Enhancing efficiency: Strain gauge based smart sensors with IO-Link interface

Strain gauge based sensors have reached a high level of maturity with strain gauge technology establishing itself as the standard in industrial environments, and the clear choice when it comes to measuring mechanical quantities such as strain, force, weight, or torque.

Alongside tasks in research and development and quality assurance, it’s widely used in process applications. In classic mechanical and plant engineering, there are countless processes and applications where these quantities must be measured as primary, or secondary parameters, for process control and product quality.

Examples include:

• Force measurement technology: pressing, resistance welding, rolling, joining, crimping, end-of-line quality checks, bending processes, bearing force control
• Weighing technology: checkweighers, filling systems, silo weighing, truck and train scales, load control in production processes
• Torque measurement technology: end-of-line test cells for parts and components such as gearboxes, door handles, screwing processes

Strain gauge-based sensors often make up the core element of a system or application, playing a key role in two aspects:

1. Providing measurement values for control systems to control actuators. Accurate measurement results are crucial as any inaccuracies could lead to significant issues, including product rejection.

2. As valuable sources of information for quality assurance, their reliability is paramount.

Moreover, requirements in terms of tighter tolerances and improved product quality are increasing, and in turn demand minimised measurement uncertainty. Paired with the previous demands, strain gauge based sensors also have to adapt the general trend of easy and cost-effective integration – from the mechanical side as well as from the electrical connectivity and software OT/IT integration.

Smart sensor concept

The sensor works passively and requires a precise external voltage supply. The output signal is amplified and digitised. Both those functions take place in the bridge amplifier. The sensor and the amplifier are connected using a high-quality measurement cable.

Nowadays, measurement amplifiers usually feature signal processing capabilities such as filters, peak detection, averaging and many more. The measured signals are output in either analogue or digital format via the amplifier’s communications interface and transmitted to the higher-level controller.

The new generation of digital sensors combines the sensing element, the amplifier, analogue-digital conversion, signal processing, and the communication interface into a single unit.

In addition to signal processing (scaling, linearisation, filtering), modern microprocessors also make it possible to perform domain-specific algorithms and sensor health monitoring functions in the sensor. These can be check- weighers and filler algorithms (for weighing applications), or a power calculation (in the case of torque measurement).

Statistical process parameters such as min/max values, peak-to-peak values as well as sensor-health related parameters can also be calculated based on the sensor-specific mechanical and thermal limits stored within the sensor and the actual measurement values.

Paired with an IO-Link interface, the measured values, calculation results and additional information can be easily and efficiently transferred to the PLC system in a standardised way.

IO-Link solution overview

IO-Link is a highly suited solution to meet the needs of the evolving automation environment. It is an open standard (IEC 61131-9) and enables a point-to-point connection between IO-Link devices and IO-Link masters. The IO-Link standard applies to the communication protocol and the electrical wiring and the device data structure.

The IO-Link system solution consists of IO-Link devices (sensors, actuators, or other field devices), the IO-Link masters, and the IODD (IO Device Description) files. The IODD contains a description of all sensor and domain- specific functionalities using a standardised data format and structure. There are different sensor profiles for specific sensor categories such as simple switches or digital measuring sensors.

IO-Link masters act as gateways connecting the individual IO-Link devices via point-to-point communication and provide the interface to the higher-level control architecture (PLCs) and IT-level applications. IO-Link masters are available for all common fieldbuses and Industrial Ethernet protocols.

This makes the IO-Link system solution independent in terms of fieldbus use and provides great flexibility for companies in configuring their system architecture at the field level. This flexibility enables easy adaptation to different PLC architectures.

IO-Link offers the potential of unifying the vast signal type, cable, and connector landscape in the field-level via a standardisation of the aforementioned elements. IO-Link data is transmitted through a three-wire, unshielded cable (max. 20 m between device and master) communicating on a 24 V level.

Compared to the analogue output of passive strain gauge sensors, this signal type is significantly more robust, providing high resistance against EMC/EMV influences.

Electrical installation becomes easier as there is no need for the sensor and instrument to share the same level of electrical potential. This is required in traditional measurement chains to pre-vent current flow over the shielding of the measurement cables.

Solution efficiency

Sensors with IO-Link communicate data between themselves and an IO-Link master. The measurement values and other process-relevant values such as the status of the limit switches are transmitted cyclically. The cyclic data is called ‘Process Data’.

Additionally, ‘On-Demand Data’ is transferred acyclic. The on-demand data consists of the entire sensor parameter data, which can be accessed by the IO-Link Master, or the higher-level PLC as needed.

A further feature of the IO-Link communication protocol is the so called ‘Events’. These are standardised alerts that are triggered when sensor-specific limits are overrun.

The exact data structure of the aforementioned data types is described in the Smart Sensor Pro-file and IODD (IO Device Description) file. Each IO-Link device, whether a sensor or actuator, is assigned a unique device ID. This identifier ensures accurate sensor identification to correspond with the correct IODD file and provides an additional specific advantage.

The sensor’s parameter set can be stored on three levels – within the sensor itself, on the IO-Link master and within the PLC. This triple-level parameter storage concept enables the ‘hot’ swopping of sensors. Using a specific setting, the IO-Link master reads whether the newly connected sensor has the same Device ID as the previous one and automatically writes the stored parameter data into the new sensor if the Device ID matches.

The general process of IO-Link sensor integration and parametrisation is the same for all IO-Link devices. The IODD file enables the sensor to be interpreted by any IO-Link compliant engineering tool.

IO-Link – the way forward

Investing in a sensors with IO-Link is a step towards modernising manufacturing processes and a strategic move that promises a low total cost of owner- ship (TCO) and high re-turn on investment (ROI). This technology enables instant cost savings across various levels, including component, related, and labour costs.

HBK has created a range of wireless IO-Link solutions ideal for many automation and testing applications with accurate, reliable, and efficient sensor technology.
Visit: HBK IO-Link Sensors

Martin Schütz – Product Manager, HBK
Martin is Product Manager for Smart Sensors at HBK. Prior to this, he was a Sales Engineer for 3,5 years in HBK’s Industrial Process Control department. He holds a BSc. in Mechanical and Process Engineering, and enjoys cycling, running, and playing guitar.

FAQs: Strain Gauge Smart Sensors with IO-Link

What are strain gauge sensors used for?
They measure mechanical quantities like strain force torque and weight in industrial processes

Why are smart sensors better than traditional ones?
They integrate signal processing communication and diagnostics in one unit for higher accuracy and functionality

What is IO-Link?
An open standard communication interface that connects sensors to controllers using standard cabling

How does IO-Link simplify sensor integration?
It standardises data structure enables plug-and-play setup and supports automatic parameter loading

What are Process Data and On-Demand Data?
Process Data is continuously transmitted measurement values while On-Demand Data includes sensor settings and parameters

What happens if a sensor is replaced?
The IO-Link master identifies the new sensor and reloads its settings automatically if the Device ID matches

What advantages does IO-Link offer over analogue systems?
It offers better EMC resistance simpler cabling and does not require potential equalisation

Why invest in IO-Link sensors?
They reduce installation time increase flexibility support digitalisation and lower total cost of ownership

What types of industrial tasks benefit from IO-Link sensors?
Applications in pressing welding weighing crimping torque monitoring and quality control gain from real-time precision data

Is IO-Link compatible with existing PLCs?
Yes it supports all major fieldbus and Ethernet protocols through flexible IO-Link masters