Goal: from the tests, determine what suspension style we would like to use going forwards
The Plan
Judgment Criteria
Since the design of this test was mainly to "get a feel" for the different styles, the judgment for each drivetrain should be based on how it compares to the other prototypes.
- Traversal ability: how easily does it scale/conform to obstacles?
- Stability: how much does the chassis move from a horizontal position (parallel to flat ground) during traversal?
- Design flaws/challenges: where does the design fail? How is its failure related to the current geometry, and how easy would it be to edit on a proper model?
Test Plan
1. Scaling down a level, both wheel sides at a time
- Powered by force applied to the rear
- Tested heights…
- 1.15.875mm, or 0.16m proportionally
- 2.22.225mm, or 0.22m proportionally
- 3.38.1mm, or 0.38m proportionally → does not provide reliable results, since wheels are not powered so they end up getting caught on the group after impact then flipping over the drivetrain
2. Climbing up obstacles, one wheel side at a time
- Powered by manually turning one wheel at a time
- Tested obstacle courses…
- 1.Small LEGO obstacle (5mm tall, 5cm proportionally) to tall LEGO obstacle (11.5mm, or 11.5cm proportionally)
- 2.Tall LEGO obstacle to small LEGO obstacle
- 3.Two tall LEGO obstacles one after the other
3. Stability test → fill the cup with water, then repeat the previous tests → omitted due to timing
Prototyping Results
| Design Style | Scaling Down Tests | Climbing Tests | Design Issues |
|---|
| Triple Bogie | - Stable throughout 1.1
- Rear rocker consistently has some air time, worse the steeper the drop
- Would likely be more successful if the CoM was closer to the back half of the drivetrain
- Flipped over at high speeds down 1.2's drop
| | - Optimal 4-bar linkage position: top pins are spread apart while the lower pins are close together (i.e. upside down trapezoid)
- Lower pins need to be close together to allow front wheels to lift up heights
- Upper pins need to be spaced out to prevent locking at the lower pins (due to them interfering with each other)
- Want to find the optimal distance between upper pins such that we eliminate the over-centre linkage
- Locking → if the chassis tips too far forward, weight is balanced on just the front bogies and rear stays suspended
- Able to drive out of this position?
- Rocker attempts to twist if the load is uneven between the pair of wheels
- Rocker joint is generally weak
- Flipping → At too high of speeds, will flip over one of the 4-bar linkage pins
|
| Double Bogie | - Frequently locks up if not driving carefully
- Most successful when it goes slowly down an obstacle, reducing the chances of the chassis weight toppling forwards and locking the bogies
- Most successful when it scales down angled one way/one wheel at a time climbs down
| | - Locking → if the middle wheel becomes slightly suspended, uneven/non-centred weight distribution from the chassis (i.e. from being tipped forwards) will pull the front and rear wheels towards the centre of the drivetrain. This then pulls the middle wheel into a locked position, completely suspended in the air
- Difficult to get out of this position
- Uneven weight distribution occurs from too high of speeds → would want to operate this style of drive at a slow speed while climbing obstacles
|
| - Very stable throughout 1.1 and 1.2 → no issues
- For 1.3, it doesn't look like it would reliably make the drop even if the wheels were powered → Bogie lift/extension is the most limited out of all of the designs since it's a smaller bogie arm
- Could only compensate by making the overall rocker-bogie system taller
| | - Constrained to the depth/height of obstacle since the front bogie is limited in height
- Flipping → Bogie may flip over bogie pivot point if the wheel is jammed
|