**Giuseppe Carbone 1,\*, Marco Ceccarelli 2, Christopher Fabrizi 3, Pietro Varilone <sup>3</sup> and Paola Verde <sup>3</sup>**


Received: 18 February 2019; Accepted: 8 April 2019; Published: 11 April 2019

**Abstract:** This paper addresses the effects of electric power quality on robotic operations. A general overview is reported to highlight the main characteristics of electric power quality and it's effects on a powered system by considering an end-user's viewpoint. Then, the authors outline the influence of voltage dip effects by focusing on robotic grasping applications. A specific case study is reported, namely that of LARM Hand IV, a three-fingered robotic hand which has been designed and built at LARM in Cassino, Italy. A dedicated test rig has been developed and set up to generate predefined voltage dips. Experimental tests are carried out to evaluate the effects of different types of voltage dip on the grasping of objects.

**Keywords:** robotic hands; grasping; electric power quality; voltage dips effects

#### **1. Introduction**

Nowadays, electronic equipment and computing devices are used in most types of industrial machines and robotic devices. They are key systems for the successful implementation of most industrial processes. However, the wide use of electronics makes this equipment more vulnerable to disturbances in terms of power quality (PQ). PQ is related to several disturbances that include, among others, momentary interruptions, voltage dips or sags, swells, transients, harmonic distortion, electrical noise, and flickering lights [1]. In general, the electrical power grid is designed to deliver power reliably with the aim of maximizing the amount of power available to customers. However, PQ disturbances are not always taken into consideration despite the fact that they can significantly affect industrial production, as well as permanently damage expensive equipment, costing industrial plants millions of dollars [2]. In order to minimize these costs, it is critical for industrial customers to understand how PQ can affect the operation of their systems and how it is possible to mitigate the effects of PQ disturbances [1–3].

The international framework of the actual standards on PQ is based on the norms of the International Electro-technical Commission (IEC), which is accepted as a worldwide reference. Moreover, national or supranational committees give further indications on the maximum limits to be imposed on PQ disturbances. For example, the European Committee for Electrotechnical Standardization (CENELEC) is the European reference, while Comitato Elettrotecnico Italiano (CEI) is the Italian national reference for adopting IEC and CENELEC standards. The above-mentioned bodies have released the norm EN 50160 that defines the European and Italian standards for the PQ in terms of voltage dips and other voltage disturbances [4]. Similarly, the norms EN 61000-4-11 and EN 61000-4-34 are adopted worldwide [4–7].

PQ is gaining significance also in robotics, and specifically in applications of service robotics. Voltage dips, also defined by the equivalent term voltage sags, are recognized as one of the most severe

disturbances that can affect the operation of industrial devices. The detrimental effects of voltage dips can result both in the tripping of the protective devices with the equipment shut down and in the malfunction of a device. The latter constitutes a sort of failure that determines a far from normal or satisfactory functionality. Both these typologies of effects have significant economic impacts on a system's operation and productivity. These costs depend on many factors that are linked to the type of manufacturing activity and to the extent of the affected area [3].

Among other industrial devices, robots certainly suffer for the presence of voltage dips in the supply voltage. This is particularly critical in the case of collaborative robots which are penetrating several new applications also thanks to the publication of a collaborative robotics reference standard [8]. In fact, the recent ISO (International Organization for Standardization) norm establishes a novel regulatory framework allowing a wide spread of collaborative robots in industrial and civil environments. The close interaction among robots and humans makes safety one of the most significant aspects of robot design and operation. Clearly, the effects of the voltage dips in the supply voltage can significantly influence robot performance as well as generate potentially critical safety issues, such as missing operations or unpredictable robot behaviors.

The case of robot grasping is quite significant, since the performance of an end-effector is considered to be the most important contribution to achieving the successful manipulation of an object. Several researchers have addressed the design of grasping devices with solutions ranging from simple end-effectors (suction cups, electromagnetic devices) to finger grippers for handling specific objects, and even complex multi-purpose robotic hands [9–13]. It appears very significant to investigate the effects of power quality on a robot grasping, since a grasping failure implies a failure of the whole robotic manipulation procedure. Moreover, this can have strong safety implications, especially in collaborative robotics tasks, as mentioned in [8].

This paper addresses the effects of the voltage dips on the performance of robotic grasping. A specific case of study is reported as referring to LARM Hand IV, a three-fingered robotic hand which has been designed and built at LARM at the University of Cassino [14–17]. A dedicated test rig has been designed and set up to generate predefined voltage dips to experimentally investigate their effect on the grasping of objects with different sizes. Experimental tests are carefully analysed and discussed to demonstrate the influence of the voltage dips on the grasping performance, as well as to propose some mitigation actions to avoid safety implications during the grasping.

#### **2. Main Characteristics of the Voltage Dips**

The term PQ embraces a wide set of disturbances that can affect the voltage and/or current [3]. The disturbances are categorized in two groups: the variations and events [4]. Each group represents a different type of phenomena and different ways of treating the disturbances [5,6]. The variations and events are due to the interaction between the power supply and the devices installed at the customers' premises.

Variations are minor changes from the ideal value of voltage or current that show a relatively slow reduction in value. The level of variations can be measured continuously and at predefined instants of time. Examples of variations are the voltage amplitude variations and the waveform distortion. Events can have large deviations from the ideal value and they can occur suddenly. Events cannot be measured continuously because they may occur occasionally. A trigger condition is needed to measure these events. In the group of events affecting the supply voltage, voltage dips are one of the most severe disturbances that can affect especially industrial end-users. Several devices are significantly vulnerable to voltage dips. The main detrimental effects of the voltage dips are the tripping of protected devices and the degradation of the performance of a device.

A voltage dip is defined as a "sudden reduction of the supply voltage, below 90% and above 1% of the declared voltage, followed by a voltage recovery after a short period of time" [4]. Figure 1 plots the time of a voltage affected by a dip. In Figure 1, the main characteristic quantities of a voltage dip are expressed as the amplitude with the symbol Vr, and the duration with the symbol Δt as shown

in Figure 1. The amplitude of a voltage dip is the minimum value of the RMS voltage during an event; it is known also as the residual voltage. The duration of a voltage dip is the time elapsed when the voltage falls below the threshold value, which is assumed to be 90% of the rated value. Further quantities can characterize a voltage dip, like the number of involved phases, the phase angle jump, or the symmetry of the voltage dips on the phases.

**Figure 1.** Example of a real voltage dip in industrial frames [2].

In transmission and distribution systems, most voltage dips originate with the short circuits and further causes include the start of a large motor and the insertion of a large transformer, or of a high power load as can frequently happen in industrial systems. In the transmission and distribution systems, the dips more frequently originate with short circuits in some nodes of the electrical network. In the presence of a symmetrical solid short circuit in a specific node, two main phenomena happen. In the node where the short circuit occurs, the voltage is equal to zero and in the other nodes electrically close to it, the voltage is affected by the sudden reduction that represents a voltage dip. This phenomenon lasts until the protection device clears the short circuit.

The framework of the actual standards on the limits of voltage dips is mainly referred to the IEC and the CENELEC norms. In particular, IEC 6100-4-11 [7] states the immunity test for the devices to define its operation class with reference to the EMC (Electro Magnetic Compatibility) and two main classes are defined which are the Class II and the Class III. The main standard of the CENELEC is the EN50160 that indicates the voltage characteristics of the electricity supply by public distribution network. In particular, for the voltage dips, this standard proposes the table shown in Table 1 to classify them according to residual voltage and duration.

Table 1 refers to all the voltage dips that can be recorded in a node. It allows for immediately ascertaining the performance of a node in a considered period, typically at least one year. Actually, the trends of future standardization activities on the voltage dips are towards a limitation of the number of voltage dips that can be tolerated at any node of a system in a defined time period as the year. The limits could be expressed using a table similar to that in Table 1. In such a case, any number of cells would express the boundary of the performance of the power supply that any customer should expect. Summarizing, the most important characteristics of a voltage dip are the amplitude and the duration.


**Table 1.** Classification of the voltage dips according to residual voltage and duration [EN50160].
