Keywords

1 Introduction

In recent years, an increasing number of areas can be observed that enable human-robot collaboration in the same workspace. The demand for such robotic applications is emerging in industry, education, agriculture, medical services, security and space exploration [1]. Cobots (collaborative robots) have become an essential component of Industry 4.0 [2]. Equipping cobots with additional sensors allows them to comply with special safety measures, avoid collisions with humans and manipulate objects more agilely.

In order to ensure that the effects of collisions with humans are minimised, the construction used in cobots can be lightweight and compliant, and there is a reduction in the power and strength of such machines [3]. Another approach is to equip cobots with an electronic skin (e-skin), which will allow the detection of a human approaching the robot’s casing [4].

Detection of human proximity can be implemented in a number of ways. For this purpose, the standard approach is to measure physical properties such as the reflection of light [5], the reflection of a sound wave [6], the reflection of an electromagnetic wave in radar [7, 8], as well as the detection of changes in a selected parameter in the area around the sensor, such as the electrical permeability of the sensor [9, 10], the magnetic permeability of the sensor [9], or the temperature of the sensor [11].

The aim of the manuscript is to present selected aspects of the operation of a capacitive proximity estimation mechanism for cobot enclosure. Capacitive proximity estimation is implemented by measuring the operating parameters of an open capacitor, one plate of which is an e-skin [4]. An important aspect addressed in the paper is the placement of the e-skin on a conductive machine case, which significantly affects the proximity estimation.

The paper is organised as follows. In Sect. 2, the developed test stand consisting of e-skin placed on industrial robots metal case is presented. In Sect. 3, the tests results are described and analysed. Finally, Sect. 4 provides the summary and further investigation proposal.

2 Developed Test Stand

The main components of the hardware setup are the e-skin together with the measuring controller, a Fanuc M10iA industrial robot and a PC. The e-skin together with its electronic measurement system is described in detail in [4]. In brief, the e-skin is implemented as a rectangular array of graphene force sensitive resistors (FSR). The touch pressure value is measured via a conductive comb electrode layer. The elecronical circuit for measuring the proximity of objects to the e-skin consists of a rectangular signal generator with a frequency dependent on the capacitance of an open capacitor formed by the conductive e-skin layer and a reference plate. The NUCLEO-F44RE board with an STM32 microcontroller clocked at 90 MHz performs the counting of the number of periods of the rectangular signal in fixed time interval T. The implementation consists of counting the occurrences of a rising edge via an external interrupt at a specified fixed time interval T. The length of this interval depends on the relevant settings of the microcontroller’s internal timer such as PSC (Prescaler), CKD (Internal Clock Division) and ARR (AutoReload Register). The default length of the time interval is set to 728,178 \(\upmu \)s. The PC performs the measurement data logging transmitted serially via the USB port. The principle of measuring the proximity of an object to the e-skin is based on the properties of an open capacitor. The capacitance of an open capacitor changes when an object, with an electrical permeability different from that of a vacuum, comes into proximity. Changes in this capacitance cause a change in the frequency of the signal generated by the electronic circuit, which is measured indirectly by the microcontroller. Placing a conductive robot enclosure in the vicinity of a system operating in this way causes significant problems with proximity estimation. A previous study [4] confirmed the effectiveness of estimating the proximity distance of a human body part to an e-skin placed on a dielectric material. As part of the research described in the current manuscript, the e-skin was placed on an industrial robot arm (Fig. 1). Bringing the hand close to the robotic arm mimics the working conditions of a cobot in which the robotic arm approaches a human.

Fig. 1.
figure 1

Hardware setup consisting of: the Fanuc M10iA robot (1), the e-skin (2) and open capacitor reference plate (3).

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3 Research Results

The tests conducted on the test stand were divided into two stages. In the first stage, the estimation of the proximity of the human hand to the e-skin at a distance of approximately 20 mm and smaller was analysed without modifications relative to the tests conducted in [4]. In the next stage, modifications were made to the measurement system and the experiment was repeated. The modifications mainly concerned the operating parameters of the microprocessor chip used.

Fig. 2.
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Initial proximity estimation results.

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3.1 Initial Results

The experiment consisted in approaching an open human hand oriented parallel to the surface of the e-skin, which was placed on the robot as described in Sect. 2. The size of the hand was about 10 cm \(\times \) 20 cm. The approach velocity had a vector parallel to the normal surface of the e-skin, but its modulus was not recorded. The result of the measurement is the number of periods n of the signal generated by the electronic measurement system measured over a fixed time interval T. Changes in this number indicate a change in the relative electrical permeability in the area of the sensor which makes it possible to infer the detection of an object in its vicinity. During this measurement, the constant time interval during which the periods of the generated signal are counted was set to the default value and is 728.178 \(\upmu \)s. The results obtained are shown in the Fig. 2. The graph shows an experiment involving bringing a human hand close to the e-skin device. The e-skin proximity response with the measurement system configured by default is small and relatively difficult to distinguish from measurement noise. The determination of the signal-to-noise ratio (SNR) was calculated on the basis of the formula (1).

$$\begin{aligned} SNR=(\frac{A_{signal}}{A_{noise}})^2, \end{aligned}$$
(1)

where:

\(A_{signal}\) - root mean square aplitude of hand proximity signal,

\(A_{noise}\) - root mean square aplitude of noise signal.

In regards to initial results, the proximity of the hand to the e-skin \(A_{signal}\) was determined to be 10,282 while \(A_{noise}\) was determined to be 1,090. As a result, SNR was calculated as 89,047. Such a result does not allow the estimation of the proximity distance, but is sufficient to dectect the presence of a hand in the vicinity of the e-skin. In order to improve the quality of the detection and allow the estimation of the proximity distance of the object, it was necessary to enhance the response of the system in relation to the noise present.

3.2 Developed Improvement

In order to improve the quality of the circuit’s response, the fixed time interval T in which the n periods of the signal generated by the electronic circuit are counted was increased. This was implemented by modifying the microcontroller’s internal timer settings. The value of the PSC prescaler was increased from 0 to 3 which had the effect of increasing the value of T from 718 \(\upmu \)s to 2,872 ms. After the modifications were made, the experiment was repeated by bringing the hand close to the e-skin placed on the robot housing. The Fig. 3 shows an example of the resulting measurement system. As a result of the modifications, a more evident response of the e-skin placed on the robot housing to the proximity of a human hand was obtained. The determination of the SNR was calculated based on the formula (1). In terms of the example shown in Fig. 3 the hand proximity to e-skin \(A_{signal}\) was determined to be 66,338 while \(A_{noise}\) was determined to be 4,759. As a result, SNR was calculated as 194,335. This represents a much better result than the initial performance described in the Sect. 3.1.

3.3 Discussion

A summary of the measurement results obtained is shown in the Table 1.

Table 1. Time reactions for e-skin pressure detection.
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Fig. 3.
figure 3

Improved proximity estimation results.

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The relatively small response of the system to an approaching hand is due to the increased capacitance of the open capacitor resulting from the presence of the conductor under the e-skin layer. This capacitance has been measured and is approximately 400 pF. In comparison, the change in capacitance due to the approach of an object with the properties of a human hand is of the order of a few pF. The change in capacitance caused by the object being targeted for detection is two orders of magnitude smaller which results in little change in the frequency generated by the electronic circuit. In spite of these problems, an efficient detection of the proximity of the human hand to the robot was successfully achieved.

The negative effect of improving the quality of the response of the e-skin placed on the metal housing of the robot to the approach of a human hand is an increase in the response time of the system to an approaching object. In the target application for safe human-robot collaboration, the measurement delay is a key determinant of whether the collaborative robot will have time to stop and avoid a dangerous collision. At the example TCP speed of an industrial robot of 4,000 mm/s, increasing the time T from 718 \(\upmu \)s to 2,872 ms results, in the extreme case, in a robot response delayed by 2,154 ms which, at the above tool speed, translates into an increase in the potential braking distance of 8,616 mm. This extension of the braking distance is significant given the reaction distance of the e-skin to the human hand of approximately 20 mm. The increase in response time also results in a significant reduction in the potential for de-noising filtering of the measurement results. When applied to, for example, eliminate outliers, this will result in a further increase in braking distance.

4 Summary

Using e-skin for capacitance-based proximity estimation when placed near conductors poses problems. They are related to the occurrence of a much higher capacitance of the open capacitor resulting from the presence of a conductor in the sensor’s working area than from changes in capacitance resulting from the proximity of objects. This problem is very important, taking into account the target application of e-skin as a device supporting safe robot-human interaction - the housing of robots is often made of conductive materials. In the manuscript, in order to minimise the problem at hand, the time interval during which the signal periods generated by the electronic system are counted was increased. This improved the response of the system and increased the SNR of the measurement system. The negative cost of obtaining such results is the increased response time of the robotic system. Further work on this topic may allow a more precise and robust estimation of the proximity distance of the human to the e-skin.