# External Forces¶

## Basics¶

In pyrosim you can add external forces to any body at any time step within the simulation. For example if you wanted to throw a block at time step fifty in the z (up) direction with a force of *5 newtons, you would first create the block using box_id = sim.create_box(**kwargs) and then send the external force using sim.send_external_force(box_id, x=0, y=0, z=5, time=50).

Here x,y,z refers to the force vector applied to the body. It is important to remember that larger mass bodies will need more force to move.

Here we present our famed mascot being hit by a box thrown with external forces. This example also showcases developing synapses.

import math
import numpy as np
import math
import pyrosim

HEIGHT = 0.3
EPS = 0.05
np.random.seed(0)

def send_to_simulator(sim, weight_matrix):
main_body = sim.send_box(x=0, y=0, z=HEIGHT+EPS,
length=HEIGHT, width=HEIGHT,
height=EPS*2.0, mass=1)

# id arrays
thighs = *4
shins = *4
hips = *4
knees = *4
foot_sensors = *4
sensor_neurons = *5
motor_neurons = *8

delta = float(math.pi)/2.0

# quadruped is a box with one leg on each side
# each leg consists thigh and shin cylinders
# with hip and knee joints
# each shin/foot then has a touch sensor
for i in range(4):
theta = delta*i
x_pos = math.cos(theta)*HEIGHT
y_pos = math.sin(theta)*HEIGHT

thighs[i] = sim.send_cylinder(x=x_pos, y=y_pos, z=HEIGHT+EPS,
r1=x_pos, r2=y_pos, r3=0,
)

hips[i] = sim.send_hinge_joint(main_body, thighs[i],
x=x_pos/2.0, y=y_pos/2.0, z=HEIGHT+EPS,
n1=-y_pos, n2=x_pos, n3=0,
lo=-math.pi/4.0, hi=math.pi/4.0,
speed=1.0)

motor_neurons[i] = sim.send_motor_neuron(joint_id=hips[i])

x_pos2 = math.cos(theta)*1.5*HEIGHT
y_pos2 = math.sin(theta)*1.5*HEIGHT

shins[i] = sim.send_cylinder(x=x_pos2, y=y_pos2, z=(HEIGHT+EPS)/2.0,
r1=0, r2=0, r3=1,
mass=1., capped=True)

knees[i] = sim.send_hinge_joint(thighs[i], shins[i],
x=x_pos2, y=y_pos2, z=HEIGHT+EPS,
n1=-y_pos, n2=x_pos, n3=0,
lo=-math.pi/4.0, hi=math.pi/4.0)

motor_neurons[i+4] = sim.send_motor_neuron(knees[i])
foot_sensors[i] = sim.send_touch_sensor(shins[i])
sensor_neurons[i] = sim.send_sensor_neuron(foot_sensors[i])

count = 0
# developing synapses linearly change from the start value to the
# end value over the course of start time to end time
# Here we connect each sensor to each motor, pulling weights from
# the weight matrix
for source_id in sensor_neurons:
for target_id in motor_neurons:
count += 1
start_weight = weight_matrix[source_id, target_id, 0]
end_weight = weight_matrix[source_id, target_id, 1]
start_time = weight_matrix[source_id, target_id, 2]
end_time = weight_matrix[source_id, target_id, 3]
sim.send_developing_synapse(source_id, target_id,
start_weight=start_weight,
end_weight=end_weight,
start_time=start_time,
end_time=end_time)

# layouts are useful for other things not relevant to this example
layout = {'thighs': thighs,
'shins': shins,
'hips': hips,
'knees': knees,
'feet': foot_sensors,
'sensor_neurons': sensor_neurons,
'motor_neurons': motor_neurons}

env_box = sim.send_box(x=2, y=-2, z=HEIGHT/2.0,
length=HEIGHT,
width=HEIGHT,
height=HEIGHT,
mass=3.)

# send the box towards the origin with specified force
# proportional to its mass
MASS = 3
# sim.send_external_force(env_box, x=0, y=0, z=MASS*20., time=300)
# sim.send_external_force(env_box, x=-MASS*70., y=MASS*73., z=0, time=320)

sim.create_collision_matrix('all')

return layout

if __name__ == "__main__":

seconds = 10.0
dt = 0.05
eval_time = int(seconds/dt)
print(eval_time)
gravity = -1.0

sim = pyrosim.Simulator(eval_time=eval_time, debug=True,
play_paused=False,
gravity=gravity,
play_blind=False,
use_textures=True,
capture=False,
dt=dt)
num_sensors = 5
num_motors = 8

# our weight matrix specifies the values for the
# starting and ending weights as well as the starting and
# ending times for the synapses
weight_matrix = np.random.rand(num_sensors+num_motors,
num_sensors+num_motors, 4)
weight_matrix[:, :, 0:1] = weight_matrix[:, :, 0:1]*2.-1.

layout = send_to_simulator(sim, weight_matrix=weight_matrix)
sim.start()

sim.wait_to_finish()
`