Torque Sensor – Useful Details..

The last time you put something with your hands, whether it was buttoning your shirt or rebuilding your clutch, you used your sense of touch more than it might seem. Advanced measurement tools including gauge blocks, verniers as well as coordinate-measuring machines (CMMs) exist to detect minute variations in dimension, but we instinctively use our fingertips to see if two surfaces are flush. In fact, a 2013 study found that the human sense of touch may even detect Nano-scale wrinkles on an otherwise smooth surface.

Here’s another example through the machining world: the top comparator. It’s a visual tool for analyzing the finish of a surface, however, it’s natural to touch and feel the surface of your own part when checking the finish. Our brains are wired to use the data from not only our eyes but also from our finely calibrated Miniature Load Cell.

While there are many mechanisms by which forces are changed into electrical signal, the key areas of a force and torque sensor are the same. Two outer frames, typically made from aluminum or steel, carry the mounting points, typically threaded holes. All axes of measured force can be measured as one frame acting on one other. The frames enclose the sensor mechanisms as well as any onboard logic for signal encoding.

The most frequent mechanism in six-axis sensors is definitely the strain gauge. Strain gauges consist of a thin conductor, typically metal foil, arranged inside a specific pattern on a flexible substrate. Due to the properties of electrical resistance, applied mechanical stress deforms the conductor, which makes it longer and thinner. The resulting improvement in electrical resistance could be measured. These delicate mechanisms can be simply damaged by overloading, since the deformation in the conductor can exceed the elasticity from the material and cause it to break or become permanently deformed, destroying the calibration.

However, this risk is normally protected from the design in the sensor device. While the ductility of metal foils once made them the typical material for strain gauges, p-doped silicon has seen to show a significantly higher signal-to-noise ratio. For that reason, semiconductor strain gauges are gaining popularity. For example, most of ATI Industrial Automation’s six-axis sensors use silicon strain gauge technology.

Strain gauges measure force in one direction-the force oriented parallel to the paths inside the gauge. These long paths are made to amplify the deformation and thus the alteration in electrical resistance. Strain gauges are not sensitive to lateral deformation. Because of this, six-axis sensor designs typically include several gauges, including multiple per axis.

There are some choices to the strain gauge for sensor manufacturers. For instance, Robotiq made a patented capacitive mechanism at the core of its six-axis sensors. The goal of developing a new type of Rotary Torque Sensor was to make a method to measure the data digitally, as opposed to as being an analog signal, and minimize noise.

“Our sensor is fully digital without strain gauge technology,” said JP Jobin, Robotiq vice president of research and development. “The reason we developed this capacitance mechanism is mainly because the strain gauge is not immune to external noise. Comparatively, capacitance tech is fully digital. Our sensor has almost no hysteresis.”

“In our capacitance sensor, there are 2 frames: one fixed and one movable frame,” Jobin said. “The frames are affixed to a deformable component, which we shall represent being a spring. Once you use a force to the movable tool, the spring will deform. The capacitance sensor measures those displacements. Learning the properties in the material, it is possible to translate that into force and torque measurement.”

Given the price of our human feeling of touch to our motor and analytical skills, the immense potential for advanced touch and force sensing on industrial robots is obvious. Force and torque sensing already is at use in the area of collaborative robotics. Collaborative robots detect collision and may pause or slow their programmed path of motion accordingly. As a result them able to working in contact with humans. However, much of this sort of sensing is carried out via the feedback current in the motor. If you have a physical force opposing the rotation from the motor, the feedback current increases. This change can be detected. However, the applied force wbtbtc be measured accurately using this method. For additional detailed tasks, 3 Axis Load Cell is necessary.

Ultimately, industrial robotics is approximately efficiency. At trade shows as well as in vendor showrooms, we have seen a lot of high-tech features made to make robots smarter and more capable, but on the bottom line, savvy customers only buy as much robot as they need.

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Caroline Powell

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