python - 如何阻止皮蒙克关节的 body 弹跳？

Question

我正在使用Pymunk模拟安装在细长腿顶部枢轴上的盒子的物理特性，该腿固定在地面上。连接盒子和腿的枢轴是 PivotJoint 和 SimpleMotor 的组合，模拟原始伺服系统，用户可以打开和关闭伺服系统，以将盒子的质心转移到腿上。以下是我实现此模型的 Pymunk+Pygame 代码:

from __future__ import print_function
import sys
from math import pi

import pygame
from pygame.locals import USEREVENT, QUIT, KEYDOWN, KEYUP, K_s, K_r, K_q, K_ESCAPE, K_UP, K_DOWN
from pygame.color import THECOLORS

import pymunk
from pymunk import Vec2d
import pymunk.pygame_util

LEG_GROUP = 1

class Simulator(object):

    def __init__(self):
        self.display_flags = 0
        self.display_size = (600, 600)

        self.space = pymunk.Space()
        self.space.gravity = (0.0, -1900.0)
        self.space.damping = 0.999 # to prevent it from blowing up.

        # Pymunk physics coordinates start from the lower right-hand corner of the screen.
        self.ground_y = 100
        ground = pymunk.Segment(self.space.static_body, (5, self.ground_y), (595, self.ground_y), 1.0)
        ground.friction = 1.0
        self.space.add(ground)

        self.screen = None

        self.draw_options = None

    def reset_bodies(self):
        for body in self.space.bodies:
            if not hasattr(body, 'start_position'):
                continue
            body.position = Vec2d(body.start_position)
            body.force = 0, 0
            body.torque = 0
            body.velocity = 0, 0
            body.angular_velocity = 0
            body.angle = body.start_angle

    def draw(self):
        ### Clear the screen
        self.screen.fill(THECOLORS["white"])

        ### Draw space
        self.space.debug_draw(self.draw_options)

        ### All done, lets flip the display
        pygame.display.flip()

    def main(self):

        pygame.init()
        self.screen = pygame.display.set_mode(self.display_size, self.display_flags)
        width, height = self.screen.get_size()
        self.draw_options = pymunk.pygame_util.DrawOptions(self.screen)

        def to_pygame(p):
            """Small hack to convert pymunk to pygame coordinates"""
            return int(p.x), int(-p.y+height)
        def from_pygame(p):
            return to_pygame(p)

        clock = pygame.time.Clock()
        running = True
        font = pygame.font.Font(None, 16)

        # Create the torso box.
        box_width = 50
        box_height = 100
        leg_length = 100
        leg_thickness = 2

        mass = 200
        points = [(-box_width/2, -box_height/2), (-box_width/2, box_height/2), (box_width/2, box_height/2), (box_width/2, -box_height/2)]
        moment = pymunk.moment_for_poly(mass, points)
        body1 = pymunk.Body(mass, moment)
        body1.position = (self.display_size[0]/2, self.ground_y+box_height/2+leg_length)
        body1.start_position = Vec2d(body1.position)
        body1.start_angle = body1.angle
        shape1 = pymunk.Poly(body1, points)
        shape1.group = LEG_GROUP
        shape1.friction = 0.8
        shape1.elasticity = 0.0
        self.space.add(body1, shape1)

        # Create bar 2 extending from the right to the origin.
        mass = 10
        points = [
            (leg_thickness/2, -leg_length/2),
            (-leg_thickness/2, -leg_length/2),
            (-leg_thickness/2, leg_length/2-leg_thickness),
            (leg_thickness/2, leg_length/2-leg_thickness/2)
        ]
        moment = pymunk.moment_for_poly(mass, points)
        body2 = pymunk.Body(mass, moment)
        body2.position = (self.display_size[0]/2-box_width/2, self.ground_y+leg_length/2)
        body2.start_position = Vec2d(body2.position)
        body2.start_angle = body2.angle
        shape2 = pymunk.Poly(body2, points)
        shape2.group = LEG_GROUP
        shape2.friction = 0.8
        shape2.elasticity = 0.0
        self.space.add(body2, shape2)

        # Link bars together at end.
        pj = pymunk.PivotJoint(body1, body2, (self.display_size[0]/2-box_width/2, self.ground_y+leg_length-leg_thickness))
        self.space.add(pj)

        # Attach the foot to the ground in a fixed position.
        pj = pymunk.PivotJoint(self.space.static_body, body2, (self.display_size[0]/2-box_width/2, self.ground_y+leg_thickness))
        self.space.add(pj)

        # Actuate the bars via a motor.
        motor_joint = pymunk.SimpleMotor(body1, body2, 0)
        self.space.add(motor_joint)

        # Add hard stops to leg pivot so the torso can't rotate through leg.
        hip_limit_joint = pymunk.RotaryLimitJoint(body1, body2, -pi/4., pi/4.) # -45deg:+45deg
        self.space.add(hip_limit_joint)

        pygame.time.set_timer(USEREVENT+1, 70000) # apply force
        pygame.time.set_timer(USEREVENT+2, 120000) # reset
        pygame.event.post(pygame.event.Event(USEREVENT+1))
        pygame.mouse.set_visible(False)

        simulate = False
        while running:
            # print('angles:', body1.angle, body2.angle)
            servo_angle = (body1.angle - body2.angle) * 180/pi # 0 degrees means leg is angled straight down
            servo_cw_enabled = servo_angle > -45
            servo_ccw_enabled = servo_angle < 45

            for event in pygame.event.get():
                if event.type == QUIT or (event.type == KEYDOWN and event.key in (K_q, K_ESCAPE)):
                    #running = False
                    sys.exit(0)
                elif event.type == KEYDOWN and event.key == K_s:
                    # Start/stop simulation.
                    simulate = not simulate
                elif event.type == KEYDOWN and event.key == K_r:
                    # Reset.
                    # simulate = False
                    self.reset_bodies()
                elif event.type == KEYDOWN and event.key == K_UP:
                    if servo_ccw_enabled:
                        motor_joint.rate = 5
                    else:
                        motor_joint.rate = 0
                elif event.type == KEYDOWN and event.key == K_DOWN:
                    if servo_cw_enabled:
                        motor_joint.rate = -5
                    else:
                        motor_joint.rate = 0
                elif event.type == KEYUP:
                    motor_joint.rate = 0

            self.draw()

            ### Update physics
            fps = 50
            iterations = 25
            dt = 1.0/float(fps)/float(iterations)
            if simulate:
                for x in range(iterations): # 10 iterations to get a more stable simulation
                    self.space.step(dt)

            pygame.display.flip()
            clock.tick(fps)

if __name__ == '__main__':
    sim = Simulator()
    sim.main()

您可以通过按“s”开始模拟，然后使用向上/向下键围绕腿旋转盒子。它的行为基本上与预期一致……直到它接触地面，然后它开始猛烈地“弹跳”。 See this screen capture举个例子。

我该如何阻止这个？我通过赋予盒子很大的质量来使其变得非常重，并且我将所有东西的弹性设置为 0。文档提到，如果每次更新使用的迭代次数少于 10 次，则东西可能会变得有弹性或柔软，但我'我使用 25。我还缺少什么？

Answer 1

尝试设置电机关节的 max_force。没有它，电机关节可以使用无限的力量。

例如这样:motor_joint.max_force = 100000000

另请注意，两个形状的组设置是错误的。 shape2.group = LEG_GROUP 不执行任何操作。在以前的版本 (<5.0) 中，组的工作方式如下，但现在您应该使用 ShapeFilter:shape1.filter = pymunk.ShapeFilter(group=LEG_GROUP)