Fluid, soft and compliant actuators

Electric motors with gearboxes are the default robot actuator, but they are not the only option, and for some jobs not the best. Excavators, forestry machines and some of the most dynamic humanoid robots use hydraulics for their enormous force in a compact package. Factory grippers and end effectors run on compressed air because it is cheap, clean and naturally springy. Soft robots made of silicone inflate and bend to grasp fragile fruit. And a new generation of legged and collaborative robots deliberately puts springs into their electric actuators, or removes most of the gearing, so that they can feel and absorb forces. This lesson surveys these alternatives and the reasons for choosing them.

Hydraulic actuators

A hydraulic actuator converts fluid pressure into force. A pump, driven by an engine or electric motor, pressurizes oil to typically 10 to 35 MPa (about 1,500 to 5,000 psi), and valves meter it into cylinders or hydraulic motors. The force of a cylinder is the pressure times the piston area, and its speed is the flow rate divided by that area.

Because the pressure is so high, a cylinder a few centimetres in diameter produces tens of kilonewtons, giving hydraulic systems the highest force and power density of any common actuator, measured at the actuator itself. The fluid is nearly incompressible, so hydraulic joints are stiff, and they tolerate shock loads well. Servo valves and proportional valves allow precise force and position control at high bandwidth, which is why hydraulics has powered flight simulators, heavy manipulators and famous dynamic humanoids.

Hydraulics has serious drawbacks for most robots. The system needs a pump, reservoir, filters, accumulators, coolers and hoses, which make it heavy and complex overall, even if the actuator is small. Throttling flow through valves wastes energy as heat, so overall efficiency is often well under 50 %. Leaks are messy and an environmental hazard, high-pressure fluid jets can inject oil through skin, a serious medical emergency, and the noise is considerable. For these reasons, even the best-known hydraulic humanoid research programme moved to electric actuation in 2024, while hydraulics remains dominant in construction, mining, forestry and agricultural machines, increasingly with electrohydraulic and autonomous control.

Pneumatic actuators

Pneumatic actuators use compressed air, typically at around 0.6 MPa (6 bar, about 90 psi) from a factory supply. The same equations apply, but the low pressure means much larger cylinders for the same force, and the compressibility of air makes pneumatic actuators springy and hard to position precisely between their end stops. That is not always a disadvantage. Most pneumatic actuators in robotics are used bang-bang, driving fully between two positions, which is perfect for grippers, clamps and part ejectors: they are cheap, fast, light, tolerant of stalling and overload, and safe in wet or explosive environments. Vacuum generators driven by compressed air power the suction cups that handle most boxes in logistics.

Compressed air is also an expensive form of energy: the compressor's electrical input is many times the useful mechanical work delivered at the cylinder, and leaks in factory air systems commonly waste a substantial share of the compressed air produced. Many manufacturers now offer electric grippers as drop-in replacements for pneumatic ones for this reason, and because electric grippers can control force and position.

Soft actuators

Soft robotics builds actuators from compliant materials such as silicone rubber, fabric and elastomers. The most common is the pneumatic network actuator: a silicone finger with a series of internal chambers on one side and a stiffer, inextensible layer on the other. Inflating the chambers makes them expand, and because one side cannot stretch, the finger bends. Fibre-reinforced versions twist or extend instead. Other soft actuators use vacuum to collapse a structure, granular jamming to switch between soft and stiff, shape-memory alloys that contract when heated, electroactive polymers or hydraulically amplified electrostatic actuators.

Soft grippers conform to irregular shapes without precise sensing or planning, and they are inherently gentle, which is why they are used to pick fruit, bakery products and fish. Their limitations are low force, slow response, difficulty modelling and controlling their shape, and limited durability. They complement rather than replace rigid actuators.

Series elastic actuators

A series elastic actuator (SEA) deliberately places a spring of known stiffness k between a conventional geared motor and the load. Measuring the spring's deflection \( \Delta x \), with encoders on both sides as in the dual-encoder joint of lesson 2, gives the force directly through Hooke's law.

The spring turns a hard-to-control force problem into an easy-to-measure position problem, and it brings further benefits. It filters shock loads, protecting the gearbox when a leg lands or an arm bumps into something. It lowers the actuator's output impedance, so the joint feels soft to a person and absorbs collisions. And it can store and return energy, as tendons do in running animals. The price is reduced force-control bandwidth: the softer the spring, the better the force resolution and shock tolerance, but the slower the actuator can change force, because the motor must wind the spring up through a larger angle. Designers choose stiffness to balance these effects.

Variable stiffness actuators go a step further, using a second motor to adjust the effective spring stiffness so that the joint can be stiff for precise positioning and soft for safe interaction or explosive motion.

Quasi-direct-drive actuators

Another route to force-sensitive, robust actuators is to remove most of the gearbox. A quasi-direct-drive (QDD) actuator pairs a large-diameter, high-torque brushless motor with a low-ratio transmission, typically 6:1 to 10:1 in a single planetary stage. With so little gearing, reflected inertia and friction are small (lesson 9), so the joint is highly backdrivable and transparent: the motor current is a good estimate of joint torque, and external forces can be felt without a separate torque sensor. The actuator absorbs landing impacts without damage and can be controlled at high bandwidth. This design, popularized by the MIT Cheetah robots, now appears in most commercial quadrupeds and many humanoids.

The trade-off is mass and heat. Without high gear reduction, the motor itself must produce a large torque, which demands a large diameter and high current, so QDD actuators are heavier for a given continuous torque and need good thermal design. A direct-drive actuator, with no gearbox at all, is the limit case, used in some high-performance motion stages and robot base joints where precision and smoothness matter more than mass.

Choosing an actuator

Actuator selection starts from the task: the forces and speeds required, how precisely they must be controlled, whether the robot must interact safely with people or the environment, the power source available and the environment itself. Electric motors with gearboxes suit most precise, repeatable tasks. Hydraulics suits very high forces on machines that already carry a hydraulic system. Pneumatics suits cheap, fast, binary motions where factory air exists. Soft actuators suit delicate, variable objects. Series elastic and quasi-direct-drive designs suit robots that must feel, absorb and control forces. The capstone in lesson 12 works through the sizing calculations for one such choice.