The last time you put something with your hands, whether it was buttoning your shirt or rebuilding your clutch, you used your feeling oftouch more than you may think. Advanced measurement tools like gauge blocks, verniers and also coordinate-measuring machines (CMMs) exist to detect minute variations in dimension, but we instinctively use our fingertips to see if two surfaces are flush. Actually, a 2013 study discovered that the human sense of touch can even detect Nano-scale wrinkles on an otherwise smooth surface.
Here’s another example from the machining world: the surface 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 part when checking the finish. Our brains are wired to use the details from not only our eyes but also from your finely calibrated rotary torque sensor.
While there are many mechanisms in which forces are changed into electrical signal, the main areas of a force and torque sensor are similar. Two outer frames, typically made of aluminum or steel, carry the mounting points, typically threaded holes. All axes of measured force can be measured as you frame acting on the other. The frames enclose the sensor mechanisms and any onboard logic for signal encoding.
The most frequent mechanism in six-axis sensors is the strain gauge. Strain gauges contain a thin conductor, typically metal foil, arranged in a specific pattern on the flexible substrate. Because of the properties of electrical resistance, applied mechanical stress deforms the conductor, which makes it longer and thinner. The resulting improvement in electrical resistance may be measured. These delicate mechanisms can be simply damaged by overloading, as the deformation from the conductor can exceed the elasticity of the material and cause it to break or become permanently deformed, destroying the calibration.
However, this risk is usually protected by the appearance of the sensor device. While the ductility of metal foils once made them the typical material for strain gauges, p-doped silicon has proven to show a much higher signal-to-noise ratio. For that reason, semiconductor strain gauges are gaining popularity. As an example, most of triaxial load cell use silicon strain gauge technology.
Strain gauges measure force in just one direction-the force oriented parallel for the paths in the gauge. These long paths are designed to amplify the deformation and so the modification in electrical resistance. Strain gauges are certainly not sensitive to lateral deformation. For this reason, six-axis sensor designs typically include several gauges, including multiple per axis.
There are a few choices to the strain gauge for sensor manufacturers. For instance, Robotiq made a patented capacitive mechanism in the core of their six-axis sensors. The goal of creating a new kind of sensor mechanism was to produce a method to look at the data digitally, instead of as 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 because the strain gauge will not be resistant to external noise. Comparatively, capacitance tech is fully digital. Our sensor has almost no hysteresis.”
“In our capacitance sensor, there are two frames: one fixed and one movable frame,” Jobin said. “The frames are connected to a deformable component, which we will represent as a spring. Once you use a force to nanzqz movable tool, the spring will deform. The capacitance sensor measures those displacements. Knowing 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 the motor and analytical skills, the immense possibility of advanced touch and force sensing on industrial robots is obvious. Force and torque sensing already is at use in the field of collaborative robotics. Collaborative robots detect collision and may pause or slow their programmed path of motion accordingly. This makes them able to working in touch with humans. However, a lot of this type of sensing is done using the feedback current of the motor. If you have a physical force opposing the rotation of the motor, the feedback current increases. This change could be detected. However, the applied force cannot be measured accurately by using this method. For further detailed tasks, load cell is necessary.
Ultimately, industrial robotics is all about efficiency. At trade shows as well as in vendor showrooms, we see a lot of high-tech bells and whistles designed to make robots smarter and more capable, but on the financial well being, savvy customers only buy the maximum amount of robot because they need.