ECE 5320 Mechatronics

Assignment #01: Literature Survey on Sensors and Actuators

Topic: Hydraulic Actuators

Prepared By:

Lee GardnerDepartment of Electrical and Computer Engineering

Utah State University

Outline

– Reference list

– To probe further

– Major applications

– Basic working principle illustrated

– A typical sample configuration in application (application notes)

– Major specifications

– Limitations

– And many more relevant issues in applications (such as, how to choose, cost information, where to buy etc.)

Reference List

• Mechatronics Handbook

• HowStuffWorks.com

• http://mechatronics.ece.usu.edu/ece5320/

• http://home.wxs.nl/~brink494/frm_e.htm

Major Applications

• Large Torque and Force• Slow Moving and Large Scale• Industrial and Large Commercial• Water Flow, Motor, Pumps etc.

– Cranes

– Heavy Machinery (Caterpiller, Back Hoe, Movers)

– Brake Systems

Basic Working Principle

• Fluid actuators, whether they are linear (cylinders) or rotary (motors) are continuous systems as they can determine the positioning of the mobile component at any point in the stroke.

Basic Working Principle Cont.

• Hydraulic systems operate according to Pascals law.

• The law of Blaise Pascal says: “The pressure, in a static hydraulic fluid in a closed system is everywhere the same”.

• Pressure = Force/Area

Working Model

If the surface area of piston “B” is 10 times the surface area of piston “A”, then the force applied to piston “A” is multiplied 10 times as the force exerted on piston “B”. PSI is equal throughout the system.

Working Model

Working Model Cont…• The hydraulic pump is

driven by an electric motor or a combustion engine.

• The hydraulic pump (2) sucks oil from the reservoir (1) and pumps the oil through the pipelines

• The hydraulic pump converts mechanical energy into hydraulic energy (pressure and flow) and the hydraulic motor converts the hydraulic energy into mechanical energy again!

1 Reservoir, 2 hydraulic pump, 3 pressure gauge, 4 hydraulic motor

Major Specifications

The components of a hydraulic actuation system are:• the pump, that is, the hydraulic power generation system;• the actuator, that is, the element which converts hydraulic

power into mechanical power;• the valve, that is, the hydraulic power regulator;• the pipes for connecting the various components of the

actuation system;• the filters, accumulators, and reservoirs;• the fluid, which transfers the power between the various

circuit elements;• the sensors and transducers;• the system display, measurement, and control devices.

Pumps

• Transforms electrical or mechanical energy into hydraulic energy.

• Constitutes fluid flow generator of the hydraulic system, as the pressure is determined by the fluid resistance.

• Common Pump Types

- Gear Pumps

- Rotary Vane Pump

- Piston Pump

Motion Actuators

• Motion actuators convert the hydraulic energy of the liquid under pressure into mechanical energy.

• Volumetric hydraulic motors and are distinguished on the basis of the type of movement generated

• Common Types:

- Rotary and Semi-Rotary Motors

- Linear Actuators

Valves

• Valves are the components in hydraulic circuits that carry out the task of regulating the hydraulic power sent to the actuator.

• Turn the oil flow on or off or to divert it according to need.

• Common Types– Directional Valves– On-Off Valves– Pressure Regulator Valves– Flow-Rate Regulator Valves

Modeling of a Hydraulic Servosystem for Position Control

Modeling Cont…

The actuator is modeled by considering the equations of flow continuity in the chambers and the dynamic equilibrium equation for the rod. The continuity equation is expressed in a general form:

where

• ΣQIN = sum of the flows in volume entering

• ΣQOUT = sum of the flows in volume leaving

• ρ = density

• V = volume

• t = time

Modeling Cont…

From the definition of the compressibility modulus of the oil β, P being the pressure in the chamber considered:

The continuity equation for chamber 1 and 2 is

Modeling Cont…

Modeling Cont…

Block diagram of the linearized model of an hydraulic servosystem with position control.

Transfer Function

Modeling Cont…• The speed gain KOLV, and therefore the open loop static

gain K0, depend to a considerable degree on the flow gain KQ and increase with increases in KQ. KQ increases as PS increases, decreases as ΔPL0 increases, and does not vary with AV0. In the hypothesis of γ below 1000 Ns/m, the effect of KPQ is modest, practically negligible.

• The force constant KOLF depends on the flow-pressure gain KPQ and increases with it. |KPQ| increases with AV0 and with ΔPL0, while it decreases as PS increases; therefore |KOLF| decreases as PS increases. Leaks lead to an increase in KOLF.

Modeling Cont…

• Static stiffness depends considerably on the pressure gain of the valve and increases with it.

• KP decreases with the leaks, which leads to a reduction in static stiffness.

• |KPQ| increases with AV0 while KQ does not vary with AV0, the pressure gain decreases with the increase in AV0.

Common Types

Hydraulic servocylinder (Hanchen).

External spur gear pump (Casappa).

Axial piston swash plate pump (Bosch Rexroth).

Limitations

• Not suitable for fast paced or high speed operations

• Usually too large and/or cumbersome for smaller precision applications.

• Precision and accuracy usually out of range for highly specialized or specific applications.

• Can only use pressurized fluids.• Durability and wear-and-tear issues result in

frequent maintenance and upgrades.

Cost

• Cost ranges widely depending on application.• Smaller applications: $100-$1000’s• Larger applications: $10,000’s and higher• Highly specialized and specific hydraulics will

cost more for power/$$$.• Generic and all-purpose hydraulics have a higher

market penetration and are generally cheaper.

Where to Buy

• Available from specialty retailers and commercial outlets.

• Industrial grade hydraulics are not available at mere retails stores and require accounts and contracts with specific vendors.

• http://www.adv-hyd.com/

• http://www.directindustry.com/