Low-cost force sensor aims for safer industrial robots

A low-cost force sensor for industrial robots would allow smaller companies to use them safely alongside human employees, according to engineers working on the project.

The sensor project at Germany’s Fraunhofer Institute for Silicon Technology (ISIT)sprang from a wider European-funded initiative to develop robots suitable for small and medium-sized enterprises (SMEs).

In major industrial production environments robots are usually huge and have to work round the clock at a static one-function station, for example, carrying out welding tasks in the automotive industry.

ISIT head of department Jörg Eichholz said: ‘SMEs need smaller, smarter, lighter, more flexible robots and they have to be cheaper and simpler to program by operators rather than expert programmers.’

Many industrial robots are also required to have a fence around them and cannot work in the same area as humans due to the dangers involved.

Sensors with six degrees of motion detection and sensitive strain gauges to detect collision are available but are expensive, costing around €5,000 (£4,300) each. Only a few universities and institutes have them and they are too expensive to use widely in industrial automation.

‘Assuming a low-cost sensor is made available, a number of factors have to be included to make robots suitable for SMEs,’ said Eichholz. ‘One is the ability to easily teach them. Using the new sensors on the wrist joint of a robot, a user would be able to take it like a child’s hand and show it how to move when conducting the assigned task. From this it develops its own program to carry out the job.’

Given that in many SMEs people will need to work in close proximity to the robots, the systems need to be able to operate safely without a fence around them. The researchers’ solution is to install networked cameras around the workspace so the robot stops if anything enters the area.

A force sensor is required to reduce the impact if a robot does make contact with a person or another piece of equipment. Fraunhofer’s robots would use force-torque sensors, and if a signal over a programmed limit is reached, it would stop and retreat to a predetermined point.

The key to making these solutions available to SMEs is producing the sensors cheaply, according to Fraunhofer. ‘We think it is possible to reduce the cost by using a single piece of silicon with four bridges and four resistors in each bridge working as strain gauges,’ said Eichholz. ‘These would be made in the same way as standard CPUs and electronics using precise lithography with an accuracy of 0.1 micrometre.’

The sensor measures the forces and torques with a long wire at its core through which an electric current flows. If the wire stretches, it becomes longer and thinner, the resistance increases and so less current flows through it. Bridges on either side carry electrical resistances. If the robot arm bumps into an obstacle, the shape of the silicon changes slightly, which causes either more or less current to flow.

The transducer is glued onto a steel plate so it can be screwed in between the arm and the grabber on the outer joint of the robot’s arm.

Strain gauges can be subject to misorientation and require complex calibration. This is avoided in the new technology as it is made from a single piece of silicon with precisely aligned resistors.

Fraunhofer believes that once brought into mass production, the sensors would be affordable enough to be widely adopted by smaller manufacturing companies. The next step will be to make an enhanced working prototype, which will be available within two years.