UTILITY OF HIGH PRESSURE OIL WITH

DIGITAL PROPORTIONAL VALVES IN SERVO

APPLICATIONS

for

Naval Surface Warfare Center

Carderock Division

9500 MacArthur Blvd.

West Bethesda, MD 20817-5700

Richard J. Norcross

Intelligent Systems Division

National Institute of Standards and

Technology

Gaithersburg, MD 20899-8230

NISTIR 6896

2002
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

ii

Disclaimer

No approval or endorsement of any commercial product by the National

Institute of Standards and Technology is intended or implied. Certain

commercial equipment, instruments, or materials are identified in this report to

facilitate understanding. Such identification does not imply a recommendation or

endorsement by the National Institute of Standards and Technology, nor does it

imply that the materials or equipment identified are necessarily the best available

for the purpose.

Copyright

This publication was prepared by United States Government employees as part

of their official duties and is, therefore, a work of the U.S. Government and not

subject to copyright.

Acknowledgement

This report is partial fulfillment of sub-contract AM 02-9802001 with AmDyne

Corporation of Millersville, Maryland.
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

iii

Abstract

The Carderock Division of the Naval Surface Warfare Center is developing the

Automated Paint Application, Containment, and Treatment System (APACTS).

APACTS will apply anti-corrosive and anti-fouling paints to ship hulls in an

environmentally sound manner. To provide accurate motion over very large

surfaces, the APACTS motion system employs a self-propelled base carrying a

long reach macro-manipulator, which in turn carries a quick response

micromanipulator to maneuver the paint nozzle and containment device. The

macro-manipulator is a commercial shooting boom aerial work platform (AWP).

The AWP uses digital proportional hydraulic valves to control the oil flow to the

actuators. The APACTS control system coordinates the flow to cause the macromanipulator to move the micromanipulator along the ship’s contours. The

APACTS developer desires to operate the AWP’s hydraulic system at higher

then normal pressures. The added pressure generates the speeds and

accelerations required by APACTS micro-manipulator to maintain nozzle

trajectory relative to the surface. The experiments reported here indicate the

servo performance of the standard digital proportional valves is retained with

the increased oil feed pressures.
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

iv

Table of Contents

1 Summary..............................................................................................5

2 Introduction .........................................................................................6

3 Methods, Assumptions, and Procedure...........................................7

4 Results and Discussions......................................................................9

5 Conclusions........................................................................................10

6 Recommendations ............................................................................10

7 References ..........................................................................................10

List of Figures

Figure 1. APACTS System Concept .......................................................5

Figure 2 Lift Actuator Response to Oil Feed Pressures.......................9
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

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1 Summary

The Carderock Division of the Naval Surface Warfare Center is developing the

Automated Paint Application, Containment, and Treatment System (APACTS) to

apply anti-corrosive and anti-fouling paints onto Navy ship hulls in an

environmentally sound manner. APACTS (Figure 1) employs three motion

components: (1) a self-propelled base, (2) a long reach macro-manipulator, and

(3) a quick response micromanipulator to move the paint nozzle and

containment device along the hull surface. The combined motion trajectories of

the macro- and micromanipulators produce the paint application trajectory.

Feedback from acoustic sensors mounted on the containment device modify the

trajectories in real-time to keep the nozzle along an appropriate trajectory

relative to the surface.

Figure 1. APACTS System Concept

Micro-Manipulator

Shroud

Nozzle

Treatment

System

Macro-Manipulator

Ship’s Surface
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

6

The controller follows the trajectory by making small changes to the positions of

the AWP’s actuators. A servo controller compares the current actuator position

with the desired position and adjusts a signal to the actuator’s valve. The valve

directs oil from a high pressure feed line, to the actuator, and back to a low

pressure tank line. The actuator motion is a function of the oil flow.

Previous APACTS research developed servo controlled actuators for the macromanipulator. That system used the standard hydraulic system provided by the

manufacturer. Those hydraulics generated 12 MPa oil feed pressure. The system

produced stable motion at low velocities and accelerations.

The APACTS micro-manipulator will use higher feed pressures (> 17 MPa) to

generate the higher speeds and accelerations required to maintain the nozzle

trajectory. While the standard components are rated above 17 MPa the effects of

the increased feed pressure on the macro-manipulator’s performance are

unknown. In all situations, only a portion of the valve input range produces

controllable motions. The remainder causes either no motion or the maximum

velocity. The increased feed pressure could cause the effective portion of the

valve input to become too small for meaningful control.

These experiments compare open loop actuator velocities with feed pressures.

The results indicate that the actuator’s maximum velocity is reduced when the

feed pressure to a digital proportional valve is increased to 17 MPa. However,

the ratio between the effective input range and the velocity range remains

similar. Therefore the aerial work platform’s standard digital proportional valves

should produce adequate servo control when operated with feed pressures

around 17 MPa.

2 Introduction

To guard against the harshness of the sea, ships are covered with anti-corrosive

and anti-fouling paints that must be periodically replaced to maintain their

effectiveness. During replacement, hazardous airborne particles (HAPs) are

inadvertently discharged into the environment diminishing the air quality and

the surrounding harbor waters. The Naval Sea Systems Command, Naval

Surface Warfare Center, Carderock Division (NAVSEA NSWCCD),

Environmental Quality Department conducts research and development leading

to fleet implementation of pollution-control materials, processes, and equipment

that enable Navy ships to be environmentally responsible. NSWCCD is

responsible for providing the Navy with the technical expertise to solve existing

and emerging waste management problems. Pursuant to that responsibility,

NSWCCD is developing the Automated Paint Application, Containment, and

Treatment System (APACTS) to significantly reduce HAP discharge from the

painting operation [1].

The primary components of APACTS are the delivery, containment, treatment,

and manipulation systems. The delivery system consists of a paint mixer,
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

7

strainer, sprayer, nozzle, and associated equipment. The containment system

surrounds the paint sprayer and includes a capture shroud, recovery vacuum,

hoses, and controls. The treatment system includes waste transport, waste

isolation, filter elements, and their support equipment. The manipulation system

consists of those devices that move the sprayer and containment shroud. The

components complement each other to produce an effective, economic, and

environmentally sound system.

The Intelligent Systems Division of the National Institute of Standards and

Technology (NIST-ISD) supports APACTS development through the

investigation of new and existing technologies to carry, maneuver, and

manipulate the APACTS sprayer and containment system. Because a single

manipulator would be unable to achieve all of the Navy’s performance

requirements at an acceptable cost, APACTS uses a series of three manipulators

to position the system about the dry dock, to reach along the hull, and to

maintain proper standoff and motion. After the mobile base positions APACTS

in or around the drydock, a long reach but slow response macro-manipulator

carries a high accuracy, fast response micro-manipulator to simultaneously

provide sufficient reach and accuracy.

Commercial AWP manufacturers use digital proportional valves to provide low

cost, variable speed control to their machines. The valves have significantly

lower dynamic response than traditional hydraulic servo valves. In addition the

valves display a significant deadband around zero velocity. The valve system

includes a response feedback signal to the hydraulic pump that adds hysteresis.

In general, the valves are inadequate for servo control.

However, the valves support servo control when operated under certain

conditions. An extra feedback line generates constant feed pressure of 12 MPa

(1750 lb/in2

) and removes most of the valve system hysteresis. The valve’s

dynamic response is sufficient when trajectories are limited to slowly changing,

non-zero velocities. Implementation with these strategies and limitations

produces satisfactory servo control of a commercial AWP’s actuators and

coordinated motion.

The APACTS developers intend to use the AWP’s hydraulic pump to also power

the micromanipulator. The micromanipulator requires a much higher feed

pressure to generate that manipulator’s desired dynamic response. The

developers fear that the higher pressure will eliminate the valve’s ability to

generate unique actuator velocities. This paper reports on experiments to assess

the AWP valve’s ability to perform with higher feed pressures.

3 Methods, Assumptions, and Procedure

The performance test consists of a series of open loop velocity comparisons. The

open loop velocity is the actuator’s steady state speed for a constant input signal.

For linear servo control, the actuator’s speed must be proportional to the input
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

8

signal. Furthermore, the servo system requires a range of possible outputs. The

servo controller cannot generate smooth motion if the slope between the input

signal and output speed is too steep.

 The AWP’s valves use a 30.77 Hz Pulse Width Modulated signal to command oil

flow. The controller divides each 32.5 ms pulse into 1040 equal segments of 31.25

μs. During a pulse, the valve is “on” for the first set of segments and “off” for the

remainder. The valve’s spool has 2.7 MPa (400 psi) oil reservoirs at both ends.

When the signal is on, the oil drains from one end of the spool. The higher

pressure of the opposing reservoir drives the spool off center. When the signal is

off, the reservoir pressure returns and the spool centers with the aid of springs.

The longer the “on” segment, the further the spool moves off of the closed

position. The ratio between the number of on segments to the number of

segments per pulse is the Duty Cycle. With sufficient Duty Cycle, oil flows

through the valve to the actuator causing motion.

The actuator must move in proportion to the Duty Cycle. In order to have servo

control, the output must be proportional to the input. If an increased input

results in an increased output sometimes and a decreased output other times, a

linear servo controller cannot control the position of the actuator.

In these experiments, the controller issues an open-loop command and monitors

the actuator position. After several seconds, the experimenter commands the

actuator to stop. During the movement, the controller records the actuator

position at regular intervals. At the end of the run, the controller performs a

position versus time linear reduction on the later 90 % of values. The slope of this

reduction is the steady state velocity for the specific Duty Cycle.

The experimenter collects steady state velocities throughout the operating range

of the valve. The valve’s closed center prevents actuator movement for much of

the command input. The experiment compares the inputs for which there is

motion.

The Snorkel hydraulic system uses a variable displacement pump. A feedback

sense line and a pressure regulator control the pump’s output pressure. In

normal operation, the sense line connects the actuators with the control plate of

the hydraulic pump and the pump maintains a feed pressure that is greater then

the pressure being used by the actuators. For servo control, a constant feed

pressure helps stabilize the valve output. Thus an extra valve (one without an

actuator) provides a phantom feedback pressure. For these experiments, the

sense line connects directly to the pump output and the pressure regulator

directly controls the system’s hydraulic pressure.

The experimenter collects sets of steady state velocities for various feed

pressures and plots command Duty Cycle versus actuator speed. If the output

results at higher pressures follow the commands similar to the results for 12 MPa

(1750 lb/in2

), then APACTS can establish servo control at the higher pressures.
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

9

4 Results and Discussions

The plot in Figure 2 shows the output response to the command Duty Cycle for

system oil pressures between 10 MPa and 17 MPa (1500 lb/in2

 and 2500 lb/in2).

The figure shows two curves for 14 MPa (2200 lb/in2

). In this region, a dramatic

variation in the response occurred. In other regions, the responses of the repeat

runs were similar and only one is shown.

Figure 2 Lift Actuator Response to Oil Feed Pressures

The standard valve on a commercial aerial work platform is not a perfectly linear

control device. When the system feed pressure increases from 10 MPa, the

actuator’s response initially becomes steeper with an accompanying increase to

the maximum speed and reduction to the command resolution. Around 14 MPa,

the system’s response becomes unpredictable. Above 15 MPa, the system again

displays proportional response, although with a smaller maximum velocity.

LIFT Actuator Response

-75

-50

-25

0

2 5

5 0

7 5

-100% -75% -50% -25% 0% 25% 50% 75% 100%

PWM Duty Cycle

17 MPa

15 MPa

14 MPa

14 MPa

12 MPa

10 MPa
Utility of High Pressure oil with Digital Proportional Valves in Servo Applications

10

5 Conclusions

A comercial aerial work platform has hydraulic components designed for 20 MPa

oil pressure. However the manufacturer restricts the pump output for normal

operations. The APACTS designers intend to increase the pump output to

provide sufficient pressure for the micro-manipulator to achieve required

dynamic response. The increased pressure’s effect on the performance of the

macro-manipulator is the subject of this investigation.

The standard valves provided with the commercial aerial work platform will

perform adequately when the feed pressure is maintained above 15 MPa (2300

lb/in2

). The slope of the output versus input is similar at the higher pressures as

with the currently utilized operating pressure. However, the maximum actuator

velocity will be less then with the current coordinated control system. This

reduction will still allow the APACTS to function in the environment in which it is

intended.

The researchers suppose the speed limitation at higher feed pressures is an effect

of the constant spool drive pressure. The valves maintain 2.7 MPa pilot oil to

move the spools. The higher oil pressure in the valve’s pressure cavity may

provide increased resistance via the spool seals. The anomaly observed at 14

MPa would be the point when this factor first takes effect.

6 Recommendations

The experimenters did not investigate the anomaly observed at 14 MPa nor the

maximum speed reduction. The speed reduction will not be a factor in the

APACTS application. The anomaly effect was highly repeatable but safely

outside the range of interest. The experimenters feel further investigation and

explanation is not warranted at this time but cautions APACTS developers of the

anomaly’s presence. As a precaution, the control system should execute an

orderly shutdown if the feed pressure falls.

7 References

[1] Carderock Division, Naval Surface Warfare Center, SOL N00167-97-SS-R1,

“Mechanical Ship Hull Paint Application System For Use in Drydock”, Commerce

Business Daily, April 10, 1997.

[2] R. Norcross, "One Degree Micro-Macro-manipulator Integration Test", NIST Technical

Report 6562, Gaithersburg, MD 2000.