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Functionality and technology of inductive sensors

 

Sensing distance

The international standard EN 60947-5-2 defines the sensing distance as follows: the sensing distance is the distance at which a standard target moving toward the sensing face of a proximity switch causes a signal change.

Standard target

The standard target is defined as a square plate, 1 mm thick, made of Fe 360 (mild steel). The length of its side is defined as the larger of either the sensing face diameter or three times sn (nominal sensing distance).

Nominal sensing distance Sn

Nominal sensing distance Sn is a type classification parameter and does not take into account tolerances during machining or changes due to external conditions such as voltage or temperature.

Effective sensing distance Sr

Effective sensing distance of an individual proximity switch which is measured at a defined temperature, voltage and installation conditions. For inductive proximity switches it must be between 90% and 110% of the nominal sensing distance at 23 ±5 °C.

Assured sensing distance S

Distance from the sensing face at which the operation of the proximity switch is ensured under defined conditions. For inductive proximity switches the assured sensing distance is between 0% and 81% of the nominal switching distance.

Usable sensing distance Su

Sensing distance of an individual proximity switch measured over the temperature range and at a supply voltage of 90% and 110% of the rated value. For inductive proximity switches it must be between 90% and 110% of the effective sensing distance.

 

Baumer inductive proximity switches are non-contact electronic sensors. Inductive sensors will recognize any conducting metal target.

The oscillator creates a high frequency electromagnetic field, which radiates from the sensing face of the switch. When a conductive metal object enters this electromagnetic field, eddy currents are induced within the metal, causing a change in the amplitude of the oscillations. The result is a voltage change at the output of the oscillator, which causes the trigger to change state and alter the output state.

 

Hysteresis

At approach and removal of the target, there is a difference between operating and release point which is defined as Hysteresis.
Hysteresis is designed into a sensor's characteristics to guard against possible incorrect pick-up due to vibration.

 

Switching frequency

Meeting EN 60947-5-2 standards, the switching frequency is the highest possible number of switchings per second.

 

Correction factor for standard inductive sensors.


Comparison of standaard sensors and factor 1 sensors.

With standard sensors, the measuring range for non-ferromagnetic metals is reduced by up to 70%. Factor 1 sensors feature a micro-controller as well as a temperature-stabilized reference circuit which compensates this undesirable effect. As a result, factor 1-sensors do not have a material-dependent reduction factor and thus offer the same maximum measuring range for all metals. They are therefore particularly suitable for measurements on non-ferromagnetic metals such as aluminum or ferrous metals.
Thanks to the integrated micro-controller, they offer the same advantages as linearized AlphaProx sensors: negligible production lot variations, low temperature drift, linearized characteristic curve as well as different teach options for adapting the measuring range to the specific application or eliminating mechanical installation tolerances.

 

 

Output

Digitally switching sensors are available with a PNP, NPN or Namur output; measuring sensors come with 0 … 10 V or 4 ... 20 mA.

Series switching

3-wire DC (PNP circuit shown)

The voltage drop across each conducting sensor reduces the voltage available to drive the load. The number of proximity switches which can be connected in series is therefore limited and may be worked out by summing the individual voltage drops plus the load requirement. Keep in mind of the power-on / response times!

 

Parallel switching

3-wire DC

3-wire DC sensors may be connected in parallel as shown. A parallel connection, however, must  incorporate a decoupling diode.

 

 

Connection diagrams

   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).

 

 

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.
 

Typical applications
 

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.
 

 
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

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.

 

 

 

 

Accessories

Diverse accessories, including an external teach-in adapter is available for analog sensors.