Unit 9: Study of Actuators - Subjective Questions

Actuator: An actuator is a device that converts supplied energy into controlled mechanical motion.

In fluid-power systems, actuators convert the pressure energy of a fluid into linear or rotary motion.

• Pneumatic actuators use compressed air and provide rapid, clean, and lightweight motion.
• Hydraulic actuators use pressurized liquid and produce high force with accurate load control.
• Linear cylinders can be used to demonstrate pushing, pulling, lifting, gripping, or joint-assistance actions in a humanoid robot setup.
• The actuator receives commands through control valves, while sensors may provide position or end-of-stroke feedback.

Thus, actuators form the link between the robot's control system and its physical movement.

A pneumatic single-acting cylinder contains a cylinder barrel, piston, piston rod, one air port, seals, end covers, and a return spring or external return mechanism.

Working principle:

• Compressed air enters through the single port.
• Air pressure acts on one side of the piston and produces an extension or retraction stroke.
• The theoretical force is given by , where is the air pressure and is the effective piston area.
• When the control valve connects the port to exhaust, the spring or an external load returns the piston to its initial position.
• Only one stroke is powered by compressed air; therefore, it is called single-acting.

It is suitable for simple clamping, pushing, ejecting, or return-to-safe-position operations.

A pneumatic double-acting cylinder has a barrel, piston, piston rod, seals, two end covers, and two air ports located on opposite sides of the piston.

Extension:

• Compressed air is supplied to the cap-end port.
• The rod-end port is connected to exhaust.
• Pressure acting on the full piston area moves the rod outward.

Retraction:

• Compressed air is supplied to the rod-end port.
• The cap-end port is connected to exhaust.
• Pressure acting on the annular area moves the rod inward.

A directional control valve alternates the pressure and exhaust connections. Since air powers both directions, the cylinder offers better motion control and can perform useful work during both extension and retraction.

A single-acting cylinder is preferred for simple one-directional work, while a double-acting cylinder is preferred when controlled force and motion are required in both directions.

A hydraulic single-acting cylinder consists of a cylinder barrel, piston or ram, piston rod where applicable, one hydraulic port, seals, and a return arrangement.

Operation:

• Pressurized hydraulic oil enters through the port.
• Oil pressure acts on the effective piston or ram area and generates linear motion.
• The output force is approximately before accounting for losses.
• When pressure is released and oil is allowed to return to the reservoir, the piston returns through gravity, a spring, or an external load.
• Hydraulic oil is nearly incompressible, so the cylinder can generate high force and maintain a load more effectively than a comparable pneumatic cylinder.

Typical uses include lifting, pressing, and other high-force one-directional operations.

A hydraulic double-acting cylinder has two ports, one at the cap end and one at the rod end. A directional control valve sends pressurized oil to either side of the piston.

Forward stroke:

• Oil enters the cap end.
• Oil from the rod end returns to the reservoir.
• Pressure on the full piston area extends the rod.

Return stroke:

• Oil enters the rod end.
• Oil from the cap end returns to the reservoir.
• Pressure on the annular area retracts the rod.

Flow-control valves regulate speed, and a pressure-relief valve limits unsafe pressure. Since hydraulic fluid is only slightly compressible, the motion is stiff and capable of handling large loads. Both strokes can perform useful work.

The final choice depends on required force, mass, safety, available power, motion quality, and operating environment.

Let the piston diameter be , rod diameter be , supply pressure be , and opposing back pressure be neglected.

The full piston area is

During extension, pressure acts on the full piston area. Therefore,

During retraction, the rod occupies part of the pressurized face. The effective annular area is

Hence,

Because , the theoretical retraction force is lower than the extension force at the same pressure. Actual forces are smaller due to seal friction, pressure losses, and back pressure.

Given:

Piston area:

Extension force:

Annular area:

Retraction force:

Thus, the theoretical extension and retraction forces are approximately and , respectively.

Directional control valves determine the path followed by compressed air or hydraulic oil.

• A 3/2 valve has three ports and two positions. It is commonly used with a single-acting cylinder. In one position, it connects supply to the cylinder; in the other, it blocks supply and connects the cylinder to exhaust or tank.
• A 4/2 or 5/2 valve is commonly used with a double-acting cylinder. It alternately supplies one cylinder chamber while releasing fluid from the other.
• A 4/3 valve can provide a central condition such as stopping, unloading, or holding, depending on its center configuration.
• Valves may be operated manually, mechanically, pneumatically, hydraulically, or electrically using solenoids.

In a robot laboratory setup, the controller energizes the valve solenoid to select cylinder extension, retraction, or an allowed neutral state.

The speed of a cylinder depends mainly on the volumetric flow rate supplied to or discharged from it. Ideally,

where is piston velocity, is volumetric flow rate, and is the effective piston area.

• Meter-in control restricts fluid entering the cylinder. It is useful for loads that resist motion, but it may give unstable motion with overrunning loads.
• Meter-out control restricts fluid leaving the cylinder. It creates back pressure and generally provides better control of varying or overrunning loads.
• One-way flow-control valves often combine an adjustable restriction with a check valve, restricting flow in one direction and allowing free flow in the other.

Proper adjustment prevents excessive speed, impact at the end of the stroke, and abrupt movement of the robot mechanism.

A typical pneumatic cylinder circuit includes:

• Compressor: Produces compressed air.
• Receiver: Stores air and reduces pressure fluctuations.
• Air preparation unit: A filter removes contaminants, a regulator sets operating pressure, and a lubricator may add oil mist when required.
• Pressure gauge: Displays the regulated pressure.
• Directional control valve: Selects extension or retraction.
• Flow-control valves: Adjust cylinder speed.
• Cylinder: Converts air pressure into linear motion.
• Tubing and fittings: Carry compressed air between components.
• Silencers: Reduce exhaust noise.
• Sensors and controller: Detect cylinder position and command the valve.

These elements must be connected according to the circuit diagram and checked for leakage before operation.

A basic hydraulic system contains:

• Reservoir: Stores and cools hydraulic oil while allowing contaminants and air to settle.
• Pump: Converts mechanical input into hydraulic flow.
• Prime mover: Drives the pump, usually using an electric motor.
• Pressure-relief valve: Limits maximum system pressure.
• Directional control valve: Routes oil to the required cylinder chamber.
• Flow-control valve: Regulates actuator speed.
• Filters: Remove particles from the oil.
• Cylinder: Converts hydraulic energy into linear force and motion.
• Pipes, hoses, and fittings: Carry the fluid.
• Pressure gauge and sensors: Support monitoring and control.

In a double-acting circuit, return oil from one chamber flows to the reservoir while pressurized oil enters the other chamber.

Procedure:

1. Inspect the robot test setup, cylinder, tubing, fittings, valve, pressure regulator, and electrical connections.
2. Ensure the robot mechanism is supported and the cylinder's path is free from obstructions.
3. Connect the two cylinder ports to the appropriate directional control valve ports.
4. Connect flow-control valves and position sensors according to the laboratory circuit diagram.
5. Set the regulator to minimum pressure and switch on the air supply.
6. Increase pressure gradually to the prescribed safe value while checking for leakage.
7. Command extension and observe piston direction, stroke, speed, sensor response, and load movement.
8. Command retraction and record the same parameters.
9. Adjust the flow controls incrementally and compare extension and retraction speeds.
10. Isolate and release stored pressure after completing the experiment.

The observations should relate valve state, pressure, flow adjustment, actuator direction, and robot motion.

Procedure:

1. Inspect the reservoir level, oil condition, hoses, fittings, cylinder mounting, valve, and pressure gauge.
2. Confirm that the return path to the tank is correctly connected and that the relief valve has an approved setting.
3. Keep the control valve in its neutral or return state and start the pump.
4. Bleed trapped air using the specified procedure.
5. Operate the valve so pressurized oil enters the cylinder port.
6. Observe the powered stroke and record pressure, displacement, stroke time, and load.
7. Release the valve so oil returns to the reservoir and observe the spring-, gravity-, or load-assisted return.
8. Repeat the trial for approved loads without exceeding pressure or load limits.
9. Stop the pump, isolate power, and release stored pressure before disconnecting any line.

Record: supplied pressure, stroke length, extension time, return time, load, leakage, vibration, noise, and return behavior.

Cylinder integration requires coordination among the robot controller, driver circuit, control valve, actuator, and sensors.

• The controller generates a command for extension or retraction.
• A suitable electrical interface drives the solenoid of the directional control valve.
• The valve routes compressed air or hydraulic oil to the cylinder.
• Limit switches, magnetic reed sensors, pressure sensors, or position transducers report actuator state.
• The controller uses feedback to confirm that the desired position has been reached before starting the next action.
• Software interlocks prevent conflicting valve commands and stop movement when a fault is detected.
• Mechanical linkages convert cylinder displacement into the required motion of the robot mechanism.

This arrangement supports sequencing, end-position detection, fault handling, and coordination with other NINO V2 motions.

Safety precautions include:

• Inspect hoses, fittings, seals, mountings, and guards before operation.
• Keep hands and tools away from pinch points and the cylinder's line of motion.
• Support robot links and suspended loads before maintenance.
• Begin with minimum pressure and increase it only to the approved value.
• Never exceed the rated pressure, load, or stroke limits.
• Use a pressure-relief valve in hydraulic systems and a properly adjusted regulator in pneumatic systems.
• Do not search for hydraulic pinhole leaks with bare hands because high-pressure fluid can penetrate skin.
• Isolate electrical power and fluid supply before changing connections.
• Exhaust pneumatic pressure and discharge hydraulic stored energy before servicing.
• Use an emergency stop and verify that limit sensors and interlocks operate correctly.
• Wear prescribed eye protection and follow the laboratory operating procedure.

Troubleshooting should begin only after isolating energy and making the mechanism safe.

Cushioning slows the piston near the end of its stroke so that it does not strike the end cover at full speed.

• In adjustable pneumatic or hydraulic cushioning, a cushion spear or sleeve restricts the exhaust or return flow during the final part of the stroke.
• The trapped fluid creates a controlled deceleration force.
• External shock absorbers or controller-based speed reduction may also be used.

Importance in a humanoid robot:

• Reduces impact, vibration, and noise.
• Protects cylinder seals, end covers, joints, and linkages.
• Prevents abrupt movement of robot limbs.
• Improves repeatability and mechanical life.
• Reduces the chance of load displacement or instability.

Cushioning must be adjusted carefully; excessive restriction may prevent full stroke, while insufficient restriction permits damaging impact.

For an incompressible-flow approximation, the volume displaced in time is

where the piston displacement is and the displaced volume is actually

Since volumetric flow rate is ,

Therefore,

For a stroke length , the ideal stroke time is

During extension, . During retraction, , which is smaller. At equal flow rates,

Thus, retraction is ideally faster than extension. Actual pneumatic speeds also depend strongly on air compressibility, pressure changes, exhaust restrictions, friction, and load.