Robotics Fundamentals
The Anatomy of a Robot
Every robot, however different, is built from the same subsystems: sensors, controller, actuators, transmission, power and frame. Learn what each does and how they connect.
beginner · 12 min read
Lesson content
A surgical robot and a robot vacuum have nothing in common to look at. Inside, they have almost everything in common. Learn the subsystems once and you can take apart any robot you meet. That includes working out which subsystem is at fault when it misbehaves.
- Sensors
Devices that measure something: a joint angle, a distance, an acceleration, an image.
They are the robot's only contact with reality, including with its own body. Everything downstream is limited by what they can measure and how well.
- Controller
The computer (or several) that reads sensors, decides and issues commands.
The one subsystem you can change without a screwdriver. Separating it from the actuators is precisely what makes a robot reprogrammable.
- Actuators
Motors, cylinders and the like that convert stored energy into motion.
The only way a robot changes anything. Without them you have a very expensive sensor.
- Transmission
Gears, belts, screws and linkages between an actuator and the joint it drives.
Motors are fast and weak. Joints need slow and strong. The transmission does that trade. Its backlash and compliance then shape how well the joint can be controlled.
- End effector
The tool at the end of the chain: a gripper, a welder, a nozzle, a camera mount.
The part that actually does the job. Change it and the robot's purpose changes entirely.
- Power source
Battery, mains supply, hydraulic pump or compressed air.
Usually the single biggest constraint on a mobile robot's mass, size and runtime. Fixed robots trade that constraint for a cable.
- Frame / structure
The links, chassis and bearings everything else bolts to.
It sets stiffness and stiffness sets accuracy. A control loop cannot correct for a structure that flexes.
Two separate networks run through every robot and mixing them up causes a lot of confusion.
The signal path carries information. It is thin, low-power and easy to disturb: sensors → controller → actuator commands.
The power path carries energy. It is thick, high-current and noisy: source → distribution → amplifiers → actuators.
They meet in exactly one place: the actuator. It takes a small signal in one hand and a large current in the other, then turns them into motion.
The power path, in more detail
Source
A battery, a mains supply, a hydraulic pump or a compressor. Its capacity sets the runtime.
Distribution and regulation
Converting one supply into the several voltages the robot needs. Motors at 24 or 48 V, logic at 5 or 3.3 V, all with their own protection.
Power amplifier / driver
Takes the controller's small command signal and modulates the large power flow that reaches the motor. For a DC motor this is an H-bridge switching with PWM.
Actuator
Converts that regulated power into torque or force.
Transmission
Converts fast and weak into slow and strong, delivering it to the joint.
Why separate the controller from the actuators at all? Wouldn't one integrated unit be simpler?
Simpler to build. Much worse to live with.
That separation is the whole reason the machine counts as a robot rather than a mechanism. The same physical body can be given a completely new job by changing only the program.
It also means each part can be developed, tested and replaced on its own. You can swap a motor driver without going anywhere near the planner.
Modern robots split the controller further still: a hard real-time loop sitting right at the joint and a slower, richer computer above it doing perception and planning.
| Subsystem | In a robot vacuum | In an industrial arm | In a Mars rover |
|---|---|---|---|
| Sensors | Bump switch, cliff IR, optical flow | Joint encoders, force sensor, camera | Cameras, IMU, spectrometers, wheel encoders |
| Controller | One microcontroller | Real-time cabinet + a PC for planning | Radiation-hardened CPU, flight software |
| Actuators | Two wheel motors, a brush motor | Six servo motors with brakes | Wheel motors, arm motors, drill |
| Transmission | Small spur gearbox | Harmonic drives | High-ratio gearboxes, rocker-bogie linkage |
| Power | Li-ion pack, ~1 hour | Mains, unlimited | RTG or solar, years |
| Frame | Moulded plastic shell | Cast aluminium links | Titanium chassis, rocker-bogie |
Sensors
- In a robot vacuum
- Bump switch, cliff IR, optical flow
- In an industrial arm
- Joint encoders, force sensor, camera
- In a Mars rover
- Cameras, IMU, spectrometers, wheel encoders
Controller
- In a robot vacuum
- One microcontroller
- In an industrial arm
- Real-time cabinet + a PC for planning
- In a Mars rover
- Radiation-hardened CPU, flight software
Actuators
- In a robot vacuum
- Two wheel motors, a brush motor
- In an industrial arm
- Six servo motors with brakes
- In a Mars rover
- Wheel motors, arm motors, drill
Transmission
- In a robot vacuum
- Small spur gearbox
- In an industrial arm
- Harmonic drives
- In a Mars rover
- High-ratio gearboxes, rocker-bogie linkage
Power
- In a robot vacuum
- Li-ion pack, ~1 hour
- In an industrial arm
- Mains, unlimited
- In a Mars rover
- RTG or solar, years
Frame
- In a robot vacuum
- Moulded plastic shell
- In an industrial arm
- Cast aluminium links
- In a Mars rover
- Titanium chassis, rocker-bogie
Example
Diagnosing by subsystem
The arm stops short of its target. Do not guess. Walk the list.
Sensor: is the encoder reading what the joint is actually doing?
Controller: was the commanded setpoint right in the first place?
Power: is the supply sagging under load?
Actuator: is the motor drawing current but not turning?
Transmission: is there backlash, or a belt slipping?
Frame: is the link simply bending under the payload?
Every one of those looks identical from across the room. Every one of them needs a completely different fix.
That is the real reason to learn the anatomy: it is also the fault tree.
The same anatomy, all the way up
You already own one of these.
Sensors: eyes, skin, inner ear. Controller: brain and spinal cord. Actuators: muscles. Transmission: tendons. Power: metabolism. Frame: skeleton.
The comparison goes further than it first looks. Your spinal reflexes and your conscious thoughts are the same split as a robot's fast joint loop and its slow planner. Touch something hot and your hand is moving before you have decided anything. The reflex is faster precisely because it never travels all the way up.
Forgetting the transmission when reasoning about a joint.
Backlash, compliance and gear ratio change everything a control loop sees. Many "tuning" problems are really mechanical.
Sizing the power system last.
On a mobile robot, batteries are often the heaviest single item. Mass drives motor size, which drives power draw. Start that loop early.
Treating the end effector as an accessory.
It is payload the arm must carry, it is where every process force enters and it is usually where the job actually succeeds or fails.
Key takeaway
- Almost every robot has sensors, a controller, actuators, a transmission, an end effector, power and a frame.
- Sensors sense, the controller thinks and actuators act. Transmission, power and frame make all three possible.
- Two networks run through the machine: a signal path and a power path, meeting at the actuator.
- The same anatomy scales from a robot vacuum to a Mars rover.
- Separating the controller from the actuators is what makes a robot reprogrammable. The subsystem list doubles as a fault tree.
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