Tactile sensing for mechatronics – a state of art survey

M.H. Lee, H.R. Nicholls

Mechatronics, No. 9, 1999

Summary

                   In industrial applications, contact interaction are an important feature of physical manipulation systems, but research in the field of tactile sensors has undergone a drastic slowdown after the ’80, when is was supposed that it would had been a fundamental sensor for the following decades. A tactile sensor is therefore defined as a device that can measure a property of an object through the contact with it. The other 4 sensing modalities are basically all advanced today in technology and even computer vision has became cheaper.

The difficulties which have somehow stopped research in tactile sensors is due to the fact that in human being this sensing modality isn’t localized, it’s complicated to transduce and it’s difficult to imitate. In industry some basic forms of sensing, such as “spatial switches” are quite common and easily accessible, therefore they are not in the matter of this stat of art survey. In the 90’ the studies directed the transducing methods to the following technologies, not basically available: Resistance and Conductance, Capacitance, Piezoelectric and Pyroelectric, Magnetic, Magnetoelectric, Mechanical, Optical, Ultrasonic and strain gauges. Interesting researched have been performed in cutaneous sensor, which are basically divided in to extrinsic (mounted at or near the contact interface) and intrinsic sensing (which consist in derivation of contact data from force sensing within a mechanical structure), this study covers about the former one, which doesn’t deal with force/torque sensors. An important method for obtaining cutaneous sensor is by using array of integral sensing elements, which have been demonstrated to be capable of having a spatial resolution of 2-4 mm (Beebe). Gray and Fearing reported an 8 ⨯ 8 capacitive fabricated array of 1mm² area.

One of the major problems regard inverse analysis, which is the issue of computing the changes on the surface from the sensed data gathered remotely through the elastic medium, since there isn’t a unique solution.

Artificial sensing fingers appear to be another interesting application for exploration and grasping, in this field at least two types of tactile sensors are considered: one for contact point localization and one for detecting more spatially diffuse dynamic events, such as contact slip.

Soft materials are becoming an interest matter for tactile sensing research and gels, followed by powders appear to be the best material in terms of impact and strain energy dissipation, conformability to surface and hysteresis effects. Also the fact that human tissue is composed by electrolytic materials have inspired researched such as Sawahata, Gong and Osada to use polyacrylamide, which, with similar mechanical properties, can capture the electrical change (piezoelectric effect). Tactile sensor can also reduce kinematic errors in stiffness control by locating precise contact point and tracking changes, being useful for dexterous multi-fingered hands (Howe).

Whiskers have also objects under study, in fact they appear to be fast, accurate and cheap, essentially being single point sensors. Son, Cutkosky and Howe demonstrated how intrinsic and extrinsic tactile sensors can be effective with less than 1 mm error contact location. Tactile sensors appear to have application also in haptic perception (integration of cutaneous surface sensing with information from position and movement variable of the manipulator), teleoperation (remote human operating a robot) and virtual reality (for which multi-sensor gloves or other actuators have been creator to provide tactile sense to the operator). Processing of the data my be with fuzzy logic, rule-based systems or model-based systems, but neural networks appear to be the fastest.

Key Concepts

Tactile Sensors

Key Results

Toyota ins an example of organization pushing workers to have “safe partnership” with robots, in order to achieve this either intrinsically safer equipment must be provided (Tobita et al.) or there must be comprehensive collision avoidance (Suita et al.), therefore tactile sensors would be a fundamental tool for human robot interaction ensuring reliability and safety conditions.

Design of Tactile Sensing Systems for Dextrous Manipulators

Stephen C. Jacobsen, Ian D. MacCammon, Klaus B. Biggers, Richard P. Philipps

Control Systems Magazine, 1988

Summary

                  Tactile sensors appear to be extremely important for a certain amount of information they can deliver, although research on the design of tactile sensors is still on the way due to the simplicity in mechanics that grippers have. The main issue appear to be: understanding the ways which contact information can be used to control grasp and the development of the overall system itself. Technologies at the mere level of transducing appear to be mainly on hand, while problems appear to be on the higher level of organizing the overall system in which the sensors are applied. The overall mechanical manipulation system is schematically divided in 6 subsystems: command source, control, effector, observers, models and the physical environment.

The paper takes as an example the Utah/MIT Dextrous hand with tactile sensors applied on it, although it wasn’t intended for industrial application, the hand appears to be good for this kind of testing in order to verify speed, strength, range motion, capability for graceful behavior, reliability and economy. The authors propose a hierarchical structure of general requirements, for this at the first level there is transduction, which is at the most simple and contact level. At the second level there is preprocessing, which is strongly dependent on the transducer and influences reliability, size and mechanical behavior of a tactile sensing array. At a third level there is the multiplexing and transmission, where data is collected and forwarded to the following steps. At the forth level there is tactile data selection, in fact data must be filtered, since only part of the information is really interesting for practical purpose (as for vision sensors). At the fifth level there is tactile data interpretation, where information is mapped and sent to the sixth level, the multisensory fusion, where information is blended with the output of other sensory systems. At the seventh level there is the world model construction, where  multidimensional image is constructed from the data; finally at the eight level there is control of grasp and manipulation. At the transducing level the aim is to obtain a stable grasp, it is considered that at least 10 bits of force must be achievable. An important issue consist in data selection, mainly 3 methods are considered: Full Scan (good for small sensors and considering mainly all sensors activated continuously, being energy consuming and providing too much data if the system is too big), Reactive Scan (sensor work only in annotating a change in the system) and Anticipatory Scan (the sensors scan patches in different moment with high frequency).

Two main examples can be taken in the field of tactile systems: site-addressable sensing systems and line-addressable sensing systems. In the first case each sensor can be individually accessed via address and data lines. Computations lead to demonstrate that full scan be accomplished by each scan at a rate of f_l=35.5 Hz.

The second systems consist in sensors connected in an matrix, they addressed according to the row and column they are positioned, a similar system is in normal computer keyboards, which in fact are basic tactile sensors (with each key being a 1 d.o.f. sensor). In this case computations lead to demonstrate an operating frequency of 34.5 Hz, being little bit low, but justified by a system which a more efficient system which allows multiple sensors in the system.

Key Concepts

Tactile Sensors, Manipulators

Key Results

The authors decided 3 steps for implementation: designing a binary sensing network for many sensors connected, introducing proportional contact sensing network and in the end making a multi-parameter sensing system.

The system may include totally 2000 sensors, the bandwidth will therefore approximately be 1 MHz over a single data line, in the first step mechanical, electrical simplicity are searched together with virtually no delay of signal transmission. For the second phase magnets and Hall-effect sensors can be applied to achieve 6 dof, in this case the use of anticipatory scanning may be preferred. For the last step the concept of modularity becomes important for the integration of other sensor systems.