WOBBLfourbar
Does this wiper linkage sweep 95 degrees, and can a 45 rpm wiper motor drive it over dry glass?
MARGINAL
The rocker swings 94.3 degrees, from 50.1 to 144, as the crank turns, and the transmission angle stays between 35.5 and 130 degrees. Outside 40 to 140 degrees it drives stiffly and wears its pins. The motor turns the crank at 39 rpm on average and peaks at 5.41 N m.
sweep
94.3 deg
transmission
35.5 to 130 deg
worst advantage
1.1x
peak torque
5.41 N m
time ratio
1.05 : 1
Confidence high
- passed hand calculation, virtual work, time step convergence, energy account, constraint drift, virtual work from the weights and loads
Simulation
Checks
| status | check | result |
|---|---|---|
| pass | hand calculation | Freudenstein's equation puts the rocker within 6.62e-04 degrees of MuJoCo's at every crank angle through the turn |
| pass | virtual work | against a steady 6 N m on the rocker, the torque MuJoCo needs at the crank matches load times rocker speed over crank speed to 0.24% of its peak |
| pass | time step convergence | peak crank torque 5.41 N m with the normal step and 5.41 N m with half the step (0.0052% apart) |
| pass | energy account | the work of the forces (drives 49.7 J, loads -48.7 J, constraints -0.00142 J) matches the change in kinetic and potential energy, 0.931 J, to 6.9e-05% of the largest energy flow |
| pass | constraint drift | the linkage loop held together to 0.00211 mm at worst (0.0026% of 80 mm) |
| pass | virtual work from the weights and loads | turning crank slowly, the weights, loads and dampers do 13.7 W at most, so T = sum(F . v) / w asks 5.45 N m at peak, and MuJoCo's drive gives 5.44 N m (0.32% of the peak apart) |
How to fix it
Make the crank 82 mm, the coupler 290 mm and the rocker 112 mm
With the ground at 300 mm, the toggle positions where crank and coupler line up give a 94.3 degree sweep with the transmission angle between 40.8 and 139 degrees, and the crank still turns fully. Running it again with these lengths, the crank peaks at 5.18 N m against 5.41 N m today.
Events
| event | time | detail |
|---|---|---|
| motor on | 0.000 s | |
| rocker at an end | 0.070 s | 50.1 deg |
| rocker at an end | 0.851 s | 144 deg |
| crank turn 1 | 1.552 s | |
| rocker at an end | 1.602 s | 50.1 deg |
| rocker at an end | 2.382 s | 144 deg |
| crank turn 2 | 3.083 s | |
| rocker at an end | 3.134 s | 50.1 deg |
Assumptions
- Links 300 mm ground, 80 mm crank, 300 mm coupler and 110 mm rocker, a crank rocker linkage, flat and stacked so the links never touch.
- The rocker's load is a drag of 6 N m against its motion, as from a wiper blade or a door.
- The motor, which stalls at 25 N m and turns 45 rpm with no load, drives the crank at full voltage, with its rotor felt through the gearbox as 1061 kg cm2, a 20 ms mechanical time constant.
- Pins are frictionless and have no play.
- The motor's current is not known, so none is reported. Give stall_current or a supply voltage with the motor's values to see it.
- Bodies are rigid, so links do not bend. A long or thin printed link flexes and rings more than this shows.
- Contacts are stiff springs that let parts sink into each other by the depth the contact check reports.
- Joints and gears have no backlash, and friction acts only where the inputs give it.