Technology of inductive sensors

Connection diagrams

  • Explanatory notes on the connection diagrams

    The specified diagrams indicate the undamped output. A sensor is in a damped state when an object is located in within its scanning range. In the diagrams Z denotes the typical load resistance position; Uz denotes the voltage applied to this load resistance. If Uz = high (≈ +Vs), then current flows; if Uz = low (≈ 0 V), then no current flows via the load resistance. Load resistance between output and +Vs is referred to as pull-up resistance, load resistance between output and 0 V as pull-down resistance.

     

    PNP or NPN output

    Sensors with a PNP or NPN output have a 3-wire design (+Vs, output and 0 V) and operate with direct current (DC). The load resistance of PNP sensors is between output and 0 V (pull-down resistance), while load resistance of NPN sensors is between +Vs and output (pull-up resistance). As a result, the PNP output is connected to the positive voltage supply during switching (positive switching output), whereas the NPN output is connected to the negative voltage supply during switching (negative switching output). Normally open contacts and/or normally closed contacts define the switching function. Normally open contacts are referred to as normally open (NO), normally closed contacts as normally closed (NC). During damping with an object, sensors with normally open function establish contact connections (Uz = high), while sensors with normally closed function disconnect connections (Uz = low).

Typical applications

  • Function

    Inductive sensors with analog output signals are characterized by their short response times, high resolution and linearity as well as their outstanding repeat accuracy. The output current and voltage values are proportional to the distance between the sensor and the object being detected. In other words, they represent absolute measured values corresponding to the distance between the active surface and the object. These properties make inductively measuring linear sensors extremely interesting for numerous applications in the area of measurement and control engineering.

     

    Typical applications

    Travel / position / displacement

     

    Diameter / eccentricity

     

    Deflection / deformation

     

    Size comparison / measurement tolerance

     

    Taper / ramp

Resolution

  • Resolution in general

    Resolution represents the smallest possible change in distance which will produce a measurable signal change at the sensor’s output. Resolution can be impaired by high-frequency electrical interference (noise) or by the resolution of digital/analog converters.

     

     

     

    Dynamic resolution

    Signal noise exerts full effect on signal processing during very rapid measurements (high scan rates). Filtration without influencing the useful signal is only possible to a limited extent, if at all.

     

     

     

    Static resolution

    Very slow object movements (low scan rates) such as the temperature expansion of shafts allow the high-frequency interference to be filtered. The carrier signal is not influenced by this filtration. Using this technique significantly increases the resolution when compared to dynamic measurements. 

     

     

    Repeat accuracy

    Repeat accuracy means the difference between the measured values of successive measurements within a period of 8 hours at an ambient temperature of 23 °C ± 5 °C.

     

     

     

     

    Response time

    The time which the signal output of a sensor requires to rise from 10 % to 90 % of the maximum signal level is called the response time.

     

     

     

     

    FS Linearity

    Linearity defines the deviation between the output signal and a straight line. It is given as a percentage of the measuring range end value (FS or Full Scale). The following alternatives are available for applications where the indicated linearity is insufficient:

    • Sensors with linearized output curves
    • Polynomials for the mathematical linearization of the sensor curve’s in the controller

Linearization

  • Signal linearization with polynomials

    Polynomials represent a mathematical function which converts a typical inductive analog sensor output curve into a linear signal. For example, it is integrated in the software of a programmable logic control to change the s-shaped sensor signal into a straight line.


    Polynomials are used when …

    • a linear signal progression is required across the entire signal range
    • rapid measurements are to be performed
    • no sensors with linear output curves are available for the intended measuring range
    • an economical solution is desired

    A variety of sensors with linear output curves are also available as alternatives.

Teach-in-functions

  • Functions

    The following parameters can be altered using the Teach-in function:

    • Analog output (measuring range)
    • Digital output (switching window)

     

    Procedure

    The same Teach-in function menu structure is used for all sensors from Baumer, with the primary emphasis being on the simplest operation possible. This function allows the measuring range to be freely programmed within predefined limits. If, for example, a small measuring range with a large signal amplitude is desired, the range can be limited to merely a few millimeters. If necessary, the direction of the analog output’s flow can be inverted, too. In addition, the points at which one of the digital outputs switches on and off can be specified. These can lie either within or outside the individually programmed measuring range.

     

    Programmable output curve

     

    Accessories

    Diverse accessories, including an external teach-in adapter as well as a 3-point converter which enables conversion of an analog signal into several digital signals, are available for analog sensors.