Balance box on wheels with springs (lock rotation relative to box) pymunk

Question

I want to be able to lock the angle of the wheels relative to the car's chassis. In between the wheels, there are springs, that should allow the car to suspend, but right now, the angle is not locked. I am using pymunk's function "RotaryLimitJoint"

A behavior like this is the goal (gif)

Right now it looks like this:

My code:

car_pos = Vec2d(100,500)
mass = 30
radius = 10
moment = pymunk.moment_for_circle(mass, 20, radius)
wheel1_b = pymunk.Body(mass, moment)
wheel1_s = pymunk.Circle(wheel1_b, radius)
wheel1_s.friction = 1.5
wheel1_s.color = wheel_color
space.add(wheel1_b, wheel1_s)

mass = 30
radius = 10
moment = pymunk.moment_for_circle(mass, 20, radius)
wheel2_b = pymunk.Body(mass, moment)
wheel2_s = pymunk.Circle(wheel2_b, radius)
wheel2_s.friction = 1.5
wheel2_s.color = wheel_color
space.add(wheel2_b, wheel2_s)

mass = 100
size = (80,25)
moment = pymunk.moment_for_box(mass, size)
chassi_b = pymunk.Body(mass, moment)
chassi_s = pymunk.Poly.create_box(chassi_b, size)
chassi_s.color = chassi_color
space.add(chassi_b, chassi_s)


#Positions
chassi_b.position = car_pos + (0,-15)
wheel1_b.position = car_pos + (-25,0)
wheel2_b.position = car_pos + (25,0)

#Joints

spring1 = pymunk.DampedSpring(chassi_b, wheel1_b, (-25,0), (0,0), 20, 100000, 1)
spring1.collide_bodies = False
spring2 = pymunk.DampedSpring(chassi_b, wheel2_b, (25,0), (0,0), 20, 100000, 1)
spring2.collide_bodies = False

wheelAngle1 = pymunk.RotaryLimitJoint(wheel1_b, chassi_b, 0, 0)
wheelAngle1.collide_bodies = False
wheelAngle2 = pymunk.RotaryLimitJoint(chassi_b, wheel2_b, 0, 0)
wheelAngle2.collide_bodies = False



space.add(
    spring1,
    spring2,
    wheelAngle1,
    wheelAngle2
)
speed = 20

space.add(
    pymunk.SimpleMotor(wheel1_b, chassi_b, speed),
    pymunk.SimpleMotor(wheel2_b, chassi_b, speed)
)

Andreas G.D Petersen · Accepted Answer · 2021-06-27T22:44:01.033

First off credits to @viblo.

What makes the following code work, is that the GrooveJoint (see docs) is created perpendicular to the car's chassis. The GrooveJoint is defining a line, where a body is free to slide on. Defining the GrooveJoint it is attached to the car's chassis and to the wheel (for both the front and back wheel).

It looks like this now:

I converted the code (from @viblo) to python and here it is:

def car(space, speed, add_car):
    car_pos = Vec2d(100,500)

    #bodies
    wheel_color = 0,0,0
    chassi_color = 255,0,0
    wheelCon_color = 0,255,255

    #Wheel 1
    mass = 25
    radius = 10
    moment = pymunk.moment_for_circle(mass, 20, radius)
    wheel1_b = pymunk.Body(mass, moment)
    wheel1_s = pymunk.Circle(wheel1_b, radius)
    wheel1_s.friction = 1.5
    wheel1_s.color = wheel_color

    #Wheel 2
    mass = 25
    radius = 10
    moment = pymunk.moment_for_circle(mass, 20, radius)
    wheel2_b = pymunk.Body(mass, moment)
    wheel2_s = pymunk.Circle(wheel2_b, radius)
    wheel2_s.friction = 1.5
    wheel2_s.color = wheel_color

    #Chassi
    mass = 30
    size = (80,25)
    moment = pymunk.moment_for_box(mass, size)
    chassi_b = pymunk.Body(mass, moment)
    chassi_s = pymunk.Poly.create_box(chassi_b, size)
    chassi_s.color = chassi_color

    #Positions
    chassi_b.position = car_pos + (0,-15)
    wheel1_b.position = car_pos + (-25,0)
    wheel2_b.position = car_pos + (25,0)

    #Joints
    spring1 = pymunk.constraint.DampedSpring(chassi_b, wheel1_b, (-25,0), (0,0), 15, 5000, 250)
    spring1.collide_bodies = False
    spring2 = pymunk.constraint.DampedSpring(chassi_b, wheel2_b, (25,0), (0,0), 15, 5000, 250)
    spring2.collide_bodies = False

    groove1 = pymunk.constraint.GrooveJoint(chassi_b, wheel1_b, (-25,0), (-25,25), (0, 0))
    groove1.collide_bodies = False
    groove2 = pymunk.constraint.GrooveJoint(chassi_b, wheel2_b, (25,0), (25,25), (0,0))
    groove2.collide_bodies = False

    if add_car:
        motor1 = pymunk.SimpleMotor(wheel1_b, chassi_b, speed)
        motor2 = pymunk.SimpleMotor(wheel2_b, chassi_b, speed)
        space.add(
            spring1,
            spring2,
            groove1,
            groove2,
            motor1,
            motor2,
            chassi_b, 
            chassi_s,
            wheel2_b,
            wheel2_s,
            wheel1_b,
            wheel1_s
        )

score 2 · Answer 2 · answered May 06 '20 at 19:29

The RotaryLimitJoint is not what you need. It will constrain the angle between the wheel and chassis, but the wheel needs to rotate so it wont work.

What you can try with instead is a GrooveJoint.

This is how the code looks like in c code, it should be fairly easy to convert it to python:

Full source: https://github.com/slembcke/Chipmunk2D/blob/master/demo/Joints.c#L263

The relevant part:

boxOffset = cpv(0, 0);
cpBody *wheel1 = addWheel(space, posA, boxOffset);
cpBody *wheel2 = addWheel(space, posB, boxOffset);
cpBody *chassis = addChassis(space, cpv(80, 100), boxOffset);

cpSpaceAddConstraint(space, cpGrooveJointNew(chassis, wheel1, cpv(-30, -10), cpv(-30, -40), cpvzero));
cpSpaceAddConstraint(space, cpGrooveJointNew(chassis, wheel2, cpv( 30, -10), cpv( 30, -40), cpvzero));

cpSpaceAddConstraint(space, cpDampedSpringNew(chassis, wheel1, cpv(-30, 0), cpvzero, 50.0f, 20.0f, 10.0f));
cpSpaceAddConstraint(space, cpDampedSpringNew(chassis, wheel2, cpv( 30, 0), cpvzero, 50.0f, 20.0f, 10.0f));

If you cant figure it out I can try to convert it to python, but unfortunately Im not on a computer with Python & Pymunk setup for that now.

Ahhh, I understand. I will try to do it in python and then post it here, so other people can use it too — Andreas G.D Petersen, May 06 '20 at 20:24

Balance box on wheels with springs (lock rotation relative to box) pymunk

2 Answers2