**2. Challenges in interplanetary drilling**

In general, if neglecting the economic factors, terrestrial drilling can be conducted with advanced auxiliary facilities to investigate the in-situ drilling formations and can automatically apply liquid lubricant to improve the drilling conditions [32, 33]. Compared with terrestrial drilling, interplanetary drilling and coring restricted by the extreme environmental conditions on the planet will have to solve several unique challenges. To assure the operability of the required drill tool and its control strategy, it is thoroughly necessary to comprehend the in-situ environment conditions and existing applicable resources. The following drilling and coring characteristics investigation and recognition based drilling strategy will be both based on these understandings. The following subsections will discuss four main challenges in interplanetary drilling.

#### **2.1. Long-distance between planet and Earth**

At present, wireless teleoperation is widely used in the monitoring and control of spacecraft operation status. For example, in the second phase of China lunar exploration, based on acquired visual images the lunar rover completed the entire inspection survey mission by means of ground teleoperation [34]. However, different from rover's navigation control, the buried drilling and coring activity is a quite dynamic and rapid process and any signal delay caused by long-distance teleoperation may directly result in a serious drilling fault. Once the drilling faults happened, specialists from Earth also need a long time to diagnose and recovery, making the drilling and coring process last for hours or days. Even though the drilling faults can be handled successfully, the final coring quality and core's stratification could be destroyed during this long time recovery process. Considering for future deeper space explorations, for example, the round-trip delay between Mars and Earth will be as long as 40 min, this long time delay by teleoperation will definitely not acceptable for interplanetary drilling and coring operations [35]. Hence, in general only when a serious abnormality occurs in the sampling process, the sampling device could be automatically forced to stop drilling and wait for the ground specialists to make a fault judgment and determine the corresponding treatment plan. Otherwise, the sampling device should work in a thoroughly autonomous condition.

#### **2.2. Complicated and uncertain drilling formations**

uncertain formations and finally retrieve valuable core samples. Since planetary regolith has a considerable number of geological and mechanical properties, it is rather difficult to identify all the parameters individually online. Hence, the authors proposed a control strategy based on planetary regolith drillability (PRD) recognition [31]. Herein, the drillability of formation is a consolidated index to stand for drilling difficulty. A recognition model based on support vector machine (SVM) has been established to evaluate the drillability of current formation and subsequently control the algorithms that can tune drilling parameters to adapt to the cur-

The remainder of this paper is organized as follows. The unique challenges in interplanetary drilling and coring are discussed first. Next, the specific drilling and coring characteristics containing the drilling loads characteristics and soil flowing characteristics are elaborated. A drillability recognition method is proposed based on monitoring the signals then. Finally, an intelligent real-time drilling strategy is achieved based on drillability recognition and drilling experiments in multi-layered drilling formations indicated that this unmanned control method could effectively reduce the drilling loads and keep a relatively complete stratification.

In general, if neglecting the economic factors, terrestrial drilling can be conducted with advanced auxiliary facilities to investigate the in-situ drilling formations and can automatically apply liquid lubricant to improve the drilling conditions [32, 33]. Compared with terrestrial drilling, interplanetary drilling and coring restricted by the extreme environmental conditions on the planet will have to solve several unique challenges. To assure the operability of the required drill tool and its control strategy, it is thoroughly necessary to comprehend the in-situ environment conditions and existing applicable resources. The following drilling and coring characteristics investigation and recognition based drilling strategy will be both based on these understandings. The following subsections will discuss four main challenges

At present, wireless teleoperation is widely used in the monitoring and control of spacecraft operation status. For example, in the second phase of China lunar exploration, based on acquired visual images the lunar rover completed the entire inspection survey mission by means of ground teleoperation [34]. However, different from rover's navigation control, the buried drilling and coring activity is a quite dynamic and rapid process and any signal delay caused by long-distance teleoperation may directly result in a serious drilling fault. Once the drilling faults happened, specialists from Earth also need a long time to diagnose and recovery, making the drilling and coring process last for hours or days. Even though the drilling faults can be handled successfully, the final coring quality and core's stratification could be destroyed during this long time recovery process. Considering for future deeper

rent drilling conditions.

20 Drilling

in interplanetary drilling.

**2. Challenges in interplanetary drilling**

**2.1. Long-distance between planet and Earth**

Given the short execution time of Mars and asteroid exploration compared with the lunar exploration, the data of soils on Mars and asteroids are rarely found yet. Herein, this chapter mainly focuses on the physical properties of lunar soil. According to previous investigations on the material returned from the moon, the terrestrial term "regolith" is also used for the interplanetary exploration [36]. Regolith has been defined as a general term for the layer or mantle of fragmental and unconsolidated rock material. According to the published literature, lunar regolith ranges from granular soil to hard rocks [37, 38], and it mainly consists of five types of material: rock detritus, mineral dust, breccia, agglutinate and impacting molten glass. The physical characteristics of above lunar soil components are quite different and the distribution of different components of lunar soil in the depth direction at the sampling site is also uncertain. During future planetary drilling processes, either soil or rock will be randomly encountered, resulting in that the final coring quality and drilling loads may both be influenced by unpredictable properties. There are numerous parameters, including cohesion, friction angle, relative density, compression ratio, particle size distribution, etc., to describe the physical properties of lunar soil [39], further increasing the difficulty to identify the physical parameters of lunar soil at different depths one by one. Therefore, it is necessary to simplify the mechanical parameter identification of lunar soil.

#### **2.3. Lacking of prior investigation on sampling site**

Similar to terrestrial mining, prior investigation on the sample site will extraordinarily guide the following drilling and coring activities. In the second phase of China lunar exploration, a novel lunar penetrating radar (LPR) has already been applied to detect the morphology of the lunar surface and stratification information of subsurface lunar regolith for supporting further detector's landing site's selecting, however, it should be noted that until now due to the mass and power constraints its detection accuracy can only reach to about 30 cm [40]. Considering that any unclear detected drilling formation may bring out a serious drilling fault once inappropriate drilling parameters are operated. Therefore, it is still difficult to apply the LPR's detecting geological layering information to guide the sampling drill before drilling begins. It indicates that the drill tool should better work in a passive adaptive control mode, in which the drill tool during the whole drilling and coring process should online switch suitable drilling parameters according to the recognized current drilling formation on the drill bit, nor in the active control mode.

#### **2.4. Limited on-orbit sensor resources**

According to the discussion in above subsection, the control architecture of unmanned interplanetary drill should better work in a passive adaptive control, in which the drill tool will totally rely on the feedback data by sensors. However, compared with the planetary rover's surface navigation control, the planetary unmanned drilling has a more limited sensing resources. In addition to the constraints of quality, power consumption and high and low temperature vacuum environment, the sensors used for drilling condition's monitoring also need to overcome the restrictions like small installation space of drilling tools and the prevention of sample contamination as well as the high frequency vibration caused by the impact of drilling tools, etc. Combing above tough working conditions together, perhaps only traditional load cells and displacement transducers can be applied to the interplanetary drilling. Hence, to realize the intelligent drilling control the sample drill need to fully integrate the existed sensors' information, which should all be imported to the controller to decide its online strategy.

Besides above challenges, there also exists some negative factors affecting the interplanetary drilling. For example, the non-water environment on the planet surface that will cause the drill tool will work in a dry condition without any liquid lubricant to improve the drilling conditions. The only effective removing cutting chips solution is the spiral auger flute. Due to the fact that drilling loads or power consumptions are highly dependent on the removal condition [41], during this dry drilling process drilling loads will be more sensitive to the drilling parameters. Overall, these harsh working conditions will definitely aggravate the risk of the interplanetary drill, which all require a more robust and reliable control strategy.

a certain degree, the height of core index *H*<sup>s</sup>

**Figure 1.** Illustration of drilling and coring process in flexible tube coring.

height *H*<sup>s</sup>

monitored in real-time.

or the coring ratio *K*<sup>c</sup>

Intelligent Drilling and Coring Technologies for Unmanned Interplanetary Exploration

http://dx.doi.org/10.5772/intechopen.75712

23

to drilling depth *H*d) can represent the core flowing characteristics and should be

It can be also found that there inevitably exists a vertical distance between the bottom of the flexible tube and the bottom of the drill bit, connecting the internal core to the external cutting chips, as shown in **Figure 1**. Due to the fact that the external cutting chips' removal flowing characteristics is heavily determined by the operated drilling parameters [43, 44], the removed cutting chips may have a negative influence on the inner coring soil and make the coring results drop correspondingly. Therefore, besides monitoring the coring characteristics, the soil removal characteristics should also be online detected. As shown in **Figure 2**, in order to comprehend the drilling and coring characteristics, a noncontact soil flowing characteris-

Since the cored soil is wrapped into the closed space, it's fairly difficult to measure the cored soil without affecting soil's original states. To solve this problem, an ultrasonic displacement sensor is deployed into the hollow flexible tube, as shown in **Figure 2(a)**. To assist measurement, a protective hollow tube is installed at the front of the sensor, allowing the sonic wave to pass through it without disturbance. Besides that, avoiding unnecessary disturbing reflection from the uneven upper surface, one Teflon made reflect board with a small mass (4 g)

indirectly calculated by acquiring the ultrasonic sensor's online value *H*u, its initial value *H*uo, and the online drilling depth *H*d. Apart from the coring states, soil removal characteristics are acquired by measuring the accumulation morphology on a PE plastic wrap by an external camera, as shown in **Figure 2(b)**. By converting the colorful images into binary images, the outline of accumulation soil can be obtained thereby. Meanwhile, by searching the right, left, and upward points of current outline and summing up each accumulation volume, the total

is elaborately designed to put on the in situ soil. As a result, the online coring ratio *K*<sup>c</sup>

tics monitoring method has been proposed for experimental verification.

index (the ratio of coring

can be
