08-08-2014, 11:03 AM
Multi-Actuator Switch-Mode Hydraulic System
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Abstract
Current hydraulic systems involving multiple actuators and a single hydraulic power
supply generally have poor efficiency. Using throttling valves to control multiple actuators
requires meeting the highest pressure requirement and the total flow of all of actuators. When
there is a large difference in the pressure requirement of the actuators, fluid throttling results in
significant energy losses. The purpose of this project is to implement switch-mode control in a
multi-actuator circuit and demonstrate the improvement in efficiency over a traditional hydraulic
system with throttling valve control. To accomplish this task a hydraulic crane arm powered by
two actuators was designed and constructed. One actuator provides a pivoting motion utilizing
low pressure and high flow, while the other provides a lifting motion utilizing high pressure and
low flow. Using a simple feedback control loop, the crane arm lifts, rotates, and lower a
load. After designing and testing the hydraulic crane, the team concluded that high-speed switch
technology with multiple actuators is feasible. This new technology, once implemented on a
larger scale in realistic applications, will reduce losses in hydraulic systems that depend on
multiple actuators to function.
Introduction
Throttling Systems
The concept of fluid power has many applications ranging from high precision robotics to
critical systems in the vehicles that many rely on every day. The basic purpose of a fluid power
circuit is to convert mechanical energy into fluid energy. Once this conversion is made, the
system must deliver the power to where it is needed in the system and convert the fluid energy
back into mechanical energy. In many cases, it is possible to achieve the desired end result
without any conversion to fluid energy. When the energy needs to be delivered to a location far
away from the energy source, however, a complex system of gears, chains, belts, and linkages
would be necessary. This often comes at the expense of the cost, simplicity, and reliability of the
system. A typical fluid power circuit is comprised of a hydraulic pump, to convert mechanical
energy to fluid energy, a series of hoses to carry the fluid to its required location, and an actuator
to convert the fluid energy back into mechanical energy. The remainder of the system is
comprised of simple and generally inexpensive control components, such as valves and
accumulators. The chief advantage of fluid power is its ability to deliver substantial amounts of
energy to remote locations without the need for cumbersome mechanisms.
This ability to deliver energy to a remote location comes at the price of efficiency. In
Fluid Power Circuits and Controls, Cundiff (1) lays out a simple example of a hydraulic circuit
used to lift a mass with a linear actuator. In his analysis, Cundiff accounts for pressure drops in
hydraulic lines and across valves in the system. He demonstrates that these losses require the
pump to provide a 23% higher pressure than it would in a system without losses. Compared with
a simple single reduction gear box which operates at 98-99% efficiency, a hydraulic system
performing the same task has a relatively poor efficiency. Furthermore, typical pumps convert
between mechanical and fluid energy at roughly 85% efficiency. Because any hydraulic circuit
will need to do this conversion twice, once at the energy source and once at the application, this
results in an efficiency of only 72%. When these conversion losses are combined with the losses
due to pressure drops within the system, it becomes clear that fluid power circuits provide poor
efficiencies compared to their mechanical system counter parts.
Switch-Mode Control
A new technology called switch-mode control provides a more energy efficient
alternative to throttling valve control. This method of control uses a high speed valve to rapidly
switch an actuator on and off. The valve is turned on and off at a constant frequency. Its duty
cycle, defined as the ratio of on time to the switching period, is varied according to the pressure
requirements of the actuator to achieve the desired result. Higher pressure requirements are
satisfied by higher duty cycles. When this basic technology is applied to multi-actuator circuits,
the energy savings become substantial.
Consider again the example of a crane lifting and rotating a load. When this circuit is
controlled by high speed on/off valves, the pump still needs to provide pressures equal to or
slightly exceeding the maximum pressure required by the lifting actuator. The flow requirements
of the pump are far lower than in the throttling valve controlled alternative. To provide the high
pressure for lifting, the duty cycle of the lifting valve is set to close to 100%, providing nearly
100% of the pressure output of the pump, with nearly 100% of the flow coming from the pump.
For the rotation, however, a much lower pressure is needed. If the pressure requirement
of this actuator is 20% of the pump pressure, the duty cycle of the valve for this branch of the
circuit would be set to 20%. With flow coming from the pump for 20% of the cycle and from
the tank for the remaining 80%, the addition of a second actuator in the circuit only increases the
flow requirement of the pump a small amount compared to a circuit controlled by throttling. In
the conventional throttling configuration, the entire flow requirement for each actuator must be
provided by the pump continuously. Figure 3 below shows a schematic of a typical switch-mode
controlled hydraulic circuit and demonstrates the reduced pump requirement using pulse-width
modulated signal control.
Background Research
In order to implement switch-mode control in a multi-actuator circuit, a mechanism was
needed for the hydraulic circuit to control. Two potential applications were considered at the
outset of the project. The first was a test stand that would simulate the power train of a hydraulic
hybrid vehicle. This would incorporate hydraulic motors to drive multiple wheels, as in a
vehicle powered by hydraulics. Different pressure and flow requirements would simulate high
speed and low torque, or low speed and high torque situations.
The other option was to design a crane arm. This application was a much simpler
mechanism with the two degrees of freedom required to demonstrate the switch-mode control.
Due to the fact that the purpose and focus of the project was to implement switch-mode control
in a multi-actuator circuit and demonstrate the improvement in efficiency, the simple crane arm
application was the most logical choice.
Mechanical Crane Arm Design
As an application for a multi-actuator switch-mode controlled hydraulic circuit, a small
scale crane arm mechanism was designed to lift a 50 lb. dumbbell. The mechanism has two
degrees of freedom which simulate a crane lifting and rotating a load. This application fits well
with the switch-mode hydraulic circuit as the cylinder that lifts the mechanism requires high
pressure and low flow, while the cylinder that rotates the mechanism requires low pressure and
high flow. The group iterated through several preliminary designs and performed an analysis of
the stresses, deflections, and natural frequencies on the final design to ensure its ability to
withstand the forces in the system.
The preliminary mechanism design incorporates a central housing containing ball
bearings which allow the shaft to rotate about a fixed axis. The arm and hydraulic cylinders are
then connected to the housing with a set of brackets. This configuration, shown in Figure 4
below, allows the mechanism to lift and rotate independently
Dynamic Force Analysis
After settling on a practical design concept for the arm and housing assemblies, detailed
design was completed for each component of the mechanism. In order to properly size each
component, a kinematic and dynamic analysis of the motions of the mechanism was performed.
This analysis provided information about the loads on each component, which was then used to
determine stresses, deflections, and natural frequencies as appropriate.
The first step in the kinematic analysis of the mechanism was specifying the desired
motion for the lift and rotation of the mechanism. To ensure a smooth trajectory with zero
velocity and acceleration at the beginning and end of each motion 3-4-5 polynomial functions,
most widely used for cam design applications, were used to specify the position of each
hydraulic actuator at a given point in time. The general form of the 3-4-5 polynomial
displacement function, equation 8.24 in Norton’s Design of Machinery (5) is as follows:
Additional Component Detail Design
In addition to the analysis performed on the arm, the shaft which supports the housing
was also examined. This shaft was treated as a cantilever beam supporting a moment generated
by the 50 lb. load and dynamic forces calculated above. The load is supported at a distance of 16
inches from the shaft, which therefore generates a moment at the root of the shaft of 800 in-lbs.
A 20 mm shaft diameter was initially selected for its reasonable overall fit in the assembly. The
maximum stress due to the bending moment was then calculated using the flexure formula. For
a 20 mm shaft, the 800 in-lb. moment generates a maximum tensile stress of 16,691 psi. The
most readily available steel alloy, 12L14, has tensile yield strength of 60,000 psi, giving a static
safety factor for the shaft of 3.6. This confirms that the selected material and diameter is
appropriate for the main shaft.
Design of Hydraulic Circuit
In the multi-actuator switch mode hydraulic system used to drive the crane arm, a Festo
fixed displacement pump is used to charge a one liter bladder style Festo hydraulic accumulator.
Fluid is discharged from the accumulator and continues to the custom manifold which houses the
high speed valve, DCV, and check valves. Fluid first flows to the Hydraforce 3-way solenoid
high speed valve which switches at a constant frequency, sending pressure pulses through the
rest of the system. Inserta check valves were installed within the manifold to control the
pressure spikes caused by the pulses. The upper check valve allows flow from the outlet of the
high speed valve to the accumulator, and blocks flow in the opposite direction. The lower check
valve allows the actuator to draw flow from tank to maintain a steady pressure when the loaded
arm is moving and the high speed valve is closed.
When the high speed valve is energized, it allows flow from the pump and the
accumulator to the DCV. When the valve is de-energized, in the off state, flow is blocked by the
valve. This allows time for the pump to charge the accumulator if needed. Additionally, inertia
from the moving mass creates suction within the manifold channels. This suction force pulls
fluid directly from tank and through the lower check valve to the DCV. This keeps the pressure
in the actuator stable and allows the mass to continue its motion even when not getting flow from
the accumulator.
From the high speed valve, fluid enters the DCV and is sent to either extend or retract the
actuator depending on the DCV’s state. The DCV has three states: open, closed, or crossed.
When the DCV is open, fluid flows back into the manifold and out a side port to the cap end of
the actuator, causing it to extend. When the DCV is in a closed state, the valve is blocked and no
Design of the Control System
With both the mechanical crane arm and hydraulic circuit designed and in the process of
being constructed, the team’s focus shifted to the control system that would control the hydraulic
circuit. The control system is comprised of two major components: a software component,
LabVIEW, and a hardware component, the electrical circuit. These components were designed
in unison with changes in one affecting the other. Prior to building an electric circuit the
software component of the control system, LabVIEW, was addressed to define what parts would
be required in the electrical circuit.
Electrical Circuit
An electrical circuit was designed around the control program written in LabVIEW. The
directional control valves and the high speed valves in the system require 24 volts to operate.
The DAQ board is only capable of outputting five volts, so an additional power source was
required. The electrical circuit was constructed to provide this additional power to the required
parts, to use transistors to connect and disconnect the individual components of the circuit, and to
operate the high speed valve with a specified duty cycle. The circuit consists of a power supply,
breadboard, rotary and linear potentiometers, transistors, and a USB 6009 DAQ board.
An Extech DC regulated power supply was selected for the circuit. The +24 volt variable
output on the power supply was hard wired to one rail on the breadboard to supply the required
24 volts for the high speed valves and directional control valves, while the -24 volt variable
output was wired to the common ground on the breadboard. Transistors were used as switches to
activate each of these switches as designated by the LabVIEW code. The power supply also
provided the five volts called for by the potentiometers by means of a hardwired five volt
connection.
Conclusions
The purpose of this project was to implement switch-mode control in a multi-actuator
circuit and demonstrate the improvement in this system’s efficiency over a traditional hydraulic
system. With the designed system limited in its ability to demonstrate the efficiency of the
switch-mode technology, verifying a smooth output motion through the lift, swing and lower
demonstrates the potential capability of this emerging technology. Testing on the crane arm was
performed on the order of 200 psi with valves and hoses rated for up to 3,000 psi. This means
that the design detailed in this report could be scaled up significantly to 2,000 to 3,000 psi before
requiring the significant redesign of many of the components in the system.
The testing platform developed to experimentally test this theoretical technology
successfully demonstrated the function of switch-mode technology by performing the lift,
rotation, and lower applications with a smooth motion. The motion demonstrated by this project
will improve significantly once the 100 Hz high speed valve, currently being developed by
Professor Van de Ven at Worcester Polytechnic Institute, is installed.
While the testing platform does demonstrate the ability of switch-mode control to
function, the inability of the system to lift a 50 lb. weight as designed was disappointing. With
cavitation occurring within the manifold due to the setscrew blocking the path for fluid to flow
from tank, lifting a load cannot be accomplished until the manifold is redesigned with the bottom
check valve installed so that fluid can flow from tank.
In the future, switch-mode control could be implemented in almost all multiple actuator
hydraulic applications to improve efficiency. Current hydraulic systems, like the backhoe
example in the introduction, could implement this technology to reduce the total required flow
that must be generated by the pump, and therefore greatly increase the total efficiency of the
entire system. Even more exciting, is the possibility of implementing switch-mode control in
hybrid vehicles where each tire is controlled by a hydraulic motor. This would not only be
advantageous for technologies like traction control, or ABS, but also for regenerative braking
where energy typically lost as heat could be recaptured through the re-pressurization of
accumulators in the hydraulic system.