The last time you put something together with your hands, whether it was buttoning your shirt or rebuilding your clutch, you used your sense oftouch more than you may think. 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 check if two surfaces are flush. In fact, a 2013 study found that the human sense of touch can also detect Nano-scale wrinkles on an otherwise smooth surface.
Here’s another example from the machining world: the outer lining comparator. It’s a visual tool for analyzing the conclusion of the surface, however, it’s natural to touch and notice the surface of your own part when checking the finish. Our brains are wired to use the information from not only our eyes but also from our finely calibrated torque sensor.
While there are many mechanisms through which forces are changed into electrical signal, the primary parts of a force and torque sensor are similar. Two outer frames, typically made from aluminum or steel, carry the mounting points, typically threaded holes. All axes of measured force could be measured as you frame acting on the other. The frames enclose the sensor mechanisms and any onboard logic for signal encoding.
The most typical mechanism in six-axis sensors is the strain gauge. Strain gauges include a thin conductor, typically metal foil, arranged in a specific pattern on a flexible substrate. Because of the properties of electrical resistance, applied mechanical stress deforms the conductor, rendering it longer and thinner. The resulting change in electrical resistance could be measured. These delicate mechanisms can be easily damaged by overloading, as the deformation from the conductor can exceed the elasticity in the material and make it break or become permanently deformed, destroying the calibration.
However, this risk is typically protected by the design of the sensor device. While the ductility of metal foils once made them the standard material for strain gauges, p-doped silicon has proven to show a significantly higher signal-to-noise ratio. Because of this, semiconductor strain gauges are becoming more popular. As an example, all multi axis load cell use silicon strain gauge technology.
Strain gauges measure force in one direction-the force oriented parallel to the paths in the gauge. These long paths are made to amplify the deformation and so the alteration in electrical resistance. Strain gauges are not understanding of lateral deformation. For this reason, six-axis sensor designs typically include several gauges, including multiple per axis.
There are several options to the strain gauge for sensor manufacturers. For instance, Robotiq created a patented capacitive mechanism at the core of their six-axis sensors. The aim of making a new form of sensor mechanism was to create a way to look at the data digitally, instead of being an analog signal, and minimize noise.
“Our sensor is fully digital without any strain gauge technology,” said JP Jobin, Robotiq vice president of research and development. “The reason we developed this capacitance mechanism is because the strain gauge is not really safe from external noise. Comparatively, capacitance tech is fully digital. Our sensor has virtually no hysteresis.”
“In our capacitance sensor, the two main frames: one fixed and one movable frame,” Jobin said. “The frames are affixed to a deformable component, which we will represent as being a spring. When you use a force to nanzqz movable tool, the spring will deform. The capacitance sensor measures those displacements. Learning the properties of the material, it is possible to translate that into force and torque measurement.”
Given the need for our human feeling of touch to our motor and analytical skills, the immense prospect of advanced touch and force sensing on industrial robots is obvious. Force and torque sensing already is at use in collaborative robotics. Collaborative robots detect collision and may pause or slow their programmed path of motion accordingly. This makes them competent at working in contact with humans. However, a lot of this kind of sensing is carried out using the feedback current of the motor. When there is a physical force opposing the rotation from the motor, the feedback current increases. This change can be detected. However, the applied force can not be measured accurately using this method. For more detailed tasks, load cell is needed.
Ultimately, industrial robotics is about efficiency. At industry events as well as in vendor showrooms, we see plenty of high-tech features designed to make robots smarter and more capable, but on the financial well being, savvy customers only buy just as much robot since they need.