The development of wireless local-area networks (WLAN per IEEE802.15.4 and the ZigBee standard) is paving the way for the wider use of sensors in applications where they had been difficult to apply. For example, ZigBee lets networked sensors (nodes) in manufacturing areas share information for immediate feedback that might improve a machine's speed and precision. A load cell or torque sensor could communicate with a temperature sensor or accelerometer within the same environment. With a temperature reading, for instance, the load cell might make an adjustment so the machine output stays accurate over a wide temperature range, or alert maintenance people that the machine needs attention now. In bottling machines, a line worker could place a fake bottle on line with a torque sensor in the cap (a torque auditor) and let it run through the capper. The torque it reads would be wirelessly sent back to the machine controls for comparison and adjustments. The sensors also provide recordable values for a particular operation, something FDA regulations often require. And if one node in the WLAN is Wi-Fi capable, the machine might use the Internet to tell a factory manager how well it's working.

Wireless features are more useful when a machine has many sensors. Wiring them for power is often not difficult but dealing with a mass of signal wires can be bothersome and a maintenance problem. Wireless sensors clean up the mess. These capabilities open new ways to control equipment.

A few applications

Torque sensors found work in auto factories in the mid 70s and quickly spread to other machines outside the industry. Examining a few of these applications may give control designers ideas for the next assignment.

Constant flow pumps can be driven by stepper motors. A sensor on the motor's end monitors its torque (a flow-rate indicator), transmits changes, and alerts operators when torque falls outside a specified window. Torque can indicate when the seal is worn or if the inlet is clogged. Proper maintenance can then lengthen the life of the pump.

Automated winding machines are frequently used in production of textiles and woven medical materials. One production-line challenge is to maintain constant tension in the fiber. Using a load cell behind a bobbin pulley or feed-roller mechanism makes this possible. The same challenge exists in tape and wire winding equipment. Some manufacturers use reaction sensors to track torque on the servomotor for feedback. This compensates for material slippage, and ensures smooth repeatable operations along with increased speed and reliability.

Stamping operations use machines with load cells and torque sensors to make low cost, high-volume brackets for consumer and medical products. During assembly, a conveyer moves sheets through various operations such as stamping, die cutting, bending, forming, drilling, and tapping. Many of these operations rely on sharp tools such as dies, drills, and tapping bits. In the past, experienced operators would have to listen for telltale sounds or felt unusual vibrations to detect problems. But this is not practical or possible with automated high-speed lines.

Load cells in-line with the dies and reaction-torque sensors behind or in-line with servomotors monitor and record loads and torques, and tell operators when new or sharper tools are needed. This increases yield without labor-intensive quality checks or costly recalls. Sensors also let engineers send live data from operations to an SPC program for further analysis.

Verifying the cap torque on bottles after filling is a common challenge in pharmaceutical and food-and-beverage companies. It is crucial to ensure proper capping torque by using servomotors and a torque-adjustable cap-tightener. Production lines still need quality-assurance programs and check points even though many systems can reject bottles for an insufficient cap torque.

Considering that some bottling machines operate at up to 1,200 bottles/min., it is essential to detect and verify malfunctions early. One verification method uses bottle auditors, reaction-torque sensors and data loggers mounted in bottles that are run through a capper. These are mixed in with other bottles to collect and record torques during operation. This can be enhanced with a ZigBee network that reports real-time data and instantly detects failures.

Laser welding makes aircraft lighter by replacing rivets. Riveting equipment works at about 3 to 7 mm/sec while laser welders run at 100 mm/sec. But laser welding would not be possible without precise motion controls, as well as sensors to monitor force and torque. Weld quality relies on the sensors for proper feedback.

A filling machine for radioactive-fluid capsules uses automation to increase throughput without spilling the fluid. It uses a biaxial indexer with a linear motor to move a tray of capsules on a horizontal axis. A stepper motor and lead screw raise and lower the filling head. A torque sensor and load cell smoothes the motions and makes measurements more precise, while feedback and data collection continues for later verification and analysis. Selecting a stepper rather than a linear motor for the vertical axis helps prevent a sudden drop of the filling head should power fail. Residual torque in the stepper motor holds position.

A torque-controlled positioner uses a self-magnetizing servomotor with a built-in torque sensor developed to move an 18-m dia. parabolic antennae. The controls provide fail-safe, continuous, and accurate positioning of a critical early detection system for the U.S. military.

The large antennae, in a windy area (gusts to 130 kph), makes for severe control challenges. Measuring and monitoring peak torque lets the system recharge its magnets during high winds with a magnet-charging circuit.

Meet Teds

It takes sensors and feedback to build automated systems. Advancements in the technology have made sensors more commonplace. Smart sensors have also let manufacturers recognize the need to standardize an entire enterprise. To assist, sensor manufacturers initiated an alliance which resulted in Transducer Electronic Data Sheet, or Teds, according to the IEEE1451.4 standard and an option promoted in the sensor industry. This first step of standardization lets sensors and related instruments more easily interface with each other. Yes, some incompatibilities in different application layers still exist. But these wrinkles will be ironed out by further alliances with players such as the servo or stepper motor industry.