The Advantages of Digital Interfaces for Displacement Sensors

Inductive displacement sensors are a reliable measurement tool, which can be found in several industries. With the introduction of amplified digital sensors – containing IO-Link Interface – the sensors become more precise and costs savings are achieved in combination with improved reliability. 

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Inductive Wheatstone Bridge Displacement Sensors

Inductive displacement sensors can be found for monitoring applications, as well as for production purpose or experimental use. 

Several technologies are available for the measurement of positions. There are optical systems, potentiometer systems or so-called LVDT’s, which work with an inductive differential transformer, to name just a view. Another principle is working with the Wheatstone bridge circuit.

The sensor features two coils which are connected to an inductive half bridge. Two resistors are used to complete a Wheatstone bridge, see figure 1

Pic. 1: Inductive Wheatstone bridge sensors. Left: Loose plunger, the mounting of the moveable core with two nuts; right: Probe version: The moveable core is fixed to the object with the force of a spring

A ferromagnetic core is moved inside, if the system of coils – and therefore an output voltage occurs – if this core is placed outside of the center of the coil system. Lastly, a voltage can be measured, which is depending on the position of the core in the coil system. 

The introduction of the IO-Link interface, in combination with the proven reliable Wheatstone bridge inductive sensors, is an easy and cost effective way to perform position measurements in industrial applications.

This principle can be used for loose plungers, as well as for probe versions. 

The advantages of those sensors are:

• Practically no wear: It is a non-contact measurement system so that the sensors are reliable measurement tool for years
• Works under harsh environment: All electrical components can be placed in a hermetical sealed housing, the moving ferromagnetic coil is the only part of the system in direct contact with weather or other outer conditions. 
• Precise measurement system: A linearity adjustment  of the core element allows to reach linearity errors better than 0.1 % and very little temperature related errors
• Works with carrier frequency bridge amplifiers: The sensors can be used with any bridge amplifier with carrier frequency of 4.8 kHz (+/- 8 %) and a suitable measurement range

Amplifier systems

Like all inductive sensors, so called carrier frequency amplifiers are required. Those systems come with an alternating voltage of 2.5  to 10 Volt, to supply the sensor. The output signal is amplified and demodulated in a second step. A low pass filter is used to suppress the carrier frequency so that – finally – the output signal reflects the displacement. 

The typical output signal of  Wheatstone bridge based inductive displacement sensors is 80 mV/V for full scale. Nevertheless, most of the instruments for displacement sensors can be used for strain gauge based sensors as well. Also the output signal of those sensors is much lower than with inductive displacement sensors. 

The integration into automation systems becomes easy as IODD files and function blocks are available for time and cost saving.

Amplified displacement sensors

Due to the fact that inductive sensors need a certain carrier frequency, a suitable instrument or DAQ – system is not always in place. This explains the trend to offer displacements sensors with an fixed mounted amplifier – in many cases as a so called “Inline- Amplifier”.

Traditionally a voltage or current output is used as a common interface. Those measurement chains have standardised industrial interfaces, can be used in all setups – 0-10 V inputs are available for testing purpose as well as for PLC systems.

IO-Link: Cost effective industrial interface

IO-Link is a serial interface, which is established in industrial production and proved a massively increasing number of nodes in the last decade.

IO-Link is a standardised technology that connects sensors and other filed devices, by using a single cable for data transmission, as well as for power supply. With cycling times of less than 1 ms, more than 1,000 sets of data can be transmitted in one second – it is a suitable technology for static measurements as well as for dynamic measurement tasks.

The field devices are connected via a so called IO-Link Master to the fieldbus. As IO-Link masters are available for all fieldbus systems, it is a solution for connecting displacement sensors with IO-Link interface to any fieldbus environment.

Displacement sensors with IO-Link

The inline amplifier module is connected to the sensor. In some cases this is a hardwired arrangement, in other cases, a clear labeling indicates which sensor belongs to which electronic unit. 

The digital part does not work differently from known amplifier systems. The difference is that sensor and instrument are a fixed combination. This allows saving of all technical details from the sensor in the instrument, also those parameters that are different from sensor to sensor, such as the sensitivity or linearisation data. The advantage is that the device gives correct readings in mm directly after the connection between the IO-Link master and the measurement device is established.

The signal flow is shown in Pic. 2. After amplifying and demodulation the raw signals is calculated to measurement values. A linearisation takes place at this step which means that the linearisation error is just half in comparison with traditional displacement sensors. As mentioned above, no set-up process is required to start the measurement. Furthermore, the firmware allows use of low pass filters with free choice on cutting frequencies. Limit switches as well as peak detection are available as well. 

The device is working with an internal sampling rate of 40 kHz, which means that the peak detection recognises fastest signals – even in the case of longer cycling times. This values can be read as in-demand data at any point in time, so long as the statistic are not reset.

With cycling times of less than 1 ms, more than 1,000 sets of data can be transmitted in one second – it is a suitable technology for static measurements as well as for dynamic measurement tasks.

Another important factor is the so called “Nominal Measurement Range monitoring”. The mechanical movement of any displacement sensor is larger than the measurement range; the core can move before and behind the measurement range.

With the traditional setup this needs to be considered and it is important that zero setting is not active during the mounting process, to ensure the core is in a suitable position.

IO-Link offers a bit that is set to “0” inside of the measurement range, independently from the zero setting of the measurement chain. This gives the user a reliable indication that the measurement chain is working without any restrictions and full accuracy.

Integration, Function blocks

Digital displacement sensors offer much more information and have extra features in comparison to traditional counterparts. Therefore, it is possible to read out maximum or minimum values, set filters and so on. There are two methods to parametrise  a IO-LINK sensor:

• So called engineering software are available from many different vendors of IO-Link Masters. Engineering software runs on a conventional PC, and it is possible to see measurement values or to parameterise the sensor. All settings – with exception of the zero point – can be saved in the displacement sensor. When you have finished the setup process, the transducer can be installed mechanically and is ready for operation. IODD files are available on the HBK´s homepage that make IO-Link sensors from HBK to a plug and play system in combination with any engineering software.  
• Users can set all parameters via their PLC system as well. IO-Link function blocks are standardised PLC software components available for all relevant PLC systems that encapsulate cyclic and acyclic communication with IO-Link sensors, enabling structured access to process data, parameters, and diagnostics via an IO-Link master. Function blocks are available on HBKworld.com for all HBK sensors and for displacement sensors as well. 

Conclusion

The introduction of the IO-Link interface, in combination with the proven reliable Wheatstone bridge inductive sensors, is an easy and cost effective way to perform position measurements in industrial applications. The mechanical integration remains unchanged, as the dimensions of the sensors do not change. The integration into automation systems becomes easy as IODD files and function blocks are available for time and cost saving. 

FAQs

What are IO-Link displacement sensors?

IO-Link displacement sensors combine inductive position sensing with digital communication allowing accurate measurement simple configuration real-time diagnostics and easy integration into industrial automation systems.

How do Wheatstone bridge displacement sensors work?

They use two inductive coils arranged in a Wheatstone bridge circuit. As a ferromagnetic core moves within the sensor the bridge output changes allowing precise displacement measurement.

What are the advantages of IO-Link over analogue sensors?

IO-Link provides digital communication automatic parameter transfer remote diagnostics higher measurement accuracy and simplified integration with PLCs and fieldbus networks.

Why are inductive displacement sensors suitable for harsh environments?

Their non-contact measuring principle minimises wear while sealed housings protect the electronics from moisture dust vibration and other demanding operating conditions.

How does IO-Link reduce installation costs?

IO-Link simplifies wiring eliminates manual calibration stores sensor parameters automatically and enables plug-and-play commissioning using standard IODD files.

Can IO-Link displacement sensors detect faults?

Yes. They provide diagnostic information including measurement range monitoring peak detection operating status and sensor health helping prevent unexpected downtime.

Which industries use IO-Link displacement sensors?

They are widely used in manufacturing automation test equipment structural monitoring machine building process industries and quality control applications.

Why are digital displacement sensors becoming more popular?

They offer greater measurement accuracy easier integration better diagnostics lower maintenance