Planificación específica

Figure 3.4.2: Velocity as a function of wheel diameter

Based on [CWh] guidelines, the driving power is scaled with the maximum velocity and the rover total mass. There is one motor for each of the six wheels and the required power for each motor is sized to provide a thrust equal to half the weight of the rover on Mars (Eq. 3.4.5).

P

= N

M

g

V

2 η

Where P_Drive is the overall power required to drive the wheel motors, g_Mars is the gravitational acceleration on Mars, VMax is the rover’s maximum speed, and η^Gear is the efficiency of the mechanical transmission and gearing. This formula is compared to empirical data in the Validation section. Motors are sized based on MER and Sojourner values.

Figure 3.4.3: WEB geometry

3.4.4.2 WEB size

When sizing the WEB, the objective is to minimize its volume to save mass so that it actually fits inside the rover footprint. The dimensions of the rover footprint are driven entirely by the wheel diameter. The WEB size is also constrained by the volume of hardware that must fit inside and by the dimensions of the solar arrays that it must support. Therefore sizing is done in three steps: first the WEB is dimensioned so that it can contain all the required hardware; second, it is compared to the solar array constraint and expanded if necessary; third, it is compared to the footprint constraint.

Algorithms to arrange a set of volumes in order to maximize the compactness are very complex and computationally expensive. Therefore, the problem of sizing the WEB is simplified through the following assumptions. The area of the WEB (of its bottom and top plates) and its height are calculated

independently. The area is estimated according to the equipment present inside the WEB, which is assumed to be lying on the bottom plate (no shelving of hardware is allowed). It is then assumed that all the pieces of hardware fit in a rectangular area a little bigger than the sum of all the individual hardware areas. The WEB area is set equal to this summed area times a factor defined by the user which accounts for imperfect arrangement of the pieces into a rectangular area. In the same manner, the height of the WEB is set equal to the largest height of all hardware pieces times the same factor (refer to Default Values).

The WEB top area must be large enough to support the solar panel, if one is present in the design. If the ratio of the solar panel area to the WEB top area is less than a threshold set by the user, then the design is valid. However if the area ratio is greater than the threshold, the size of the WEB is increased in order to reduce the area ratio to the threshold value. In this manner the WEB is sized to support the solar panel and made to be large enough to contain the required equipment.

The WEB is assumed to have the same aspect ratio (width divided by length) as the rover footprint.

Since the size of the footprint is based entirely on the wheel diameter, then given a particular WEB size, it is possible to check if the WEB actually fits inside the rover footprint. For the design to be valid, the following system of equations must be verified (refer also to Default values).





W_WEB ≤ W_Rover − C₁× D_Wheel

(3.4.6) L_WEB ≤ L_Rover − C₂ × D_Wheel

W and L represent the WEB width and length, respectively. C1 and C2 are parameters set by the user. If either of these equations is not satisfied, the rover design is invalid.

3.4.4.3 WEB structural design

The top plate, bottom plate and walls are modeled separately. For all plates and beams, the boundary condition is that all edges are clamped. Both the bottom and top plates have to support normal loads and are designed for bending. The thickness of these plates is sized in order to meet a maximum deflection requirement set by the user (refer to Default Values). The top plate is assumed to be a simple metallic plate, but the user can choose the option of a sandwich structure for the bottom plate.

The connection between the WEB and the suspension is assumed to be located at the center of the WEB side walls; it is at this location that the weight of the WEB and the attached equipment is transferred to the suspension system. In addition to the loads from the deck and bottom plate equipment, the walls also have to support the hardware attached to them, such as arms or cameras. As a first approximation, the upper half of the walls is assumed to be under compression between the load of the deck plus the wall-attached hardware and the ground reaction. It is therefore designed for buckling. The lower half is assumed to be under tension between the ground reaction and the bottom plate equipment. The walls have three components: a structural element (carries the loads), aerogel for insulation, and fiberglass sheets for containment [HWS]. The user can change the total thickness between the two fiberglass sheets. There are also three types of structural elements that one can choose from: a simple plate, an

‘H’ shape column, or a Z-spar shape (see Figure 3.4.4). The Z-spar case is chosen by default because it was used on Sojourner [HWS]. No information was found on the wall design of the MER rovers. The thickness of the structural element is calculated to satisfy both bucking and tensile stress requirements.

The calculations capture the axial loads but not the torques that the wall-attached hardware applies to the walls.

Figure 3.4.4: Wall with Z-spar structure type

R is the ground reaction force, F1 is the weight of the deck equipment plus the WEB top plate and the equipment attached to the walls, and F2 is the weight of the equipment inside the WEB plus the bottom plate.

The mass of attachments, bolts and other fixations is estimated to be a fixed fraction of the total calculated mass. The mass of the mobility system differential, which is located within the WEB, is currently ignored.