Obstacle Climbing

Step

As mentioned in Section 2.6, HADES should be capable of climbing stairs as well as traversing the track rails which frequently run through mine tunnels. The test to evaluate this is a step obstacle that is 220 mm high as defined in Section 2.6. The simulated step platform should be wide enough for both wheels and must be free of protrusions or handles which could assist the spoked-wheels climbing. The setup is shown in Figure 9.1.

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Runs were carried out incrementing step heights by 20 mm, using wooden blocks under the platform, seen in Figure 9.1. Runs were successful if HADES could pull both front and back wheels over the step, in at least four out of five attempts. The step was first approached perpendicularly. HADES is able to climb a 250 mm high step, but struggled to overcome 270 mm, only managing two of five attempts. The pitch and roll data, measured with an IMU, was collected for a typical 250 mm high run, displayed below in Figure 9.2.

Figure 9.2: Pitch and roll data during climbing a step

HADES begins moving after 1 second. During build up to the obstacle, some oscillation occurs due to spoke collisions, discussed further in Section 8.1.2. The chassis tilts by 20° as the front wheels ascend the step. The 20° tilt in the opposite direction at 8 seconds, illustrates HADES dropping sharply down the other side of the step platform. It is not immediately obvious, but there are short 4° pulses in the roll axis, at 3 and 8 seconds. These pulses indicate HADES did not approach the step perpendicularly. Instead, one wheel ascended/descended the step slightly earlier than the other, causing a small rotation in the roll axis. The influence of the roll axis is discussed later in this section.

As mentioned in Section 2.2, spoked-wheels can climb obstacles 1.5 times their wheel radius. HADES should therefore be able to climb 300 mm high obstacles. However, results show a 50 mm deficit. This disparity of 50 mm is due to the underside of the chassis bottoming out,

158 rather than the spoked-wheels’ inability to climb the step. The underside of the chassis is 150 mm off of the ground. HADES’ front wheels always ascend the step, but as the step is greater than 150 mm, the step edge catches the underside of the chassis. As the back wheels continue to push the chassis (against friction) up the step edge, the front wheels become suspended and lose contact with the ground, as seen in Figure 9.3. The back wheels are unable to overcome the friction and push the chassis all the way over the step. This problem is reduced when climbing multiple steps such as a staircase. So long as the horizontal distance between steps is short enough, the front wheels reach the second step before the chassis bottoms out. As the front wheels ascend the second step, they lift the underside of the chassis higher, which prevents it catching on the first step.

Figure 9.3: Bottoming out on the step

The step edge, catching the chassis and causing it to bottom out, is avoided by modifying

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Figure 9.4 displays the gyroscope data from a run where the step was approached roughly 20° from normal. By increasing the angle of approach from perpendicular, the wheels encounter the step individually, reducing the dead zone shown in Figure 9.5.

Figure 9.4: Pitch and roll data during an angled approach

The green zones illustrate areas where the wheels can overcome obstacles. The red areas are dead zones, which the wheels cannot reach, and where obstacles can cause bottoming out. Additionally, when the wheels encounter the step individually, larger rotation in the roll axis

160 Figure 9.4. In comparison to Figure 9.2 (4° roll rotation), the additional 8° moves the underside of the chassis away from the step edge, avoiding catching and preventing bottoming out.

Figure 9.5: Varied angle of approach over an obstacle

Although bottoming out prevents perpendicularly climbing steps greater than 250 mm, the largest known step obstacle in an underground mine tunnel is 220 mm as mentioned in Section 2.3.5. As HADES can climb 250 mm high steps, this specification is exceeded. If debris or rubble greater than 250 mm are encountered, an increased angle of approach reduces the dead zone and increases roll axis rotation to inhibit bottoming out.

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The 300 mm obstacle height specification holds for a single obstacle as well. A single obstacle, without protrusions or handles to assist the spoked-wheels, is assumed to be worst case, as shown in Figure 9.6. The single obstacle is also incremented by 20 mm, similarly to the step.

Figure 9.6: HADES climbing a single obstacle

The angle of approach for a single obstacle has little effect on HADES’ ability to overcome it, provided the wheels, rather than the chassis, make contact with it. The maximum climbable obstacle height is limited by the pivot’s 30° rotation lock, the design for which is discussed in Section 3.1.2 As a result, the maximum climbable obstacle is 530 mm. Figure 9.6 displays an obstacle at 550 mm which lifts the rear axle off the ground due to pivot lock.

As discussed in Section 2.3.4 spoked-wheels are capable of climbing obstacles 1.5 times their wheel radius, which is 300 mm for HADES. But this 300 mm height only accounts for spokes locking on top of obstacles to pull the wheel over. The high torque motor-gearbox assemblies and rubber wheels use friction to overcome objects as high as 530 mm, exceeding

162 the 300 mm specification. As long as sufficient friction is available, only the pivot (rather than the spoked-wheels) limits the maximum obstacle height. Increasing the size of the slot angle, mentioned in Section 3.1.1 would increase the maximum obstacle height.