Automation and drones

Why this matters

A drone that must patrol an area all day needs its battery swapped regularly. Swapping by hand means someone on duty at all times, so an automatic battery-swap station is where industrial automation (sensors, PLCs, actuators) meets the drone system (flight controller, missions and links). This module brings the whole course together.

The sequence as a state machine

A state machine describes a system as a set of states, and the events that move it from one state to another. The SFC language in IEC 61131-3 is designed for this kind of task.

• IDLE: wait for a drone to land
• LOCK: clamp the drone in position when the sensor sees it on the pad
• SWAP: the gripper removes the old battery and inserts a new one
• CHECK: confirm the new battery is seated and its voltage is correct
• RELEASE: unclamp, leaving the drone ready to take off
• FAULT: if any step times out or the emergency stop is pressed, stop all motion and wait for a person to inspect

TIMEOUT = {"LOCK": 5, "SWAP": 20, "CHECK": 5, "RELEASE": 5}   # maximum seconds per state
NEXT = {("IDLE", "landed"): "LOCK", ("LOCK", "locked"): "SWAP", ("SWAP", "swapped"): "CHECK",
        ("CHECK", "voltage_ok"): "RELEASE", ("RELEASE", "released"): "IDLE"}


def run(events):
    state, entered = "IDLE", 0
    for t, event in events:
        if event == "estop" or (state in TIMEOUT and t - entered > TIMEOUT[state]):
            reason = "E-stop" if event == "estop" else f"timeout in {state}"
            print(f"t={t:>3} s  {state:<8} -> FAULT ({reason})")
            return "FAULT"
        new = NEXT.get((state, event))
        if new:
            print(f"t={t:>3} s  {state:<8} -> {new:<8} on '{event}'")
            state, entered = new, t
    return state


print("final:", run([(0, "landed"), (2, "locked"), (15, "swapped"), (17, "voltage_ok"), (19, "released")]))
print("final:", run([(0, "landed"), (2, "locked"), (30, "swapped")]))

t=  0 s  IDLE     -> LOCK     on 'landed'
t=  2 s  LOCK     -> SWAP     on 'locked'
t= 15 s  SWAP     -> CHECK    on 'swapped'
t= 17 s  CHECK    -> RELEASE  on 'voltage_ok'
t= 19 s  RELEASE  -> IDLE     on 'released'
final: IDLE
t=  0 s  IDLE     -> LOCK     on 'landed'
t=  2 s  LOCK     -> SWAP     on 'locked'
t= 30 s  SWAP     -> FAULT (timeout in SWAP)
final: FAULT

Modbus: a common language for devices

The drone mission system must ask the station’s PLC whether it is ready. Modbus is a widely used industrial protocol, storing data as coils (bits) and registers (16-bit values). Under Modbus specification V1.1b3, the TCP variant uses port 502, function 01 reads coils and function 03 reads holding registers. OPC UA (IEC 62541) is a newer standard with richer data structures and security.

import struct

transaction_id, protocol_id, unit_id = 1, 0, 1
function_code, start_address, quantity = 3, 0, 2
pdu = struct.pack(">BHH", function_code, start_address, quantity)   # function + data
mbap = struct.pack(">HHHB", transaction_id, protocol_id, len(pdu) + 1, unit_id)
frame = mbap + pdu
print("request bytes:", frame.hex(" "))
print("length:", len(frame), "bytes; send to TCP port 502")

request bytes: 00 01 00 00 00 06 01 03 00 00 00 02
length: 12 bytes; send to TCP port 502

Automation inside the drone

A flight controller uses several nested feedback loops (a cascade). In PX4’s diagrams, the position loop commands velocity, the velocity loop commands attitude, the attitude loop commands rotation rate, and the rate loop commands the motors, with inner loops running faster than outer ones. ArduPilot follows the same principle: an angle P controller produces a desired rate, and a rate PID controller drives the motors.

A drone can also command external equipment. ArduPilot has relays triggered by a transmitter switch or the DO_SET_RELAY mission command, and a servo gripper commanded with DO_GRIPPER, for example to release a load at the destination. This is the same automation studied throughout the course, just on an aircraft.

Module lab

Common mistakes

Summary

• A state machine describes a sequence as states and events, and needs time limits and a FAULT state
• Modbus TCP uses port 502 and function 03 reads holding registers, on a controlled network
• A flight controller is a cascade of feedback loops from position down to rotation rate
• ArduPilot can command relays and grippers from missions, linking drones with automated equipment

Check your understanding

1. The SWAP state has a 20 s limit and was entered at 2 s. If it has not finished at 25 s, what happens?
2. Which Modbus function reads holding registers?
3. Which port does Modbus TCP use?
4. In a cascaded controller, which loop runs fastest?
5. Which ArduPilot mission command operates a gripper?