LUNAR ROVER

motors

One of the key components of the Lunar X-Prize is for the rover to travel at least 500 m, and an additional $5 million will be awarded to the team that travels at least 5 km. Our goal is to earn the additional bonus money, which adds considerable difficulty to the design process. The lunar day provides fourteen days of continuous sunlight to power the rover, followed by an equally long lunar night when the rover must go into hibernation mode. Although surviving the lunar night will net additional prize money, our design needs to ensure that the rover can cover the full 5 km in the first lunar day in order to maximize the probability of success. Fortunately, the small mass of the rover, combined with the low gravity of the moon, leads to a minimal amount of power and motor size to accomplish this goal. DC brushless motors are the ideal choice to power the rover due to their ability to take DC power directly from either the solar panel or batteries. The brushless design also minimizes the risk of failure compared to a brushed DC motor. For our design, the Portescap nuvoDisc 32BF will provide sufficient torque output with minimal power consumption and size for one to be placed in the hub of each wheel. On the downside, the nuvoDisc 32BF is only rated to 80ºC and will require extra testing to ensure it’s feasibility, but the coil temperatures are rated to 125ºC. In case the nuvoDisc 32BF will not work, the Portescap 16BL provides similar performance up to 100ºC operating temperatures at the cost of increased size and mass. Finally, DC brushless motors operate at very high speeds; therefore, a combination of 100:1 and 50:1 harmonic drive gears will be necessary to provide enough torque at low enough speeds for the rover to operate.

Portescap nuvoDisc 32BF

Input Power

P_in = 3 W

Output Torque

τ_m = 8 mNm

Output Speed

n_m = 5000 rpm

Mass

26 g

Cost

$59.50

Table 1. Motor Properties

Harmonic Drive Gears

Gear Ratio 1

GR₁ = 100:1

Torque Factor 1

TF₁ = 1.42

Gear Ratio 2

GR₂ = 50:1

Torque Factor 2

TF₂ = 1.30

Mass

330 g

Cost

Unknown

Table 2. Gear Properties

Design Parameters

Rover Mass

m = 12 kg

Wheel Radius

r_w = 6.0 cm

Number of Wheels

N = 6

Gravity

g = 1.622 m/s²

Table 3. Rover Properties

Wheel Speed

Rover Velocity

Available Torque

Based upon these calculations, the rover will be able to cover the minimum required distance of 500 m in only 22 hours, 7 minutes. Furthermore, the rover will reach 5 km after only 9 days, 5 hours, 10 minutes assuming constant driving conditions, and allowing for uphill climbs and obstacle avoidance, the bonus prize will be achieved well within the fourteen days of sunlight. Prior to departure, the engineering team will attempt to map a relatively flat course for the rover; however, the rover must be able to handle inclined terrain, thicker layers of lunar soil, and the failure of one or more motors. The limiting factor in these circumstances will be the available torque. With the current design, the rover will be able maintain cruising speed on inclines up to 3.5º, through adverse soil conditions, and with only four motors operating (the failure of one motor necessitates shutting off its opposite for balance), although not all at once. If conditions worsen, or two difficulties arise at once, the motor speed can be lowered to produce more torque.

Minimum Torque

Number of Motors, N

Rolling Friction, μ_R

Grade, θ (º)

Required Torque, τ_min (mNm)

6

0.01

0

1.95

6

0.01

5

18.9

6

0.01

10

35.7

6

0.01

20

68.4

6

0.05

0

9.73

6

0.05

5

26.7

6

0.05

10

43.4

6

0.05

20

75.7

4

0.01

0

2.92

4

0.01

5

28.3

4

0.01

10

53.6

4

0.01

20

103

4

0.05

0

14.6

4

0.05

5

40.0

4

0.05

10

65.1

4

0.05

20

114