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Torque Measurement in Robotics Applications

The global market for robotics is expanding rapidly. With the advent of lower-cost robots that are easier and cheaper to deploy such as collaborative robots or cobots and warehouse robotics systems.

ISO 10218-1 is an international standard that sets out requirements for safety and design guidelines in industrial robots, in order to comply with the requirements cobots use position and torque sensing embedded into their joints to allow automated control and safe operation.

This typically requires the use of at least one force-torque sensor in addition to monitoring the electric motors in the joints. Some more advanced cobots now have torque sensing in every joint such as the cobots made by Doosan and the latest KUKA LBR iiwa machines.

In addition to helping meet the ISO 10218-1 safety standard, the use of multiple torque sensors also allows the robot manufacturer to improve system control with advanced kinematics using inputs from the torque sensors to compensate for the mass of the robot's limbs and the payload to deliver fast, controlled and safe movements of the robot through complex 3-dimensional paths. There is also a growing market for sensor systems as a standalone accessory or built-into robotic tools to help control the operation of the tool.

There are currently 2 key technologies for torque measurement in robotics applications:

Strain-Gauge Sensors

This is the conventional method for measuring rotating torque by using a strain gauge attached to a flex plate in the joint of the robot and is the dominant technology in use in cobot systems today. Changes in strain, because of torque, are recorded as variations in an electrical signal.

The advantages of strain gauges are they are relatively low cost and simple to apply in low volume like a test lab.

However, the disadvantages of strain gauges in robotic joints include:

They require the use of flex structures and four gauges being arranged in a Wheatstone bridge circuit to provide a measurable strain, this compromises the mechanical integrity of the system making the robotic arm less stiff than it could be without the sensors
Strain gauges are typically not robust or resilient to harsh environments, their output is affected by temperature and,
Susceptible to interference from background electromagnetic radiation and magnetic fields

Displacement Sensors

This method typically uses a pair of measurement disks attached at opposite ends of a shaft the ‘twist angle’ of the shaft is measured from the phase difference between them through an optical or magnetic measurement. This enables torque to be calculated.

The main advantage is the ability to overload the sensor to the maximum load capacity of the “twist shaft”.

The disadvantages of this method include:

Requires a reduced diameter section of the shaft known as a torsion bar to enhance the twist angle (several degrees at most for a length-to-diameter ratio L/D = 5) which can have a negative effect on the mechanical stability of the system
Sensitive to temperature
Limited measurement accuracy
Requires a larger packaging volume due to the need to measure shaft twist along the torsion bar length

Both of these existing technologies require an element of twist or flex in the robot's joints, this means that the robotic arm can flex in operation, and this can be a factor limiting the performance and repeatability of the cobot.

SAW Torque Sensors

There is now an alternative sensing technology, Transense's patented sensor technology based on Surface Acoustic Wave (SAW) technology that provides an improved way to measure torque, rotation, and temperature in a robotic system compared to conventional sensor technology.

SAW technology can facilitate improved robot performance and design by virtually eliminating joint flex, creating a higher-performing and more repeatable robot with more compact joints than has been possible before.

In mobile robots, it can be used to provide accurate torque data from the electric motor drive or final drive systems. This can enable closed-loop control of the electric motors to improve efficiency. Improved robot positional control and acceleration or deceleration, with accurate drive torque feedback and greatly enhancing torque vectoring capability compared to conventional drive systems. The technology can also be used to reduce servo-drive harmonics, extending the operating envelope of the machine.

Transense SAW technology is, a wireless, passive, non-contact sensing system consisting of two main components:

SAW sensing elements connected to an antenna or RF coupler mounted on the rotating shaft and
An electronic interrogation unit called a reader that is connected to its stationary RF Coupler. The reader creates an interrogation signal which is transmitted to the rotating shaft through the RF coupler.

The sensing elements on the shaft do not require any other power source and work as a passive device reflecting an interrogation signal back to the reader electronics. The very small amount of power required being contained in the RF signal. This system also allows the robot joints to fully rotate beyond 360 degrees, which can be particularly useful in the wrist joint. The RF coupler can also be used as an accurate position resolver. If full rotation is not required, for example in the lower joints of the system for example then the RF coupler can be eliminated and the SAW device can be hard-wired to the reader electronics.

The backscattered signal coming from the SAW device has a frequency of oscillation that is affected by a physical measurement such as strain and temperature.

The reader electronics analyse the received back-scattered signal and calculates the value of the physical strain and temperature, the shaft surface strain gives an accurate measurement of shaft torque. Measurement of temperature in the same unit allows for temperature compensation across a wide range of operating temperatures.

The advantages of this approach are:

SAW technology measures micro-strain so the shaft or joint can be very stiff, no need to compromise the mechanical integrity of the system
Immune to background RF and magnetic interference, so can be packaged very near to the electric motor inside the joint
More accurate than displacement sensors with +/-1% accuracy achievable in a typical application
Completely wireless installation is possible with small, light, and passive components only on the shaft
Delivers a high-speed measurement of torque that can then be used in system control
Compact dimensions that can be easily packaged into a complex system reducing the size of or eliminating entirely the flex plates and torsion shafts required by conventional sensors
Can provide torque and multi-axis bending load detection
Temperature sensing and compensation built-in
Robust and reliable proven in aerospace and motorsport environments

The disadvantages of this technology are:

Not available as an off-the-shelf sensor must be designed into a system
Requires proprietary Transense SAW devices and ASIC

In conclusion, the market for robotics systems is expanding rapidly in traditional markets such as manufacturing but also in new markets such as warehouse automation. The utility and safety of robotic solutions are driven by their ability to sense the environment in which they are operating, with force and torque measurement becoming more commonplace as a key control input. The SAW sensor technology developed by Transense offers a new way to measure torque and temperature in the robotics market that could help to unlock higher performance from robotic systems.