Engineering Design Guide (Science Module) Part 2

A CAD crosssection of the Rover science module

In the last blog post, we have gone through functional requirements (FR) listing, Morph chart brainstorming, constraints listing and design parameters (DP) selections. You can check out the previous installment here.

In this blog post, we are going to show you how we visualise and adjust the main science module design base on the previous steps.

Step 5 Visualisation of Design Parameters & Adjustments

After selecting the DPs, we are going to design parts for the selected DPs and integrate them into actual applications (Assemblies). In this step, we are going to visualise the DP concepts for each part by considering working mechanisms, size, attachment methods, positioning (components, attachments, holes etc.) and feasibility etc. For example, the main science module is comprised of various parts: main sample sensors, main samples sensor printed circuit boards (PCBs), generic PCB, the main sample box and module box. We will make the following adjustments for each part in the main sample box.

Main points to consider:

  • 4 separate main sample PCBs should fit in each chamber of the main sample box
  • The main sample box should fit in the module box
  • Additional components for rotation function and the sealing requirement
  • Additional sensing design requirements

Analysis Method

Even after we selected all our individual DPs, we still need to consider the practical design that can make all our DPs work together. Generally, there will be a few design options for each DPs. In the following section, I(Nora) am going to show you how we select the main science module box designs by considering constraints and pros and cons.
These will includes working mechanisms, size, attachment methods, positioning(components, attachments, holes etc.) and feasibility etc.

I.  Main sample PCB adjustment

science sample PCBs with moisture sensor, RGB sensor, magnetometer and temperature sensor conneted
Science module sample box PCB

In the last blog post, Jessica has introduced the science sample PCB from the electrical perspective. In this part, I (Nora) am going to show you how to determine the mechanical dimensions of PCB before and after we work on the electrical design.

Main things to consider for PCB mechanically are dimensions of the PCBs and how the PCB components interface with the sample box.

a) Determine the outline size of the PCBs.

Outline dimensions depend on PCB positions, containers dimensions etc.

Where to put the PCBs.  (Outside vs Inside & PCB on side vs PCB at the bottom)

PCB
outside
Pros
1. More space
2. Protect PCB components from soil
Cons
1. Complex attachment method
PCB
inside            
Pros
1.Easier to assemble
Cons
1. Less space
2. PCB water-proof spray needed
3. Can’t fix PCB when covered by spray
PCB on
side
Pros
1. Larger PCB space
Cons
1. Do not have enough room for a moisture sensor. Have to cut moisture sensor and recalibrate.
2. Sensing components do not have enough contact surface area with sample soil
3. Smaller soil capacity
PCB at the bottomPros
1. All the sensors will have enough contact with soil to retrieve data
Cons
1. Smaller PCB space

Analysis

  1. All PCBs need to be tested frequently
  2. we need enough space for surface mount electrical components and routing
  3. Attachment method can be improved by sophisticated designs.
  4. Sensing efficiency is more prior than PCB space.

Conclusion Put PCB outside the chamber under the sample box.

PCB Outline dimension

Requirements

  1. All the PCBs should be attached at the bottom of the sample box
  2. PCBs should not interferes with each other
  3. Make PCBs square to leave flexibility for later on design modifications and assembly

Analysis

  1. Each PCB should be smaller than 1/4 of sample box horizontal section.
  2. Space and position for attachments: It’s more convenient to assemble if we put all the attachments inside chambers

Conclusion: make PCBs the same size as chamber size

b) Consider attachment methods

Nuts and Bolts
The most secured method but require large space
Clips Less space required but difficult to assemble
Silicon Least secure, save the largest space, have to clean Silicon every time take PCBs on and off

Analysis

  1. Bolts and nuts: Standard M3 nuts width across corner is 6.01mm. At least two nut holders with diameter of 9mm are required. We would not have enough space for routing if we go with the plan.
  2. Assembling complexity: Clips> Bolts and nuts> Silicon
  3. PCB only need to be attached to the rover when going on field testing or during competition doing science tasks.

Conclusion: Leave 2 holes for clips for each chamber, Silicon for back up.

c) Electrical components positions and electrical routing

CAD front view of science sample PCB with component connection holes and wiring.
Sample PCB CAD

Constraints

Components that have to stick in the chamber from the bottom per chamber:
1.    4 sensors(moisture, temperature, RGB and magnetometer )
2.   2 Clips holes

Back components:
1.    2 power connectors
2.   4 JST pins(signals)
3. a multiplexer

Clearance
1.   fillets & attachment holes position
2. Clearance for sensors and surface mount component

II. Module box adjustment

Constraints

  1. Module box inner width: 105mm
  2. Module box top lid width: 98mm
  3. Sensor holes at the bottom should fit PCB sensors exactly.
  4. Smallest PCB we can have is 45.5mm by 45.5mm
  5. Chamber top should be big enough for the 25mm top lid circular holes

a) Box dimensions

Analysis

  1. After considering clearance, top lid side width should be no more than 94mm
  2. The bottom width should be no less than 98mm

Conclusion

  1. Shape of sample box: small side width on top, big side width at the bottom
  2. Dimensions of sample box: 94mm for the top half, 98 mm for the bottom half

b) Holes dimensions

Holes dimensions are determined by position of sensors on the PCB as well as the clearance fit size for each sensors.

We get the final sensor positions and size by various laser cut testing.

III. Rotation and Sealing adjustment

a) Sealing method

Components

  1. O-ring (Additional requirement: compression)
  2. Spring(To provide compression force for O-rings)
  3. Top spindle lid and bottom o-ring lid design

O-ring lid design

Requirements

Sample box is at open positon. The chamber holes on the bottom lid align with holes on the top lid.
Sample box is at Open position
  1. O-rings around the holes
  2. Space for top lid hole parking
  3. Holes positions should always within the range of the chambers
  4. The o-ring slot should fit a standard size o-ring

b) Rotation

Driving top lid with motor require a fix point and a rotate point. The position of the servo motor will change our design massively.

Plan A: Motor at the bottom

Fix point: motor itself
Rotation point:  shaft

CAD model of the sample box with the motor at the bottom.
Plan-A half  section view

Plan B: Motor on the top lid (Final plan)

Fix point: shaft
Rotation point: motor

Constraints: The distance between the top lid of the sample box and module box should be no more than 30mm

Conclusion: Plan B

CAD model of the sample box with motor on top
Plan B half section view

IV. Sensing environment adjustment (Load cell)

Constraints

  1. Sample box cannot touch the wall
  2. Load cell working mechanism

Load cell working mechanism: The bar load cell we are using work base on measurement of bending moment. To measure the weight of the soil sample, we put the load cell under the whole device and weigh by the difference.

Conclusion

  1. Load cell attachment position: one end attach on module box bottom, the other end attach on all the device
  2. A support is required for all the devices including sample box, shaft, PCBs, clamps

Components material selections and manufacturing to be continued in the next blog post

Special thanks for Anita Smirnov and Harry J.E Day.

Author: Nora Deng

Nora Deng is science squad lead of BLUEsat Mars off world robotic team, BLUEsat general manager and a mechanical engineering student in UNSW.

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