable, the following points should be considered:

    1. Cable cross-section according to max. sensor current
    2. Consider wire resistance for long cables (Attention voltage drop through cable)
    3. Consider cable requirements according to specifications e.g. Shielded twisted pair cable for RS-422 / RS-422 or SSI interfaces. Specifications according to Technical Reference Manual, see Downloads.

    When is a termination recommended for RS-422 / RS-485 or SSI interfaces?

    A proper termination of the data lines is recommended in any case. However, for very short data lines and data rates up to approx. 200 kBit/s, a termination is not absolutely necessary.
    The specifications of the Technical Reference Manual must be observed, see Downloads.

    Where can be found FAQ’s for Industrial Ethernet interfaces?

    The FAQs for the Industrial Ethernet interfaces are listed in the corresponding Technical Reference Manual (see Downloads).

    How long could be the RS-232 data cable?

    The RS-232 interface is not designed for long data cables (no differential signals). The cable length depends mainly on the data rates of the RS-232 interface.

    See the following guidelines:

    • 19’200 Baud → up to 15m
    • 115’200 Baud → up to 2m

    The cable length is also influenced positively / negatively by other factors:

    • Quality of the cable (Shielding, cross-section, wire resistance, etc.)
    • Environmental conditions (Disturbance sources like motors, etc.)
  • How should the orange reflective target plate be mounted?

    How should the orange reflective target plate be mounted?

    When assembling the orange reflective target plate or reflective foil, the following points must be observed for trouble-free operation:

    • Only use the Dimetix reflective target plate (see accessories)
    • Mount the reflective target plate at an angle of 1-2 ° (see figure)
    • Do not scratch the reflective target plate
    • Scatter light must not be reflected in sensor optics
    • The entire laser spot must fit on the reflective target plate (spot size depending on the measuring distance)

    What is an optimal measuring surface?

    An optimal measurement surface has the following properties:

    • Flat, fine and not porous
    • Diffuse reflective (not glossy / reflective)
    • Bright and stable / low vibration
    • Bigger than the laser spot

    The topic can be divided into two use cases:

    1. Natural measurement surfaces: None / Low Influence on Measurement Surface.
    2. Selectable measuring surface:
      • Short ranges → White-matt surface (E.g. white matt sprayed board as an economical solution), or Dimetix orange reflective target for more performance (see Accessory)
      • Wide ranges → Dimetix orange reflective target (see Accessory)

    The possible measuring range of the sensors must be checked according to the specifications (see Products).

    What optical disturbance sources can influence the sensors?

    The sensors operate in the wavelength range of 620 … 690nm (corresponds to red light in the visible electromagnetic spectrum). This means that all optical light sources in the same color range can act as potential sources of disturbance.

    Possible disturbance sources that have to be eliminated / minimized:

    • Other optical sensors in the same wavelength range
    • Rotary lights / flashing lights / flashlights
    • Sunlight

    Possible approaches to minimizing such disturbance sources:

    • Separate sensor from other sensors in the same spectrum
    • Shield sensors with housing / covers

    Does dust influence the measuring process?

    The influence of dust on distance measurements depends on the density of dust. If the main part of the laser beam is reflected by dust particle, the distance measurement will be influenced in a negative way (measurement errors). However, such conditions can only be found in extreme dusty environments like cement silos.

    On what kind of targets can be measured?

    All opaque targets can be measured if they do not have highly reflective surfaces. Reliable measurements on transparent targets are not possible.

    Is it possible to measure through glass?

    Is it possible to measure through glass?

    It is not recommended to measure through glass, since there is a signal lost and reflections on the glass may have a negative impact on the accuracy.

    However, if a glass is the only option for a specific application, check the following points:

    • Use coated glass to reduce the reflections and signal losses
    • Install the glass with an angle to the front of the sensor of 5° min.
    • Keep the glass clean all the time

    Is the Laser Light eye safe?

    The Dimetix sensors belong to laser class 2. Sensors of this laser class have visible laser light and a laser power less than 1mW (<1mW).

    Realize that in normal case the bright light of a class II laser beam into your eyes will cause a normal reaction to look away or close the eyes. This response is expected to protect you from Class II Laser damage to the eyes.

    If class II laser beams are directly viewed for a long period of time damage to the eyes could result. Avoid looking into a class II laser beam or pointing into another person’s eyes. Avoid viewing class II laser beams with telescopic devices.