Torque sensing

Citation

(1999), "Torque sensing", Industrial Robot, Vol. 26 No. 5. https://doi.org/10.1108/ir.1999.04926eaf.003

Torque sensing

Torque sensing

Keywords Robots, Torque, Sensors

As "direct-drive" robots become increasingly popular, there is a need for high-performance motion controllers. A direct approach to control is possible if joint torque sensors are used to measure the external loads on all joints. Positive feedback of the torque signal can then compensate for the dynamics of the robot manipulator.

In the design of robot manipulators, much of the torque/force reaction of the link load on the joints comes in the form of non-torsional components because actuation then takes less effort. Since the torque sensor is directly attached to the motor's distal link, it has to bear those potentially large non-torsional components of the generalised force/torque vector at the joint. The first challenge is to measure the torsion torque faithfully without influence from any of the non-torsional components. The second challenge relates to the sensor stiffness. To increase the signal-to-noise ratio of the sensor, it is desirable to design a structure that generates a large strain for a given load torque and therefore has a high sensitivity. However, the resulting compliance introduces a joint angle error that should be minimised. Thus there are two conflicting requirements - high mechanical stiffness and high torque sensitivity. Strain gauges exhibit variations in their gauge factor, which is the ratio of the fractional change in resistance to strain. The strain gauges are also placed on areas with high strain gradients, which makes the gauge outputs sensitive to placement errors.

These contradictory requirements can be captured by defining a performance index, called structure efficiency, as the ratio of the (local, maximum) strain to the (overall) torsional deflection caused by the same torque. This dimensionless index is a decisive factor in the sensor design and should be maximised. It is independent of material properties, and captures the ratio of the local and global strains.

The index is maximised in elastic structures that produce high strain concentration in torsion. In theory, there is no limit on the strain concentration in an elastic body. However, high strain concentrations occur in very small areas, which may be smaller than the physical size of available strain gauges. Moreover, since strain gauges average the strain field over their area, the detected strain my be significantly lower than the calculated maximum. Therefore, it is important to generate high strain over a sufficiently large area.

Introducing a torque sensor into a robot joints adds flexibility. Although torsional flexibility can be compensated for by sophisticated controllers, deflection in the other axes is more problematic. Consequently, another design criterion dictates high stiffness in non-torsional directions. Addition of a torque sensor to a robot joint must not require redesign of the joint and should result in a minimal change in the manipulator's kinematics, in particular the link offset. Hence, a shape with a small width is desirable.

The body of the sensor should be designed for ease of manufacture. It should be a monolithic structure; that is, the body should be machined from a solid piece of material. This decreases the hysteresis and increases the strength and repeatability of the sensor.

The sensor design must optimise the trade-off among several conflicting design criteria. Also, many design iterations are required to arrive at a final design. Despite this complexity, it is possible to arrive at a novel basic sensor design.

Similar to a thin-wall beam, the structure should have a large second moment of area around its x axis compared to that around its z axis. As a comparison it is worth noting that a simple cylinder is 0.77 times less stiff in bending that in torsion.

Thin-section rectangular bars experience high stress/strain concentrations under torsion loads, which yield high sensitivity without sacrificing stiffness. This fact suggests that an appropriate structure should be primarily stressed by torsion. The elastic structure exhibits high bending stiffness around the x axis. However, its poor stiffness around the y axis is a drawback. This problem can be solved simply by adding more wing pairs. For maximum sensitivity, strain gauges should be located where the maximum induced strains occur. Since the strain field is averaged over the area covered by the strain gauges, it is very important first to determine the loci of the peak strain, and second to ensure the creation of a sufficiently large strain field. The sensor body is modelled by solid elements. Since the body is symmetrical in geometry and boundary conditions, it suffices to analyse half of it, provided that adequate position constraints are imposed on the nodes of the cutting plane.

In order to characterise the linearity and sensitivity of the sensor, static torsional and unbending torques were applied in an experimental apparatus built for these static tests. One side of the torque sensor was affixed to a bracket, while two aluminium bars were attached radially and axially to the other side. The ends of the bars were connected to mechanical levers via ropes in which load cells (MLP-50 from Transducer Techniques) were installed.

Motivated by the need for joint torque sensing in robots, a new sensor has been designed. Its key features are its extremely high stiffness and its insensitivity to non-torsional force/torque components. It has been shown that the maximum strain sensitivity to torsion can be maintained without sacrificing torsional stiffness, if elastic body exhibits strain concentration to torsion loads. The sensor also has been designed so the effect of non-torsional moments and forces on the local strains is minimal, which has been achieved by a combination of mechanical design and electrical summation of strain gauge signals. The sensor has been tested extensively. Tests confirm that the sensor meets all its design goals and is well-suited as a torque-sensing device in robots or other industrial applications.