Actuation and Power Systems

A robot that can only sense and compute but cannot move is an expensive sensor array. Actuation is what makes a robot a robot: the ability to exert forces and produce motion in the physical world. But actuation is inseparable from energy — every movement costs power, and the power budget is almost always the tightest constraint in mobile robot design. Understanding actuators means understanding both their mechanical characteristics and their energy demands.

Actuator Types and Their Characteristics

The dominant actuator in robotics is the electric motor, but several technologies compete depending on the application. DC brushed motors are the simplest: voltage applied across the terminals produces torque proportional to current. They are cheap, easy to control, and deliver smooth motion. The mechanical brushes that commutate the current wear over time, limiting lifetime in high-cycle applications. Brushless DC motors (BLDC) eliminate the brushes by using an external electronic speed controller (ESC) to switch current through the motor's phases electronically. They are more efficient, more powerful per unit weight, and have far longer service life. Virtually all high-performance robot joints and drone propulsion systems use BLDC motors. Servo motors add a position sensor (encoder or resolver) and a closed-loop controller to a motor, creating a package that holds a commanded angular position with high accuracy. Hobby servos (RC-car style) control a small angular range at low torque. Industrial servos drive precision CNC machines and robot manipulator arms with sub-millimeter repeatability. Hydraulic actuators use pressurized fluid (typically oil) to produce force. They have exceptional power density: a hydraulic cylinder the size of a soda can can produce thousands of newtons of force. Boston Dynamics' Atlas humanoid robot used hydraulics in its first generation because no electric motor could match the power-to-weight ratio needed for dynamic jumping. The trade-off is complexity — pumps, valves, and fluid lines add mass, failure points, and maintenance demands. Pneumatic actuators use compressed air. They are fast, clean, and inexpensive but hard to control precisely because air is compressible. They dominate in industrial pick-and-place machines where fast, on/off motion is needed but fine position control is not.

Torque, Speed, and Gearing

Electric motors produce their peak torque at low speed and are most efficient at moderate speeds — yet many robot joints require high torque at low speed (lifting a heavy arm link) or high speed at low torque (spinning a drone propeller). Gearboxes bridge this gap through mechanical advantage. Torque and angular velocity obey the conservation of power: Power = Torque x Angular_velocity. A gearbox with a reduction ratio of N:1 multiplies torque by N and divides speed by N (ignoring friction losses). A motor producing 0.1 N·m at 1,000 RPM, coupled to a 100:1 gearbox, outputs 10 N·m at 10 RPM — suitable for a heavy robot arm joint. The choice of gear ratio is a critical design decision. A high reduction ratio delivers large torque but slow motion and also increases the effective inertia seen by the motor, which can hurt dynamic response. A low reduction ratio gives fast motion but insufficient torque for heavy loads. Backdrivability — whether a force on the output shaft can back-drive the motor — also depends on gear ratio and gearbox type; this matters for safety in human-robot interaction. Planetary gearboxes, harmonic drive gearboxes, and cycloidal drives are the common choices in precision robot joints. Harmonic drives (used in NASA space robots and most modern collaborative robot arms) achieve gear ratios of 50:1 to 320:1 in a compact, zero-backlash package — backlash is the small angular slop when a gearbox reverses direction, and eliminating it is critical for positional accuracy.

Power Budgets and Energy Storage

Every component in a robot consumes power. The power budget is an accounting of all power consumers and the energy source that must supply them. A power budget for a hypothetical delivery robot might look like this: four drive motors (peak 200 W each = 800 W peak), onboard computer (45 W), lidar (8 W), cameras (4 W total), communication radio (3 W), lighting and auxiliary systems (10 W). Total peak power: approximately 870 W. But peak power occurs only during aggressive acceleration; average power during steady delivery walking might be 150 W. The energy storage must supply the average power for the required mission duration. A 4-hour delivery shift at 150 W average requires 150 W x 4 h = 600 Wh of usable stored energy. Lithium-ion cells have an energy density of approximately 200–250 Wh/kg at the cell level (lower at the pack level after accounting for packaging, wiring, and battery management electronics). A 600 Wh pack at 200 Wh/kg weighs about 3 kg — plus perhaps 50% overhead for packaging, yielding roughly 4.5 kg of battery pack. That mass is part of the robot's structural load and reduces its payload capacity. This cascade — mission time requires energy, energy requires battery mass, battery mass requires stronger structure and larger motors — is why power system design is often the first design driver in a mobile robot program.