Stationary tension meters with one or three rollers are industrial sensors designed for fixed installation on production lines, where they continuously monitor the tension of moving product (wire, tape, cable, sheet) for process control, automatic regulation and in-line quality control. Unlike portable tension meters, they are built into the machine and operate continuously throughout production, sending their signal to the line PLC for automatic tension regulation by means of compensation systems (brakes, brushless motors, magnetic clutches).
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Configurations
The main configurations are: single-roller, in which the sensor is built into one of the product guide rollers; the product deflects slightly around the roller and the force exerted is proportional to the tension (simple geometry, but demanding precise installation); three-roller (the most widespread), with two fixed lateral rollers and a central one connected to the force sensor, a more robust geometry that is independent of how precisely the product is positioned; and clamp type for wide tapes (with knife clamps that capture the tension at several points across the width). The load cells used are predominantly strain-gauge, with an accuracy class of 0.1-0.5% of full scale.
Technical characteristics
Stationary tension meters cover ranges from a few cN for fine yarns up to tens of kN for heavy industrial cables and tapes. Output interfaces include: analog 4-20 mA or 0-10 V for integration with standard PLCs; digital RS-485 Modbus, Profibus, Profinet and EtherCAT for integration with advanced industrial systems. The dynamic response is typically of the order of 1-10 ms, enough for real-time regulation even on fast lines. Mechanical stiffness and durability in harsh environments are essential characteristics for long-term reliability. Typical protection ratings are IP65-IP67 for the industrial environment.
Process applications
In-line applications are varied: wire drawing lines (tension control on the various drawing passes, to avoid breakage); textile spinning lines (yarn tension control at every stage of the process); extrusion and calendering lines for plastic and rubberized tapes; rolling and cutting lines for thin foil (tension control on slitting and refining lines); production of electrical and optical cables (tension control during stranding, sheathing and marking); and rotary printing lines (tension control of paper or film, to avoid creasing and misregistration).
Compensation systems
The tension value measured by the stationary tension meter is typically compared with a setpoint defined by the process; the difference feeds a PID control system that acts on actuators (magnetic powder brakes, brushless motors, electromagnetic clutches, motorized haul-offs) to keep the tension constant. The precision of the control depends on the quality of the sensor, on the response speed of the compensation system and on the mechanical inertia of the process. Advanced solutions use predictive control logics based on process models, to improve stability further.
Standards and installation
Calibration of stationary tension meters is carried out at installation, with calibrated masses or with reference force gauges applied to the product held still in the working position. Periodic verification can be performed in situ through dedicated procedures (for example by replacing the product with a calibrated cord). Compliance with the machinery safety directives (Directive 2006/42/EC in Europe) and with electromagnetic compatibility (EMC) requirements is essential for integration into industrial machinery. The interfaces must be compatible with the machine manufacturer’s systems.
Selection criteria
The choice of stationary tension meter takes into account: the working tension range (cells sized to operate at 30-70% of full scale are preferable, for maximum accuracy); the geometry of the product and the line speed; the digital interfaces required by the customer’s PLC system; the IP protection rating; the availability of standard mounting accessories for rapid mechanical integration; and the technical support available for setting up and tuning the PID control.
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