Unit 9: Study of Actuators

I. Orientation: Actuators and Fluid-Power Principles

An actuator is a device that converts an input form of energy into controlled mechanical motion. In fluid-power systems, compressed air operates pneumatic actuators, while pressurized liquid operates hydraulic actuators. A cylinder produces mainly linear motion: its piston travels inside a barrel and transfers force through a piston rod. The governing relation for both systems is pressure acting over piston area.

• Actuator: A mechanism that creates motion or force from energy supplied by air, liquid, electricity, or another source.
• Pressure: Force distributed over an area, expressed as (p = F/A), where (p) is pressure in pascals, (F) is force in newtons, and (A) is area in square metres.
• Cylinder force: For an ideal cylinder, (F = pA). Actual force is lower because of seal friction, leakage, pressure losses, and mechanical resistance.
• Piston area: The cap-end area is (A_c = \pi D^2/4), where (D) is bore diameter. On the rod side, effective area is (A_r = \pi(D^2-d^2)/4), where (d) is rod diameter.
• Flow and speed: Piston speed is approximately (v = Q/A), where (v) is velocity in metres per second and (Q) is volumetric flow rate in cubic metres per second.
• Single-acting convention: Fluid pressure produces motion in one direction; a spring or external load returns the piston.
• Double-acting convention: Fluid pressure is alternately applied to both sides of the piston, providing powered extension and retraction.
• Control principle: Directional control valves determine which cylinder chamber receives pressure and which chamber is exhausted or returned to the reservoir.
• Safety condition: Motion must be tested at low pressure or low speed first, with the rod path clear and emergency stopping available.

II. Pneumatic Cylinders — Fast, Clean Linear Actuation

A pneumatic cylinder uses compressed air to move a piston and rod. It is suitable where moderate force, high speed, simple installation, and clean operation are more important than precise high-load positioning.

A. Pneumatic single-acting cylinder

A pneumatic single-acting cylinder uses compressed air for one powered stroke and a spring or external force for the return stroke.

• Construction: The main parts are the cylinder barrel, piston, piston rod, seals, end caps, air port, and return spring. The spring is commonly located on the rod side or cap side according to the required normal position.
• Extension action: When compressed air enters the powered chamber, pressure acts on the piston face and moves the rod outward. The opposite chamber vents through the directional valve.
• Return action: When the valve removes supply pressure, the spring pushes the piston back. Return force is approximately (F_s), reduced by friction and any external load.
• Force relationship: During extension, useful force can be estimated by
  
  TEXT

    F_out = pA - F_s - F_f

  
  Here (p) is gauge pressure, (A) is piston area, (F_s) is spring force, and (F_f) represents friction and other losses.
• Operating characteristic: The spring occupies space and limits available stroke. The cylinder also consumes air mainly during the powered stroke, which can reduce energy use compared with a double-acting arrangement.
• Typical application: A clamp, ejector, or gripper that should move to a safe default position when air is removed.

B. Pneumatic double-acting cylinder

A pneumatic double-acting cylinder receives compressed air alternately on the cap and rod sides, so both directions are powered.

• Extension action: Air enters the cap-end chamber while the rod-side chamber exhausts. The full piston area produces extension force.
• Retraction action: Air enters the rod-side chamber while the cap-end chamber exhausts. Because the rod occupies part of the area, retraction force is lower:
  
  TEXT

    F_extend ≈ pA_c
    F_retract ≈ pA_r

  
  (A_c) is cap-end area and (A_r) is the smaller annular rod-side area.
• Directional valve: A 4/2 or 5/2 valve is commonly used. The first number indicates ports and the second indicates valve positions; a 5/2 valve has five ports and two switching positions.
• Speed control: Meter-out flow control is often preferred because exhausting air through an adjustable valve helps prevent erratic movement. Cushioning near the end caps reduces impact.
• Position control: Limit switches, reed switches, or position sensors can indicate fully extended and fully retracted states. Intermediate positioning requires suitable valves and feedback.
• Worked example: With (p=0.6) MPa and bore diameter (D=20) mm, (A_c \approx 314) mm², so ideal extension force is (F \approx 0.6 \times 10^6 \times 314 \times 10^{-6} \approx 188) N before losses.

C. Study the action of pneumatic and hydraulic single- and double-acting cylinders using Sierena's humanoid robot NINO V2

The pneumatic cylinder study identifies how air pressure, valve state, and load affect the direction, speed, and force of robot motion.

• Supply path: Compressed air travels from the source through a regulator, pressure-control elements, tubing, and a directional valve before reaching the cylinder.
• Single-acting observation: Energizing the valve causes one powered movement; de-energizing it allows the spring to restore the rod. The return direction therefore depends on spring orientation.
• Double-acting observation: Two valve states independently command extension and retraction. The rod should stop or reverse only after the control signal and pressure path have changed.
• NINO V2 linkage: On the humanoid platform, the cylinder output may move a joint linkage rather than a joint directly. Observe the rod stroke, linkage geometry, joint angle, and final body posture together.
• Experimental sequence: Check connections, set regulated pressure, select the cylinder mode, command extension, record travel and response time, command retraction, and compare the two strokes.
• Concrete measurements: Record pressure in bar or MPa, stroke in millimetres, travel time in seconds, and joint displacement in degrees. Do not compare speed without recording the supplied pressure and valve-flow setting.
• Interpretation: A slower stroke can result from restricted flow; a weak stroke can result from low pressure, leakage, excessive friction, or an unfavorable linkage angle.
• Observation record: A useful table contains command, valve state, chamber supplied, rod direction, joint movement, end position, time, and abnormal sound or vibration.

III. Hydraulic Cylinders — High-Force Controlled Actuation

A hydraulic cylinder uses pressurized oil or another hydraulic fluid to generate large linear forces. Compared with pneumatics, hydraulic systems are generally stiffer and better suited to heavy loads, but they require fluid containment, filtration, and careful pressure control.

A. Hydraulic single-acting cylinder

A hydraulic single-acting cylinder is powered by fluid pressure in one direction and returns through gravity, a spring, or an external mechanical load.

• Construction: The cylinder includes a barrel, piston, rod, seals, ports, and often a wiper to prevent contamination entering along the rod.
• Extension action: Pumped fluid enters the active chamber and creates force (F = pA). The rod extends until the valve changes state, the stroke ends, or the load balances the hydraulic force.
• Return action: Fluid leaves the chamber while a spring, gravity, or external force returns the piston. The return speed depends on displaced fluid volume and the return path.
• Pressure limitation: A relief valve protects the circuit from excessive pressure. Blocking a hydraulic outlet can cause rapid pressure rise and damage if no relief path exists.
• Application: Lifting, pressing, or clamping tasks where the load itself can provide the return stroke.
• Comparison with pneumatics: Oil is much less compressible than air, so the hydraulic cylinder normally gives firmer load holding and less spring-like movement.

B. Hydraulic double-acting cylinder

A hydraulic double-acting cylinder is powered in both extension and retraction by directing fluid to either side of the piston.

• Extension force: Pump pressure acts over the full cap-end area, giving (F_e = pA_c), less seal friction and pressure losses.
• Retraction force: Pressure acts over the annular rod-side area, giving (F_r = pA_r). Since (A_r < A_c), retraction force is normally lower at equal pressure.
• Flow balance: Equal pump flow does not produce equal speeds because the two effective areas differ:
  
  TEXT

    v_e = Q/A_c
    v_r = Q/A_r

  
  (v_e) and (v_r) are extension and retraction speeds, and (Q) is hydraulic flow.
• Control valve: A 4/3 directional valve can provide extension, retraction, and a centre condition. The centre condition determines whether the actuator stops, floats, or unloads the pump.
• Stability: Hydraulic fluid supports high loads, but sudden valve changes can cause pressure surges. Flow-control valves, cushioning, and gradual command changes reduce shock.
• Leakage check: Oil around fittings, seals, or hoses indicates leakage and must be treated as a safety and maintenance fault, not merely as reduced performance.

C. Study the action of pneumatic and hydraulic single- and double-acting cylinders using Sierena's humanoid robot NINO V2

The hydraulic study compares high-force fluid actuation with pneumatic movement while relating cylinder behavior to NINO V2's humanoid joints or mechanisms.

• Hydraulic supply path: A pump sends fluid through a reservoir, filter, pressure-control valve, directional valve, and hose to the selected cylinder chamber.
• Single-acting observation: The powered stroke occurs when oil enters one chamber; the return depends on the spring or mechanical loading arrangement. The return should be monitored for smoothness and uncontrolled descent.
• Double-acting observation: Switching the directional valve sends oil to opposite chambers. Record whether extension and retraction have equal travel but different speed or force.
• Load response: Apply the intended robot load gradually. Hydraulic pressure should rise in relation to load demand, while the relief valve prevents unsafe excess pressure.
• NINO V2 linkage: Observe whether a small cylinder stroke creates a large joint angle through leverage. The same cylinder force can produce different joint torque:
  
  TEXT

    T = F r sin(theta)

  
  (T) is joint torque in N·m, (F) is cylinder force, (r) is linkage distance, and (\theta) is the included force angle.
• Practical comparison: Pneumatic actuation normally gives faster, more compliant motion; hydraulic actuation normally gives greater force and stiffness. Actual performance depends on pressure, flow, bore, rod diameter, load, and linkage.
• Condition monitoring: Check for overheating, foaming, leakage, unusual vibration, and delayed response. These signs may indicate restricted flow, trapped air, contamination, or valve problems.

IV. Sierena's Humanoid Robot NINO V2 — Integrated Laboratory Study

Sierena's humanoid robot NINO V2 provides a physical system in which actuator commands can be related to visible joint movement, mechanical transmission, and control-system response. The laboratory objective is to study action, not merely identify components.

A. Study the action of pneumatic and hydraulic single- and double-acting cylinders using Sierena's humanoid robot NINO V2

The experiment is complete when the actuator type, valve command, cylinder movement, and resulting humanoid motion are connected by measured evidence.

• Preparation: Identify the cylinder, ports, valve, regulator or pump, sensor connections, mechanical linkage, and emergency-stop control before applying power.
• Initial condition: Place NINO V2 in a stable posture with the actuator unloaded where possible. Confirm that the rod can travel through its complete stroke without striking the frame.
• Single-acting procedure: Command the powered stroke, measure extension time, remove the command, and observe spring- or load-assisted return. Record the normal resting position.
• Double-acting procedure: Command extension and retraction separately. Confirm that the correct chamber is pressurized for each direction and that both end positions are detected.
• Pneumatic versus hydraulic record: Keep pressure units consistent, such as bar or MPa. Record supply pressure, cylinder bore, rod diameter, stroke, load, travel time, and joint angle.
• Control relationship: A controller output does not itself guarantee movement. The complete chain is command signal → valve action → chamber pressure → piston force → linkage motion → joint response.
• Performance indicators: Compare force capability, response speed, smoothness, holding ability, noise, repeatability, and energy or fluid consumption.
• Safety controls: Exhaust pneumatic pressure before disconnecting tubes; isolate hydraulic pressure before loosening fittings; never place hands between moving links; and use the emergency stop for unexpected motion.
• Result interpretation: A correct conclusion states the observed motion and its cause, such as “retraction was slower because the rod-side area and flow setting differed,” rather than reporting only that the robot moved.
• Laboratory outcome: The study demonstrates how actuator selection affects a humanoid robot's force, speed, compliance, posture control, and mechanical safety.