Robotic systems have developed considerably over the last ten years. New advances in actuators and materials allow the creation of more complex systems capable of conducting more advanced tasks than previously possible. Force and tactile sensing is one of the key research areas for robot systems, where such systems are required to characterise their interaction with the environment. With the ever-increasing demand to create robots that can safely interact with their environment, especially with humans, the need for robust, low-cost and miniature sensors is increasing too. It is well recognised that the acquisition of high-quality force and tactile sensor signals during a physical interaction between, say, a robot hand and an object being manipulated provides the opportunity to greatly improve the handling and manipulation capability of the robot system. Incorporating small-sized force/torque sensors in robot hands and arms has been shown to enhance obstacle avoidance, object grasping, in-hand manipulation and, generally, the meaningful interaction with the physical environment [1
]. Current force measuring sensors show clear shortcomings due to their size, manufacturability, integration incompatibility, sensitivity, measurement range and/or lack of sufficient axes of measurement [4
], with direct force measurements clearly outperforming approaches that use remote sensors to indirectly measure forces imparted on the robot structure.
The force sensing approach proposed in this paper attempts to provide a solution to this issue whilst keeping costs low and representing an enabling technology for a wide range of application areas including medical robotics, manufacturing, and areas involving human–robot interaction. The main contributions of the presented work are:
Integration of optoelectronic force sensors in a bespoke mechanical structure for the purpose of force/torque measurement;
A concept that lends itself to miniaturisation;
Simplified manufacturability and customisation to fit a wide-range of robot systems;
Low-cost fabrication and assembly of sensor structure; and
Good sensitivity and sensor range for a wide range of manipulation tasks.
A number of advanced robots have appeared recently in the research arena. Examples include dexterous, flexible snake-like manipulators that can potentially provide high dexterity and mobility in confined spaces that may not be easily achieved by traditional robotic systems [8
]. However, most of these robots lack force-sensing capabilities—An aspect that becomes essential if one wants to operate a robot in an environment where physical interactions with surrounding objects are the norm. Hence, such robots cannot exploit their dexterity capabilities to the full, since interactions (collisions) with the environment remain largely undetected [13
Using force sensors, robot hands can distinguish various kinds of tactile information including pressure and tangential forces, providing a sense of touch and enabling the equipped hands to perceive scratching, pricking, rubbing and the recognition of object shapes. The interpretation of the force sensor signals and, generally, the sense of touch have been extensively researched within the field of robotics [16
One of the most advanced force/torque sensors is probably the six-axis sensor developed by DLR (German Aerospace Center) [20
]. Despite its superior force/torque measurement capabilities, there are some shortcomings: (a) The sensor’s geometry cannot be easily adapted to a particular robot structure (because of the Stewart platform, which is the dominant feature of this sensor and cannot be altered without impacting on the measurement characteristics); (b) the sensor is relatively complex; and (c) expensive to manufacture (manually attachment of strain gauges on the sensor’s flexor beams). The superior measurement characteristics of the DLR force/torque sensor are acknowledged. Our paper is particularly interested in proposing a sensing concept that is low cost and can easily be integrated with a robotic structure—Hence, we do not see our sensor as a competitor to the DLR sensor with regards to measurement accuracy and sensitivity.
Another important and widely used force/torque sensor is the ATI Nano17 sensor (ATI Industrial Automation, Apex, NC, USA)—This sensor is often seen as an industry standard for torque/force sensors, because of its outstanding sensing capabilities with regards to accuracy, sensitivity and range [22
]. However, the sensor comes in a predefined housing (circular, 17 mm diameter, 14 mm length) which cannot be modified, and as such is of limited usefulness when further miniaturisation and adaptation to robot structures are required, although some researchers have succeeded in integrating these sensors with robot hands for improved interaction sensing. For example, during EU project HANDLE (Developmental pathway towards autonomy and dexterity in robot in-hand manipulation), the fingertips of the Shadow Robot Hand were equipped with Nano17 sensors to measure force and tactile information during robot-object interaction [23
]. Good real-time measurements along all six axes were achievable, but the robot hand’s fingertips needed to be adapted and integrating the Nano17 sensors meant increasing the size of the fingertips; also the comparatively high weight of the sensors impacted on the fingers’ inertia and required control adjustments. It is also noted that an ATI sensor is a relatively expensive item. In this paper, the ATI sensor is used for the calibration and evaluation of our sensor (see Section 3
In general, most available force sensors are fabricated without the structure that they will be used on in mind. This complicates their integration with the intended structure. This becomes particularly problematic in cases where the robot structure is small, as is often the case with manipulation devices such as robot hands and robots used for minimally invasive surgery [4
]. In such cases, having the capability of adapting the sensor geometry to the robot’s structure is essential. In some cases, there is additionally the need of extending the structural geometry of the robot at the point where the sensor is to be integrated. A number of robots or, in some cases, their links can be represented by hollow, cylindrical structures where the inner space is used to feed through auxiliary cables, tubes and/or tendons. Integrating a “conventional” sensor (which commonly does not provide empty space internally) is not possibly in such a case without significantly modifying the overall robot structure [13
]. Here, our sensor concept excels because it provides the required design freedom to align the sensor structure with the robot structure. In our chosen example, where the force/torque sensor is to be integrated with the STIFF-FLOP (STIFFness controllable Flexible and Learn-able Manipulator for surgical OPerations) arm [25
], the sensor structure is designed to be ring-shaped and, thus, easily integrated between robot segments, allowing multiple cables and tubes to be passed through (Figure 1
, Figure 2
and Figure 3
In general, our work aims at creating force/torque sensors that are most suitable for integration with a range of robot systems, particularly snake-like and highly redundant arms to measure the interaction of their individual links with the environment and robot hands to measure forces during grasping and manipulation events. Particular emphasis is on the creation of low-cost, easily manufacturable and integratable sensors for the real-time measurement of force and torque signals. With a particular interest in medical robotics, we will demonstrate the integration of our sensor with a continuum robot (the STIFF-FLOP arm), which is intended for use during minimally invasive surgery. Having such sensors integrated with these types of robots, we can provide physical interaction information such as force and tactile perception in surgical environments during an operation. Having this information readily available, compliant force control can be incorporated and used to move complex robot structures whilst preventing excessive forces from being applied to the clinical environment [31
Although there are many methods to measure deformation to calculate external force/torque, for example by attaching polyvinylidene fluoride (PVDF) films [33
], strain gauges [20
], piezoresistive materials [34
], or fibre bragg grating (FBG) [21
] on the sensor structures, we are proposing light intensity-based measurement using optoelectric technology [36
] and simply-supported beam (Figure 1
and Figure 2
]. Our proposed methods have advantages such as immunity to electrical noise, low power consumption, low-level noise, no need for electronic filtering, easy attachment into the sensor body, and low cost [40
]. In addition, recent technological advances allow diminutive sized photo sensor (1.0 × 1.4 × 0.6 mm3
]. To calculate force/torque components, optoelectronic sensors are deployed along the circumference of the sensor structure and can measure three deflections (δ1
) (Figure 1
, Figure 2
and Figure 4
). This simplifies manufacturing, makes the overall sensor size miniaturised, and secures an ample space in the centre of the sensor structure. This is in contrast with conventional methods as mentioned above, so it is hard to secure an ample space in the centre of the sensor structure.
Furthermore, today’s 3D printing technologies allow low cost fabrication of miniaturised complex sensor structures in metal. Hence, the overall size of the sensors can be much smaller than commercially available force/torque sensors. In this paper, the detailed design and calibration of a proposed multi-axis force/torque sensor with a ring-like structure and hollow inner section providing ample space for auxiliary components is presented. Employing the STIFF-FLOP manipulator as an example, we show how our sensor can be customised to fit a particular robot structure. The sensor is calibrated and, hence, through an experimental study, we obtain and validate its calibration matrix. This study also evaluates various properties such as measurement error, repeatability, hysteresis and crosstalk.
The calibration matrix was calculated analytically (Equation (16)) and compared with the empirically obtained one (Equation (19)). The analytically induced Equations (7)–(13) assumed the three deflections were measured on the centre points underneath the simply-supported beam where external force is transmitted to, as shown in the Figure 6
. However, in the real sensor application, the optoelectronic sensors and the mirrors were attached closely to the centre of the sensor structure as shown in Figure 1
and Figure 2
, and they were not positioned exactly at the centre and exactly toward the centre of the sensor structure. For these reasons, the simulation result (Figure 10
b) and the experiment result (Figure 13
b) are slightly different. Moreover, due to the hysteresis, the output voltage readings of the three optoelectronic sensing elements obtained from the loading and unloading processes are not exactly the same (Figure 13
). However, the calibration matrix is calculated using the entire set of voltage readings from both loading and unloading, therefore the calibration matrices are not exactly the same.
The errors and the crosstalk are associated with hysteresis, as shown in Figure 13
. Due to the hysteresis, the calculated calibration matrix causes the errors and crosstalk in values of the Fz
, and My
to be higher. It also negatively influences the repeatability of the sensor. As shown in Figure 14
, Figure 15
, Figure 16
, Figure 17
, Figure 18
and Figure 19
, it is observed that due to hysteresis, the magnitudes of the estimation forces/moments are getting larger in comparison with real forces/moments, as cycle numbers increase. This explains why the repeatability errors of Fz
, and My
are relatively large. The static simulation result (Figure 10
d–f) shows a low crosstalk of around less than 0.5% (note that the static simulation cannot show hysteresis properties). However, it is noted that the hysteresis level of the multi-axis force/torque sensor proposed in this study is higher than that seen in commercial sensors (such as ATI sensors). Hence, the use of metals in the fabrication of future sensors is planned, instead of the currently used ABS plastic materials (the sensor structure is fabricated from Visijet EX200 via a Projet HD 3000 3D production system.
Using our calibration device, Fz
could be applied on the sensor independent of Mx
However, the pure bending moments Mx
could not be independently exerted on the sensor. Hence, in our setup, we had to apply coupled force/moment values to our sensor to calibrate it. As such, in order to guarantee that the sensor readings are not affected by forces and torques that it does not measure (i.e., forces on the x
directions and moment around the z direction), a set of simulations were carried out that are explained in the following (Figure 20
and Figure 21
In the simulation, two opposing forces were applied to a shaft along the sensor’s z direction. The smaller force F2
was applied on the tip of the shaft (0.5 N), while an opposing force of higher magnitude F1
was applied along the shaft at different points (0.8 N). Since the two forces have different magnitudes, a net horizontal force was always resulted. However, the resulting moment depended on the point of application of F1
. When the length of the lever arm at the point of application of F1
was such that the moment equalled the one generated by F2
, the resulting moment was zero, while the resultant force was F2
= 0.3 N. It can be seen in Figure 20
that, at this point, the deflection of all three beams was negligible. Thus, this structure is not affected by these lateral forces.
Concerning the moment around the z direction, a similar simulation was carried out, which demonstrated that the sensor was also not affected noticeably by this element (Mz
). Figure 21
shows the deflection of the three beams in this situation. It can be seen that the deflection is negligible (in the order of 10−7
m, around 100 times smaller than for a moment of similar magnitude around one of the other axes).
In the future, we will propose an advanced calibration device that can apply coupled force/torque as well as pure moments. In addition, advances in optoelectronics technology have allowed photo sensors to become highly miniaturized, whilst remaining very cheap. Small-sized optoelectronics were recently achieved; an example includes the NJL5901R-2, 1.0 × 1.4 × 0.6 mm3
, by New Japan Radio Co., Ltd. (Tokyo, Japan). We, therefore, foresee that the size of our sensor structure can be further reduced [50
It should be mentioned that, in Section 3.1
, an ATI Nano IP65 six-axis force/torque sensor, which is a standard measurement device, was used for calibration. However, this sensor is not a high precision sensor, whose inaccuracies are around 1%. According to the measurement theory, the standard measurement device should be more precise than that of the calibrated sensor. Hence, in our future studies we will either use standard weights or a more precise six-axis force/torque sensor for the purpose of calibration.
5. Conclusions and Future Works
In this paper, details of an optoelectronic and simply-supported beam based three-axis force/torque sensor, which can be adapted to bespoke mechanical structures for the purpose of force/torque measurements within robot systems, have been described. An example application of our force/torque sensing concept to a flexible continuum cylindrical MIS (Minimally Invasive Surgery) manipulator (the STIFF-FLOP arm) has been presented, including its design, fabrication, and evaluation tests. In addition, a strategy to obtain the calibration matrix for reliable force prediction, applying the multiple linear regression method, is proposed. Finally, the effectiveness of the proposed three-axis force sensor has been validated through a set of experiments evaluating its properties such as crosstalk, hysteresis, repeatability and error.
Although, in this paper, we verified a sensor development method of building three-axis force/torque sensor based on optoelectronic technology and simply-supported beam, as future work, a metal structure will be considered to re-evaluate the sensor performance properties conducted in the Section 3