Overview of automation

Why this matters

A drone hovering steady in the wind, a factory conveyor that stops when a box falls across it, and an air conditioner holding room temperature all work on one principle: measure, decide, act, over and over. Anyone who understands this principle can read any automated system, from factory machines to a drone’s flight controller.

The whole course uses one hypothetical case: an automatic drone battery-swap station, with a conveyor carrying batteries, sensors that detect a landed drone and a battery, a pneumatic gripper arm, a charging cabinet whose temperature must be controlled, and an emergency stop button. Stations like this are used for drones that must fly continuously, such as area surveillance. The Python code of every module can be downloaded from /downloads/uat-202/.

Components of an automated system

• Sensor: turns what is measured into an electrical signal, such as a sensor detecting a battery on the belt or a temperature sensor in the charging cabinet
• Controller: compares the measurement with the desired value (setpoint) and decides, such as a PLC or microcontroller
• Actuator: turns commands into action, such as a motor, pneumatic valve or fan
• Process: what is being controlled, such as belt speed or cabinet temperature
• Disturbance: something that changes the process without our command, such as heavier batteries on the belt

Open loop and closed loop

An open-loop system acts on a plan without measuring the result, like a toaster set on a timer whether or not the bread is done. A closed-loop (feedback) system measures the result and corrects the command by the error $e=r-y$, the setpoint minus the measurement.

def conveyor(kp, t_end=20.0, dt=0.01):
    v, tau, k, target, u0 = 0.0, 0.5, 0.25, 0.5, 2.0   # m/s, s, (m/s)/V, m/s, V
    speeds, t = {}, 0.0
    while t <= t_end + 1e-9:
        load = 0.1 if t >= 10 else 0.0                    # extra load at 10 s
        u = u0 + kp * (target - v)                         # kp = 0 is open loop
        v += dt * (k * u - load - v) / tau
        for mark in (9.99, 20.0):
            if abs(t - mark) < dt / 2:
                speeds[mark] = v
        t += dt
    return speeds


for kp in (0, 8, 40):
    s = conveyor(kp)
    label = "open loop" if kp == 0 else f"feedback Kp={kp}"
    print(f"{label:<17} before load {s[9.99]:.3f} m/s, after load {s[20.0]:.3f} m/s")

open loop         before load 0.500 m/s, after load 0.400 m/s
feedback Kp=8     before load 0.500 m/s, after load 0.467 m/s
feedback Kp=40    before load 0.500 m/s, after load 0.491 m/s

Factory levels in ISA-95

The ISA-95 standard (IEC 62264) divides an organisation’s systems into levels, showing what each device does and whom it talks to.

Our battery-swap station has sensors and actuators (level 1), a PLC and control screen (level 2), while the system scheduling missions for the whole drone fleet is comparable to level 3.

Where people fit

Automation is not just “human does it” or “machine does it”. Sheridan and Verplank (1978) proposed a ten-level scale, from the human doing everything to the computer deciding without informing the human. Parasuraman, Sheridan and Wickens (2000) split work into four stages, information acquisition, analysis, decision and action, each of which can be automated to a different degree.

In a drone, for example, the flight controller holds attitude fully automatically, but the decision to start a mission still belongs to the pilot. Choosing the right level means thinking about the consequences when the system fails, and people must understand enough to take over.

Module lab

Common mistakes

Summary

• Automated systems have sensors, controllers, actuators and a process, working as a sense–decide–act loop
• Open loop does not measure, so it cannot handle disturbances; closed loop corrects commands by the error
• ISA-95 divides systems into levels 0 to 4, from the physical process to business systems
• Levels of automation can differ between steps, and people must be able to take over

Check your understanding

1. Is a toaster on a timer open loop or closed loop?
2. The setpoint is 0.5 m/s and the measurement 0.42 m/s. What is the error?
3. In Example 1, with $K_p=8$ and an error of 0.03 m/s, how much is the voltage command raised above $u_0$?
4. At which ISA-95 level is the PLC controlling the conveyor?
5. Why can open loop not cope with disturbances?