ROBOTICS 01PEEQW Basilio Bona DAUIN – Politecnico di Torino

Force/Torque Sensors

Basilio Bona3 ROBOTICS 01PEEQW /2016 Force/Torque Sensors

 Many Force/Torque (FT) sensors consist of strain gauges attached to a piece of steel. When a force is applied, the material bends and the resistance of the strain gauge changes.  With most modern FT sensors, the internal electronics take care of all the calculation and signal cleaning and produces a digital force signal for all six axes  However, strain gauges are not the only sensing technology used in 6-axis FT Sensors.  Capacitive sensing technology can be hugely effective for robot force sensors; capacitive 6-axis FT Sensors overcome many of the problems with strain gauge technology.  6-axis FT Sensors have two ends: a fixed casing and a floating plate. The fixed casing is usually attached to the end of the robot arm, while the floating plate is attached to the end effector or tool. Inside the FT sensor, the plate is attached to the casing by a stiff but compliant metal structure, which bends slightly when a force is applied.  Various sensors are fitted to the structure to detect the deflection of the material. In traditional strain-gauge FT sensors, this compliant structure usually consists of three metal beams, each fitted with several strain gauges.  In capacitive sensors, it consists of an intricately machined metal block, which is designed to deform in a particular way under force. This structure is attached to two metal plates (one at each end) which hold a set of capacitive sensors to measure the displacement of the plates. Basilio Bona4 ROBOTICS 01PEEQW /2016 Force/Torque Sensors

 The most basic force sensor is a simple Force Sensitive Resistor. They are cheap (some as low as 5 USD), and, in general, they are not suitable for precision measurements. They are good for detecting if a force has been applied or not; the error may be as high as 25%, but if that is good enough for your application, then it is certainly an economical option. Basilio Bona5 ROBOTICS 01PEEQW /2016 FT Types: Resistor

 Another type of sensors are flexible force sensors. While they may look similar to the previous option, and work on similar principles, they generally have a better accuracy and are also more expensive (more in the range of 20 USD for a resistive flexible sensor).  For example, the Flexi Force is a popular brand of flexible resistive force sensors which can have errors of less than 3% and support loads up to 445N. Basilio Bona6 ROBOTICS 01PEEQW /2016 FT Types: Flexible Force Sensor  They are adaptable and fit into small spaces, but only detect forces applied at one point and in one direction.

 Capacitive flexible force sensors are a similar option, which can be a bit more accurate over a smaller range, but are also more expensive Basilio Bona7 ROBOTICS 01PEEQW /2016 FT Types: Capacitive Flexible Force Sensor

 Piezoelectric force sensors are another sensing technology that works using quartz crystals which generate an electrostatic charge when a force is applied, creating a voltage which is proportional to the input force.  They can be good for dynamic force applications due to their high responsiveness. However, they are not good for static force measurements, because the voltage decays quickly, meaning you would have to employ lot of extra signal processing for use in robotic applications. Basilio Bona8 ROBOTICS 01PEEQW /2016 FT Types: Piezoelectric

 Strain gauge bases sensors are the most common force sensing technology in robotics.  These are either used individually, in a single load cell, or multiple strain gauges are used together to measure multi-axis forces, as in a 6-axis FT Sensor.  The function of a strain gauge is quite simple – the resistance of a flexible conductive foil changes when placed under strain. This resistance change is usually measured using a Wheatstone Bridge, which produces a voltage proportional to the resistance change. Basilio Bona9 ROBOTICS 01PEEQW /2016 FT Types: Strain Gauge

 The strain gauge is fixed to a material with known mechanical properties. When a force is applied to the material it will deform elastically. The strain gauge measures the deformation and this is multiplied by the Young's Modulus and cross sectional area of the material (which are both constants) to calculate the force.  As they usually rely the voltage change, strain gauges are inherently affected by noise. Various techniques exist to reduce the effect of noise on strain gauges, which designers implement in the integrated electronics of most FT Sensors. However, it is something to bear in mind if using such sensors in environments with high levels of electrical noise (e.g. near motors, AC power lines, arc welders or relays).  Strain gauge force sensors of one axis are known as load cells. Unless the load cell has integrated electronics you will have to perform some signal processing on the output of the load cell. Basilio Bona10 ROBOTICS 01PEEQW /2016 FT Types: Strain Gauge

Basilio Bona11 ROBOTICS 01PEEQW /2016 FT Types: Load Cells Load Cells

 Capacitive technology is the next step beyond the traditional strain gauge technology because it overcomes some of the inherent problems with strain gauges. Capacitive sensors are used to detect the distance between two parallel conductive elements.  An alternating voltage is applied to the elements, which produces an alternating electric current which can be then used to calculate the distance between the elements.  A set of specially positioned capacitance sensors is used. These sensors are attached to two metal plates which are connected to each other mechanically through a compliant element. The two metal plates act as the top and bottom of the force sensor. When a force is applied, the compliant structure squashes (or stretches) and the capacitive sensors can measure this deflection. We can then use the measurements to calculate the applied force in all six axes. Basilio Bona12 ROBOTICS 01PEEQW /2016 FT Types: Capacitive

 There are several advantages to using capacitive sensors over the traditional strain gauges. Because the sensors use an AC voltage, they are not affected by noise as strain gauges are, because the provided signal is inherently digital.  Capacitive sensors generally produce a stronger output, so less signal conditioning is needed, which ultimately means better accuracy.  While strain gauges must be bonded to the metal with an adhesive, which can be a point of weakness over time, capacitive sensors do not require bonding Basilio Bona13 ROBOTICS 01PEEQW /2016 FT Types: Capacitive

 There are many more technologies which are used in force sensing. These include pressure-based sensing, using pneumatics or hydraulics, which have been used in research for micro applications and can be used in restrictive environments such as fMRI machines. There are also less common techniques, like magnetic force sensors. There are even new, cheap, tactile sensors which are based on barometer technologies.  New force sensors and force sensing technologies are being developed all the time, especially in the field of micro and nano-electronics, as used in robotic micromanipulators. Basilio Bona14 ROBOTICS 01PEEQW /2016 FT Types: Other Sensing Technologies