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Load Cells and Torque Sensors in Automation
Release Date: January 2007

Today sensors provide solutions for most of the challenges facing the growing demand in mechatronics, robotics, automation, and motion control. According to the MCA (Motion Control Association), these industries currently account for more than $44 billion in annual sales. Over the past decade, significant advancements have been made in sensor technology. Smart sensors are becoming available with analog or digital output, and or wireless capability. Miniaturization and overload provision along with lower cost solutions for plug and play sensors are also contributing to increased usage. Sensors are a key component in the automation industry, behaving almost as part of its nervous system. Hence, selecting the correct sensor with expected performance & acceptable "MTBF" (Mean Time Between Failure) and using the sensor properly is extremely vital. A variety of sensors including tactile, pressure, force, torque, displacement, temperature, accelerometer, optical, vision, infrared, proximity, magnetic and many more is widely used in the automation & motion control system industries. However, the focus of this article is strictly on load cells and torque sensors.

The advancement of the wireless local area network (WLAN) per IEEE802.15.4 and ZigBee standard is paving the way for sensors to be implemented in applications where it was not possible in the past. ZigBee low power with multi nodes communication capability allows sensors to communicate among them. Through this communication process they can share information for immediate feed back control to optimize their performance in speed and precision. A load cell or torque sensor could communicate with a temperature sensor or accelerometer within the same environment to detect any anticipated problem, which could affect its performance. Since one of the possible nodes in the wireless local area network is WI-FI, communication through the Internet can be facilitated as well. In turn, this widens and amplifies the capabilities and possibilities of controlling the automation & motion control system. Engineers and their maintenance team can now remotely monitor all networks of automated activities to minimize or eliminate any rejects or malfunctions in a timely fashion. This helps avoid system failure and down time, allowing more elaborate PM (preventive maintenance) as needed.

For decades, load cells and torque sensors have been widely used for measurement & control and have been very instrumental in the operation of automated systems as a feedback source. Just like stepper and servomotors, load cells and torque sensors have become a fundamental component in the automation industry.

In the mid 70's automotive industries fully benefited by utilizing multi spindle nut-runners with built in reaction torque sensors to monitor the required torque during automated oil pan assembly. The success of the program was a result of a close alliance between a prominent air tool manufacturer and a very innovative sensor manufacturer. Below are other application examples:

Constant flow pump: In the constant flow pump, a stepping / stepper motor is used to create a pumping function and allow fluid to flow. A reaction torque sensor attached to the back end of the motor monitors the required torque and can relate the changes in torque to the flow rate or alarm the operator when the torque falls outside of its expected window or malfunctions. Using the torque feed back improves the efficiency of the pump and elongates its life.

Automated winding machine: A popular application in the textile industry is the automated winding machine. One of the challenges engineers have on the production line is maintaining constant tension in fiber during operation. Utilizing a load cell behind a bobbin / pulley or feed roller mechanism makes this procedure feasible. The same challenge exists in back up tape winding as well as wire winding industries. Some manufacturers use a reaction torque sensor to track the torque on the servomotor for feedback. This will compensate for material slippage, which results in a smooth repeatable operation and, in turn, increases speed & reliability.

Stamping operation: In order to fabricate low cost and high volume brackets for automotive or other consumer products, a fully automated stamping house is set up to meet expected commitment. The success of the program is based solely on projected yield. During the assembly line a conveyer continuously moves flat sheets of material through various operations including stamping, die cutting, bending, forming, drilling, and tapping .The quality of the operation relies on sharp tools including dies, drills & tapping bits. In order to detect a dull tool and avoid rejects at the end of the production line, load cells are placed in-line with the dies and reaction torque sensors behind or in-line with the servomotor to monitor and record the required load or torque to alarm the operator when a new or sharper tool is needed. This helps increase yield without requiring a labor-intensive quality check or costly recall. In the past, experienced operators had to listen for a specific sound or vibration during manual operation but with high-speed automated lines this is not practical or possible. With sensors, engineers can also simultaneously send live data during the operation to a SPC program for further analysis to help them meet their six-sigma quality goal.

Torque controlled antennae positioning system: A self-magnetizing servomotor with a built-in precision torque sensor has been developed to support the parabolic antennae providing fail-safe, continuous, high precision, & accurate positioning of a critical early detection system for the U.S. Military. The 18 meter diameter parabolic antennae is located in an extremely windy and harsh area as high as 130 kilometers per hr which is capable of moving the large dish antennae and causing severe control challenges by effecting the motor torque. The ability to measure and monitor peak torque makes it possible for the system to recharge the magnets with an integral magnet-charging circuit after peak condition is encountered. This in turn, makes the necessary adjustment. This sophisticated and well-designed system has helped the military achieve a very challenging and critical requirement for our country's protection and safety.

Bottle capping & on line torque verification: One major and common challenge in the pharmaceutical and food & beverage industry is capping after filling and plugging. It is crucial to ensure proper capping torque by utilizing improved servomotors and a settable cap tightening torque mechanism. Although many systems come with the capability to reject the bottles having insufficient capping torque on line during production, reliable quality assurance programs and check points are still required to ensure that this happens and provide added quality assurance.

Considering that some of these automated systems operate at the rate of up to 1200 bottles per minute, it is essential to detect & verify any malfunction early on. One possible verification system utilizes bottle auditors with an integral reaction torque sensor and data logger mixed in with other bottles to collect & record torque values during operation. The system can be enhanced further using ZigBee wireless communication to report live data for instant detection of any system break down.

Laser welding & high precision motion control: In an effort to make aircraft lighter and faster, laser-beam welding is replacing riveting where possible. Comparing the riveting speed of about 3 to 7 millimeters per second with 100 mm per second for laser welding, you can definitely appreciate the benefit of new technology. But the new technology of laser welding system would not have been possible without utilizing high precision motion control systems as well as multi-component sensors to monitor force and torque during the high-speed operation. The optimum weld quality relies on the sensor's precision performance for proper & reliable feedback.

Radioactive fluid capsule filling machine: A high precision automated system is required for increased throughput without introducing spillage. A bi-axial indexer is selected; a linear motor helps to move the tray of capsules on the horizontal axis; and a stepper motor with a lead screw raises or lowers the filling head vertically. With the use of a torque sensor and a load cell in the system, smooth operation and precision measurement is achieved while proper feedback and data collection continues for further verification and analysis. The selection of the stepper motor over linear motor for vertical axis also helps prevent the sudden fall of the filling head by utilizing the residual torque in the stepper motor should power failure occur.

Spray glazing operation on a rotating table: In another application in which sensors aren't thought to be applicable, a robot is selected to spray glaze on parts located on a rotating table in a production line. The engineers require maintaining and controlling the added weight of the glazing material on each part. A smooth operation, as well as constant and well-controlled torque, is maintained using a torque sensor with the motor. In addition a precision load cell with Zig-Bee wireless communication capability is placed under each part to monitor the added weight of the glaze. With the proper feedback, a headache free automated system continues to operate requiring low maintenance.

As demonstrated, sensors are a required source of feedback in automation and motion control. With advancements in technology, the usage of such products is becoming much more commonplace. The introduction of smart sensors had manufacturers recognizing the need to standardize an entire enterprise. Therefore sensor manufacturers initiated the necessary alliance which resulted in the creation of TEDS (Transducer Electronic Data Sheet) per IEEE1451.4 standard, an option now promoted in the sensor industry. This first step of standardization allows sensors and related instruments to interface with each other but some incompatibility in different application layers still exist which will require further alliances with major players such as the servo or stepper motor industry. Contact a sensor solution company / manufacturer at the early stage of your design to minimize this incompatibility and see how load cells and torque sensors can help improve your project's application.

Printed in Instrument Design & Technology Winter 2006 Issue.

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