**3. Drilling and coring characteristics**

The ultimate goal of interplanetary sampling exploration is to acquire as much as possible planetary regolith for further scientific analysis. Apart from the volume of planetary regolith, the stratification of the sample should also be seriously considered in the drilling process. If the geological information of soil sample was not be preserved completely, its geological value would be significantly reduced. In China Chang'e drilling and coring mission, a novel flexible tube coring (FTC) method referred from Luna 24 mission is being adopted to solve the above problem [42]. As shown in **Figure 1**, its drilling and coring process is illustrated.

In the FTC penetrating process (rotary speed *n* and penetrating velocity *v*p), the in-situ subsurface regolith destroyed by the cutting edge of drill tool can be divided into two parts: the wrapped sample into the flexible tube and the cutting chips conveyed along the spiral flute. Since the wrapped core soil is adjacent to the cutting soil through the holes at the bottom, it may result in a sudden collapse of the inner surface of the flexible tube when cutting chips are removed, resulting in a decline in the height of core. However, considering that there is no relative locomotion between the sample and the flexible tube in stable conditions, the sample can be continuous along the depth direction. Although the adopted FTC method has a great advantage in maintaining the core stratification, there still exists a considerable possibility that a very small amount of core soils are finally acquired in drilling process. Therefore, to Intelligent Drilling and Coring Technologies for Unmanned Interplanetary Exploration http://dx.doi.org/10.5772/intechopen.75712 23

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

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

22 Drilling

**3. Drilling and coring characteristics**

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.

The ultimate goal of interplanetary sampling exploration is to acquire as much as possible planetary regolith for further scientific analysis. Apart from the volume of planetary regolith, the stratification of the sample should also be seriously considered in the drilling process. If the geological information of soil sample was not be preserved completely, its geological value would be significantly reduced. In China Chang'e drilling and coring mission, a novel flexible tube coring (FTC) method referred from Luna 24 mission is being adopted to solve the above problem [42]. As shown in **Figure 1**, its drilling and coring process is illustrated.

In the FTC penetrating process (rotary speed *n* and penetrating velocity *v*p), the in-situ subsurface regolith destroyed by the cutting edge of drill tool can be divided into two parts: the wrapped sample into the flexible tube and the cutting chips conveyed along the spiral flute. Since the wrapped core soil is adjacent to the cutting soil through the holes at the bottom, it may result in a sudden collapse of the inner surface of the flexible tube when cutting chips are removed, resulting in a decline in the height of core. However, considering that there is no relative locomotion between the sample and the flexible tube in stable conditions, the sample can be continuous along the depth direction. Although the adopted FTC method has a great advantage in maintaining the core stratification, there still exists a considerable possibility that a very small amount of core soils are finally acquired in drilling process. Therefore, to a certain degree, the height of core index *H*<sup>s</sup> or the coring ratio *K*<sup>c</sup> index (the ratio of coring height *H*<sup>s</sup> to drilling depth *H*d) can represent the core flowing characteristics and should be monitored in real-time.

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 characteristics monitoring method has been proposed for experimental verification.

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

**Figure 2.** Scheme of the noncontact soil flowing characteristics monitoring method. (a) Scheme of monitoring method; (b) Acquired images in monitoring process.

volume of accumulated soil *V*acc can be finally online acquired. Hence, based on above noncontact measurement the soil flowing characteristics during the drilling and coring process can be accurately monitored without any damage.

To verify the proposed measurement, drilling experiments under the condition of *n* = 400 rev/min, *v*<sup>p</sup> = 150 mm/min are conducted. The online coring results containing the ultrasonic sensor's value, the coring height, and the coring ratio are illustrated respectively in **Figure 3**. It can be seen that during the first 105 mm drilling depth, the ultrasonic sensor's value keeps stable, meaning that the coring soil stays at the original position making the coring height climb stably to the 105 mm and coring ratio keeps around the 100%. After then, the monitored sensor's value reveals that it has a sudden increase, resulting in a turning point at the 105 mm drilling depth. According to the definition, the corresponding coring height and coring ratio both has a sharp decline. Finally, during the 200 mm depth, the coring height slips to approximately 70 mm and the coring ratio reaches to less than 40%.

Based on above founding, it can be inferred that there exists a sudden collapse of the cored soil in the flexible tube. Actually, this interesting phenomenon can be explained by the state of the cored soil. As shown in **Figure 1**, the cored soil and the conveyed soil are inevitability connected at the bottom of the drill bit. Under proper drilling parameters or penetration per revolutions (*PPR* = *v*p/*n*, mm/rev), once the drill bit drills into the regolith the cutting chips will be conveyed from the bottom by auger's spiral locomotion, which may make the cored soil stays in a positive stress, and vice versa. Since there exists a small side failure zone at the outer annular space of cored soil by the inner edges of cuter [45], once the bottom of soil become totally granular at a certain depth, cannot be able to sustain the upward positive

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Apart from the core flowing characteristics, cutting chips' removal flowing characteristics is also investigated. By identifying the outline image of the wedge-shaped of the removed soil outside the surface and calculating its 3D volume per second, the online volume of removed soil *V*acc under three different *PPR*s (1.6, 0.53, and 0.32 mm/rev) is shown in **Figure 4**. It can be

stress, it will result in a sudden broken or collapse along the longitudinal direction.

**Figure 3.** Monitored height of core and coring ratio in experiments.

**Figure 4.** Monitored volume of accumulated soil *V*acc under different *PPR*s.

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**Figure 3.** Monitored height of core and coring ratio in experiments.

volume of accumulated soil *V*acc can be finally online acquired. Hence, based on above noncontact measurement the soil flowing characteristics during the drilling and coring process

**Figure 2.** Scheme of the noncontact soil flowing characteristics monitoring method. (a) Scheme of monitoring method;

To verify the proposed measurement, drilling experiments under the condition of *n* = 400 rev/min, *v*<sup>p</sup> = 150 mm/min are conducted. The online coring results containing the ultrasonic sensor's value, the coring height, and the coring ratio are illustrated respectively in **Figure 3**. It can be seen that during the first 105 mm drilling depth, the ultrasonic sensor's value keeps stable, meaning that the coring soil stays at the original position making the coring height climb stably to the 105 mm and coring ratio keeps around the 100%. After then, the monitored sensor's value reveals that it has a sudden increase, resulting in a turning point at the 105 mm drilling depth. According to the definition, the corresponding coring height and coring ratio both has a sharp decline. Finally, during the 200 mm depth, the coring height slips to approximately 70 mm and the coring ratio

Based on above founding, it can be inferred that there exists a sudden collapse of the cored soil in the flexible tube. Actually, this interesting phenomenon can be explained by the state of the cored soil. As shown in **Figure 1**, the cored soil and the conveyed soil are inevitability connected at the bottom of the drill bit. Under proper drilling parameters or penetration per revolutions (*PPR* = *v*p/*n*, mm/rev), once the drill bit drills into the regolith the cutting chips will be conveyed from the bottom by auger's spiral locomotion, which may make the cored soil stays in a positive stress, and vice versa. Since there exists a small side failure zone at

can be accurately monitored without any damage.

(b) Acquired images in monitoring process.

24 Drilling

reaches to less than 40%.

**Figure 4.** Monitored volume of accumulated soil *V*acc under different *PPR*s.

the outer annular space of cored soil by the inner edges of cuter [45], once the bottom of soil become totally granular at a certain depth, cannot be able to sustain the upward positive stress, it will result in a sudden broken or collapse along the longitudinal direction.

Apart from the core flowing characteristics, cutting chips' removal flowing characteristics is also investigated. By identifying the outline image of the wedge-shaped of the removed soil outside the surface and calculating its 3D volume per second, the online volume of removed soil *V*acc under three different *PPR*s (1.6, 0.53, and 0.32 mm/rev) is shown in **Figure 4**. It can be seen that during above drilling and coring process, the penetrating velocity is kept constant (80 mm/min), while the rotary speed will be adjusted (50 rev/min → 150 rev/min → 250 rev/min → 150 rev/min → 50 rev/min). Meanwhile, the monitored volume *V*acc can be divided into seven stages (AB → BC → CD → DF → FG → GH → HI).

During the AB stage of the first 20 s, since drill bit constantly cuts the in situ soil simulant without spiral auger's participant, there is almost no soil accumulated upon the surface. After then, the drill bit is buried in the soil, the auger starts to remove soil from the borehole bottom with a low removal speed during the BC stage. At the 40 s moment (C point), the rotary speed is suddenly switched to 150 rev/min with the result of the sudden increase of *V*acc. It can obviously be seen that the removal speed during CD stage is higher than that during BC stage. Above phenomenon is almost same with that in conditions between DF stage and CD stage. At the 85 s moment (F point), the corresponding PPR is regulated back to 0.53 mm/rev, which results in a slow increase trend of the *V*acc. After about 5 s, the removal speed becomes normal. This slow increase trend of the *V*acc also exists in the sudden change on G point. Based on above experimental results, it can be concluded that the monitored volume of accumulated soil can reflect the online removal states well and the PPR index has a great effect on the removal states and should be optimized further.

According to preliminary experiments, the proposed non-contact drilling and coring characteristics monitoring method has been validated well. Next, to provide suitable drilling parameters database for the following intelligent drilling strategy, more drilling and coring experiments taken the drilling loads and core's quality into account will be conducted in several different drilling formations, such as limestone, sandstone, compacted soil, etc.

> at least three different formations for validation. Herein, DDAG is adopted to conduct the drillability recognition. The classification's structure diagram for four levels of lunar regolith

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As can be seen from the above algorithm structure, this method constructs a classifier with a two-way directed acyclic graph. Among them, the classifier 1 is located at the top of the root node to complete the first and second levels of drillability level 1–4 drillability comparison. By comparing the drillability level of 1 and drillability level of 4, the most samples may not belong to drillability level 1 (drillability level 4) can be excluded. After 3 times of excluding, the remaining category will be the drillability 1. Experiments indicated that by successive

In fact, model parameters in SVM play an important role in affecting recognition's accuracy. In the kernel function of SVM, scale parameter *g* and penalty coefficient *C* have the most significant effect on recognition's accuracy. When the two parameters do not match well, SVM will be overtraining or overfitting, which is an unstable situation in recognition. Herein, based on a grid search method, these two SVM model are optimized. To verify the optimized SVM model's generalization ability, drilling characteristics of different drillability samples under constant drilling parameters should be imported to conduct recognition training. Herein, a combination

comparison this classification algorithm can guarantee a higher recognition accuracy.

simulants' drillability is shown in **Figure 6**.

**Figure 6.** Drillability recognition algorithm based on DDAG.

**Figure 5.** Scheme of drillability recognition based on SVM.
