Kerbal Space Program Hovering PID Controller

hover.py

import krpc, time, math

conn = krpc.connect(name = "PID Hover")
vessel = conn.space_center.active_vessel

# Find the craft height (altitude from command module and bottom of craft are different)
bbox = vessel.bounding_box(vessel.reference_frame)
HEIGHT = max(abs(bbox[0][1]), abs(bbox[1][1]))

print("Now controlling '" + vessel.name + "'")

# We need this for later
surf_frame = vessel.orbit.body.reference_frame

# Some program constants
RATE = 0.1
TGT_ALT = 100 - HEIGHT / 2

# PID constants
K_C = 0.25
K_P = -0.03
K_I = 0.01
K_D = -5.5

# The integral and differential component data
prev_err = []
prev_alt = vessel.flight().surface_altitude

vessel.auto_pilot.engage()

timer = 0
while True:
	# Ensure we're pointing upwards
	vessel.auto_pilot.target_direction = (1, 0, 0)

	alt = vessel.flight().surface_altitude

	# Calculate the PID components
	integral = sum(prev_err) * RATE
	proportion = alt - TGT_ALT
	differential = (alt - prev_alt) * RATE

	# Sum the components and set the craft throttle
	throttle = K_C + K_P * proportion + K_I * integral + K_D * differential
	vessel.control.throttle = throttle

	# Keep pushing / pulling the integral buffer
	prev_err.append(alt - TGT_ALT)
	if len(prev_err) > 1 / RATE:
		prev_err = prev_err[1:]

	prev_alt = alt


	time.sleep(RATE)

	# Print only occasionally
	if timer == 10:
		print("Throttle = " + str(throttle) + "(P = " + str(K_P * proportion) + ", I = " + str(K_I * integral) + ", D = " + str(K_D * differential) + ")")
		timer = 0
	else:
		timer += 1