# Identification of Impact Frequency for Down-the-Hole Drills Using Motor Current Signature Analysis
## 1 Introduction
On the other hand, in [20], the impact frequency is obtained by the FFT analysis of signals captured from the strain gauge bridge, pressure sensor, and flow sensor after a wavelet transform for reducing noise and improving the resolution ratio in both time and frequency domains. Overall, the proposed frequency identification approaches are unattractive for deployment in conventional diesel-powered pneumatic or hydraulic drilling machines due to the requirement of installing, operating, and maintaining sensors that may not be readily available and must withstand extremely harsh operational environments. There is currently a growing tendency towards incorporating electrical equipment at the different stages of mining. Replacing traditional diesel-powered equipment with electrical counterparts brings multiple benefits, such as reducing exhaust gases, fumes, and particulate matter that may be harmful to workers [22; 23]. Analyses also show that electrifying the different stages of the mining process can potentially reduce operational costs [24]. The proliferation of electric trucks and loaders in mining facilities [25; 26] is a concrete example of this tendency. Given this trend, it is reasonable to expect that diesel-based drives in current drilling machines will eventually be replaced with electrical drives. Moreover, using electrical motors in drilling machines will facilitate the deployment and adoption of advanced monitoring techniques currently used in other application domains. In particular, Motor Current Signature Analysis (MCSA) techniques reported and used in other application domains could be directly applied for condition monitoring and fault detection in drilling machines, including broken rotor bars [27; 28], bearing damage [29; 30], eccentricity [31; 32], electrical stator faults [33; 34], among others [35]. In addition, MCSA can also enable the development of new low-cost methods for monitoring the impact frequency in drilling applications, which have not been reported in the literature. This paper proposes and evaluates three methods for estimating the impact frequency in a pneumatic DTH drill rig using measurements of the current in the three-phase induction motor that provides the rotational motion to the DTH hammer. The advantage of using motor current signals is that these signals are commonly monitored for safety, monitor ing, and control purposes so that the frequency identification tasks can be implemented with already deployed equipment without further intervention or modifications to the drilling rig. To evaluate the proposed MCSA approaches for impact frequency identification, we perform the following tasks: (i) we first use measurements of hammer pressure and sound signals to establish reference margins for the impact frequency of a DTH drilling rig while operating over a high-hardness rock specimen, and (ii) we then assess the accuracy of the estimated frequency obtained by applying three different functions to the measured motor current signals, including side bands, waterfall plots, and envelopes. The experimental evaluation shows that, for the considered case study, all the proposed MCSA methods enable the estimation of the impact frequency of the DTH drilling rig while operating over the rock sample. Moreover, the analysis permits establishing operational requirements for these techniques to be applied in practical scenarios. Considering the simplicity of the implementation, the results provide evidence of the practical utility of the proposed methods for promoting the adoption of this technology in field drilling operations.