analysis of hydraulics actuator speed control using ...€¦ · figure 2 proportional hydraulic...

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 7, July 2017, pp. 213–224, Article ID: IJMET_08_07_025

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=7

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

ANALYSIS OF HYDRAULICS ACTUATOR

SPEED CONTROL USING DIGITAL

HYDRAULICS

G. Kalaiarasan, Giriraj Mannayee, Boopathi M, Somanadh Mayakoti

School of Mechanical Engineering,

VIT University, Vellore, Tamil Nadu, India

K. Krishnamurthy

School of Mechanical and Building Sciences,

Kongu Engineering College, Perundurai, Tamil Nadu, India

ABSTRACT

Digital hydraulics is a forthcoming field which, in spite of the fact that being

connected in a few routes since long, brings some new thoughts, ideas and answers for

fluid power. The focal and unavoidable segments of digital hydraulic systems are on-

off valves. This paper talks about the likelihood to utilize minimal price on/off valves

rather than servo valves indeed, even in requiring servo applications. Different on/off

control procedures are first checked on and examined. Digital hydraulics makes

additionally many challenging issues for component improvement, control, modelling,

simulation and treatment of dynamical impacts. These difficulties are differentiated by

the potential points of interest of digital hydraulics which are complex: robustness,

reliability and cost savings. In any case, other genuinely digital segments, as digital

displacement pumps, digital cylinders and transformers were created to improve digital

hydraulics. The Digital hydraulic principles used here involves digital valves only. In

light of these basic parts, various ideas have been proposed. Results demonstrate that

energy efficient and speed control is conceivable with basic and minimal cost on/off

valves. In this manner, the digital hydraulics potential option for conventional servo

systems.

Key word: Digital Hydraulic, PID, Cost Function, Motion Control.

Cite this Article: G. Kalaiarasan, Giriraj Mannayee, Boopathi M, Somanadh Mayakoti

and K. Krishnamurthy, Analysis of Hydraulics Actuator Speed Control Using Digital

Hydraulics. International Journal of Mechanical Engineering and Technology, 8(7),

2017, pp. 213–224.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=7

Analysis of Hydraulics Actuator Speed Control Using Digital Hydraulics


1. INTRODUCTION

The expression “Digital fluid power” is wide and not completely characterized. “Digital fluid

power implies hydraulic and pneumatic systems having discrete valued components effectively

controlling system output”. A two-valued single component with no intelligence is not

considered as a digital system. A digital system has various discrete valued components (on or

off), a few illustrations being a microchip-transistors, a digital camera-pixels. The features of

Digital hydraulics [1] are: 1) all valves are on/off control type. 2) Performance is equivalent to

conventional proportional and servo systems, which implies on/off valves supplant servo and

proportional valves. 3) Digital flow control unit should possess separate meter-in meter-out

control which gives settled inflow/outflow proportion. 4) Flexible and programmable

controller, which uses at any rate the fundamental data of the controlled system. 5) Redundant

circuits to such an extent that the system works, perhaps with degraded performance which is

an exclusive feature of digital hydraulics.

Two basic categories of digital fluid power are systems in view of parallel connection and

systems based on switching technologies. Parallel connected systems have majority of parallel

associated parts and the output is controlled by changing the state combinations of the segments.

This system has a specific number of discrete output values. On the other hand, switching

technologies use quick and persistent switching of single or a couple of segments and the output

is balanced by e.g. the pulse-width modulation. It controls the normal flow area by high

frequency modulation and the pulse width modulation is the most widely recognized approach.

Our approach is through parallel connected systems. The figure 1 presents the parallel

associated usage of 2-way valve. The flow region of the DFCU is the total of the flow regions

of the open valves. Binary coding is the most widely recognized technique and flow proportions

are in the ratio of 1:2:4:8 and so forth. Other coding strategies incorporate Fibonacci (1:1:2:3:5

and so forth) and pulse number modulation (1:1:1 and so forth). Separately on the coding,

DFCU has 2N opening combinations, which are termed as states of DFCU. Each state has

distinctive flow area in the binary coding while at the same time changing level of redundancy

exists in the other coding strategies. Basic contrast to the switching valve is that DFCU does

not require any switching to keep up any of the opening qualities. Switching are required just

when the state changes.

Figure 1 Digital valve base DFCU

SOL10SOL09

SOL5 SOL1

SOL6SOL2

SOL7SOL3

SOL8

SOL1

SOL2

SOL3

SOL4

SOL5

SOL8

SOL6

SOL7

SOL4

G. Kalaiarasan, Giriraj Mannayee, Boopathi M, Somanadh Mayakoti and K. Krishnamurthy


The Digital Valve positioned DFCU is presented in the figure 1. Four Digital based Valves

are introduced to govern flow directions:

1. P→ A i.e. discharge from pump (P) to cylinder piston side chamber (A),

2. A→T i.e. discharge from cylinder piston side chamber (A) to Tank (T)

3. P→B i.e. discharge from pump (P) to cylinder rod side chamber (B) and

4. B→T i.e. discharge from cylinder rod side chamber (B) to Tank (T) permitting independent

meter-in meter-out flow management. The Digital based Valve is made out of 4 control

segments in parallel, each control segment incorporates one ON/OFF Valve and Orifice flow

meter.

Assumptions for our circuit

1. In our DFCU, Pressures at every point is constant.

2. Ideally, our valves are fast in operation

3. There is no leakage in the overall circuit.

4. There are no friction losses, discharge losses and pressure losses.

The Digital based Valve [2] is made out of 4 control segments in parallel, each control

segment incorporates one ON/OFF valve and one Orifice flow meter. The discharge control of

Digital based Valve is accomplished by modifying the condition of various control segments.

Theoretically there are three fundamental coding strategies for Digital based Valve: (a) Binary-

based coding which is also termed as “PCM” stands for Pulse Coding Modulation, (b) “PNM”

coding stands for Pulse Number Modulation and (c) Mixed PNM-Binary based coding. Anyway

characteristics of the mentioned coding strategies are explained earlier. Binary rooted coding

Digital based Valve is utilized in the current system since it is flexible to be acknowledged in

dealing with PCs. All the on/off valves connected in our proposed DFCU (figure-1) are assigned

to solenoid operation, which are energized by the PLC program.

For the mentioned Digital based Valve, (operating) Pressure drop of each segment can be

treated alike. In this way our binary based coding of the Digital based Valve can be

accomplished by tuning the fluid flow area of the Orifice flow meter in every segment. Like the

aggregate current in a considered electric circuit is equivalent to the summation of the current

in individual parallel connected member, the aggregate discharge in the Digital based Valve is

equivalent to the summation of the discharge in individual parallel segment. Thus the discharge

capacity of the orifice flow meter [11] in Digital based Valve can be determined by the equation

(1),

0.0851P

Q C ASG

∆= × × × (1)

Q = discharge (L/min)

C = Flow coefficient (0.64-0.76 for laminar flow)

A = Area of the Orifice (mm2)

∆� = pressure drop at orifice (KPa)

�� = specific gravity of the flowing fluid (constant)

2. SAMPLE CALCULATION FOR ORIFICE DIAMETER

For smallest valve let us say, flow rate Q= 0.4 L/min, and pressure drop across the valve, let us

say 500 KPa, then substituting the values we got the diameter of 0.5784 mm.



As we followed pulse code modulation (1:2:4:8 flow capacity), the next valve flow rate will

be 0.8 L/min, and pressure drop is constant across the all valves, then we got the diameter of

second valve as 1.1568 mm

Like this, after obtaining all the orifice diameters for four valves, feed to the circuit in our

simulation part.

2.1. Implementing Random Bit Sequence

Till now we operated our digital flow circuit unit with sequence of 4-bit combinations with the

help of PLC program. We can also provide random bit- sequence using digital electronics

principles [12]. The procedure is as follows:

• We have to select our desired random bit- sequence

• Prepare the Next state, Present state and Excitation input tables

• With the help of the KV-map, obtain the output logics and connect to Flip-Flops

• Assign the output terminals of Flip-flops to our proposed DFCU (figure 1).

2.2. Proportional valve

To control a coil on the Proportional hydraulic valve [10], with the help of supply source, the

input is attached to an amplifier system. In view that the electrical contribution from existing

sources is mainly less than the amplitude of current required to conduct the coil, because of that

input current should be amplified. This operation is fulfilled by an Amplifier based system. The

Amplifier system is placed on the desired valve, which is stated as OBE (on board electronics).

The source of the input may originate from a few devices, for example, “Potentiometer” under

the surveillance of system operator, from a program logical controller, or controlling using

joystick

The amplifier system operates the valve coil with input current. When the current passes

along the coil, an EMF is generated, making the armature of the solenoid system to set into

motion. The input from the armature operates the valve spindle considering flow path, pressure

relief, DCVs, the spool in a pressure reducing valve. The spindle is also compensated by a

spring. Subsequently, the estimated force contributed by the solenoid system is restricted due

to the drive of the attached spring.

Considering the above principles of proportional hydraulics, we can create that environment

in our software tool. Here we operated i.e. input is given from the joystick as shown in the

figure below. This circuit will act as an open-loop system since output action is not controlled

by any feedback signal. The solenoid attached proportional hydraulic valve is assigned to the

joystick motion.



Figure 2 Proportional hydraulic system

2.3. Servo valve

At the point when closed-loop hydraulic control systems initially started to appear in industry,

the applications were for the most part those in which very high accomplishment was required.

While hydraulic servo systems [9] are still intensely utilized as a part of superior efficient

applications such as the machine-tool industry, they are starting to yield wide acknowledgement

in a different type of industries. Cases are material handling, mobile machinery, plastics, steel

plants, mining, oil analysis, and automotive verification. Servo valves were advanced to

encourage the modification of fluid flow in light on the changes in the load movement.

Closed loop servo drive methodology is progressively turning into the standard in machine

automation, where the administrators are challenging high precision, rapid operation and less

complex change. There is additionally a desire that the cost of expanding the level of automation

ought to be contained within satisfactory edge.

In its elementary form a servo or a servomechanism is a control system which calculates

its own output and forces the output to rapidly and precisely follow a command signal. In this

way, the impact of irregularities in the control device itself and in the load can be limited and

additionally the impact of external noises. A servo system can be intended to control any

physical quantities such as movement, force, pressure, temperature, electrical voltage or

current. Considering the above principles of Servo hydraulics, we can create that environment

in our software tool. Here we operated i.e. input is given from the joystick as shown in the

figure below. This circuit will act as a closed-loop system since output action is controlled by

feedback signal. The solenoid attached Hydraulic servo valve is assigned to the CONTROL

block and an equation is written in the variable assignment of CONTROL variable.

0 L/min

? (Is)

? (Is)

? (Is)

JY_X



Figure 3 Hydraulic servo system

3. STEADY STATE CHARACTERISTICS

In this chapter to clarify the system behaviour, our 4 Digital based Valves on considered flow

directions are thought to be precisely the same. In this way the consistent equations of the

system (Figure. No.1) in the forward direction can be presented as equations (2), (3), (4).

�� − �� − �� = �� (2)

�� − �� − �� = −�� (3)

� = �� – �� (4)

The equations (2) and (3) represents the flow from the source to the port-A and port-B

chamber respectively of the cylinder. By using the equation (4) force can be calculated based

on our assumption we have taken F=0, where ��,�� , ��, �� are taken as flow combination

parameters of the Digital based Valve for flow directions:

1. P→ A i.e. discharge from pump (P) to cylinder piston side chamber (A),

2. A→T i.e. discharge from cylinder piston side chamber (A) to Tank (T)

3. P→B i.e. discharge from pump (P) to cylinder rod side chamber (B) and

4. B→T i.e. discharge from cylinder rod side chamber (B) to Tank (T).

The term " Q" is the discharge capacity of the least significant bit binary based Digital

Valve with unit pressure drop,

�� Stands for pump pressure,

�� Stands for cylinder piston side pressure (A),

�� Stands for cylinder rod side pressure (B),

V stands for Steady State combinations velocities of the proposed DFCU,

�� Stands for cylinder piston side area,

�� Stands for cylinder rod side area,

F stands for force.

NOTE: The flow combination parameters “ U��,U��, U��, U��” are considered to be the inputs

and V, P�, P� to be the outputs of the system. The above equations can be simplified for the

extending direction [3] as follows:

0 L/min

Control (Os)? (Is)

? (Is)

? (Is)

Control (Os)



�� = �� ! "� #/�%��!"& (5)

�� = '(��)*(�+/�,(�!'& (6)

� = -./01�% 2"� * #/�%

��!"& (7)

Where:

3 = �/�%

, 4 = -./-%5

Similarly, steady-state velocity and pressure of the cylinder can be solved in retracting

direction and the result is

�� = "�� ! "� #/�%�6�!"& (8)

�� = "&� *�6�#/�%�6�!"& (9)

� = − -/501�% 2� ! #/�%

�6�!"& (10)

3 = �/�% , 4 = -.%

-/5

4. IMPLEMENTATION OF STEADY STATE CHARACTERISTICS IN

MATLAB

This section gives us the valuable information regarding our digital hydraulic circuit. As per

the taken cylinder specifications (rod diameter-16mm, piston diameter-32mm, supply

pressure12MPa, flow capacity of smallest valve of our proposed DFCU-(1*10^-8 m3/

(sec*√�8)), we will obtain steady state velocities and pressures.

• The system (Figure.No.1) has total 15*15= 225 different U�� − U�� or U�� − U��

combinations giving non-zero velocities.

• Write a MATLAB script involving above formulae and cylinder specifications

• Create the logic such that we will obtain 15*15 velocity matrix in the workspace which

resembles the each 225 combinations for U�� − U�� orU�� − U��.

• We can plot 3-D bar graphs featuring U�� − U�� or U�� − U�� as x, y axis respectively and

velocity as the z-axis.

• From the 3-D bar graph or from the 15*15 velocity matrix in the work space we can select

U�� − U�� or U�� − U�� state combinations giving almost the same velocity.

• Then feed the corresponding bits (combinations) for both extending and retracting to our

proposed DFCU (figure-1) using programming

4.1. Observations from the velocity table of extending direction

Table 1 shows velocity table of extending direction, it can be seen that same velocity can be

achieved with some combinations. For example, velocity of 0.45 m/s for extending direction

can be achieved with five different U�� − U�� Combinations as noticed from steady state

extending velocities work space environment. Those are:



Table 1 Velocity table of extending direction

S.No. 9:; 9<=

1 14(1110) 7(0111)

2 14(1110) 8(1000)

3 14(1110) 9(1001)

4 14(1110) 10(1010)

5 14(1110) 11(1011)

3-D Bar graph for extending direction steady state velocities

Figure 4 Steady state velocities of extending direction

3-D Bar graph for extending direction corresponding steady state pressures

Figure 5 Steady state Pressure of extending direction



2-D line graph for extending direction steady-state velocities vs 225 state

combinations

Figure 6 Up – Ut Combinations of DFCU states

The same process can be followed for retraction motion (MATLAB script, velocity table,

selecting the combination for -0.45m/s)

5. CONTROL METHODS

5.1. Feed- forwarding the obtained combinations to our DFCU

All the on/off valves connected in our proposed DFCU (figure-1) are assigned to solenoid

operation, which are energized by the PLC program. There we achieved step-wise-step shaped

velocity graph. Now instead of that sequence (0000-1111), we have to assign our selected

combinations in the programming. Therefore, we will achieve constant extending and retracting

velocity of 0.45 m/s. In previous section, we performed feed forwarding technique for

controlling the velocities obtained with the help of steady state characteristics and simulation.

Here we introduced Cost-function type control method. It is basically an open-loop control

method.

A Cost-function is the performance measure of what we want to minimize. For example,

the term “cost” can be Power consumption or total error.

We can also define the “cost” as the deviation from a reference value of a signal.

It is a functional equation which assigns a group of points in a time based to one scalar

value. That value is called the “cost”.

5.2. Cost-Function type control method

A combination of control signals can be selected from our velocity matrix, obtained in previous

chapter. First we have to write cost-function equation [3] involving steady-state velocities and

downstream pressure. Then we have to minimize the chosen cost function. The developed cost-

function is given as follows:

2 2( ) [ ( ) ( )] [ ( ) ( )]r e r eJ k V k V k K P k P k= − + − (11)

eV is the calculated steady-state velocity,

rV is the reference velocity,



eP is the calculated downstream pressure (equal to the PB for extending direction and PA for

retracting direction)

K is the tuning parameter, which is used to find a compromise between velocity and pressure.

Algorithm for our open-loop control strategy

Note the present reference velocity and reference pressure

Read the calculated velocity and pressure with all combinations from the steady-state

characteristics.

Pick the combination which minimized the cost-function value.

Assign the selected control signal to the corresponding Digital based valves and go to step 1.

6. RESULTS ANALYSIS

Velocity graphs of hydraulic servo valve and our proposed DFCU, we can come to know that

we achieved step-wise-step flow control from the Digital valves. Our proposed DFCU possess

simple control electronics. For same flow rates of 120 L/min; same loads and same cylinder

piston and rod diameters, the digital valve associated actuator extends more length with almost

same velocity. The cost of on/off valves (around 16) is less when compared to the cost of the

hydraulic servo valve. The estimated price for digital- hydraulic four-way valve is 1200 €

(including manifolds, control electronics), which is only a fraction of the price of the water

hydraulic servo valve (>5000 €).

Essential challenge of our digital flow circuit unit is the capacity of the total on/off valve

system when correlated to hydraulic proportional and servo valves. Even though each on/off

valve is smaller than proportional hydraulic valve with similar capacity, the case can be inverse

when DFCU valve system with 15-25 On/Off type valves are used. There are two options for

advancement of smaller DFCUs. The first is to upgrade existing valves for ideal parallel

connectivity. Other option is micro designed valve like e.g. using silicon etching technique.

Effective improvement of hydraulic micro valve will make it conceivable to improve smaller

DFCUs with an enormous number of similar valves. This sort of valve would offer quick

reaction and good verbosity. Digital flow control unit is considered since it provides cost

effective resolution. It depends purely in application. It surely relevant for water hydraulics

since hydraulic servo and proportional valves are so costly. Two-way solenoid valves gives cost

effective solution for low-pressures (below 50 bar) and discharge rates (below 20 l/min). By

considering aggregate costs includes manifolds and control electronics which are twice the price

of the valves, a sample cost estimation for the digital flow circuit unit can be calculated. A cost-

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25

velo

city

(m/s

)

Time(sec)-0.7-0.6-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 10 20 30 40 50

Ve

loci

ty(k

m/s

)

Time(sec)



evaluate for the digital valve system with 4-way valve is similar to the cost of the pilot

associated hydraulic valve with spindle position feedback and coordinated hardware. So, the

present on/off valves technique provides less cost in oil hydraulics. This is convincing since

there is no mass production of digital based hydraulic valves. If the switching mode branch of

fluid power is taken, the switching noise of our digital valves are precisely correlated to the

response time of the valve. More noise is produced when we use fast valves. That is because of

high impact energy and quick armature movement. The noise produced in digital hydraulic

system is lesser than with PWM technique control. Digital flow control unit is not using fast

valves, noise level is low in digital hydraulics since valves are operated only if velocity changes.

The other factor is pressure peaks. They are harmful since they produce noise and cause damage

the system. In real time applications, jerky motion is rarely allowed. Our proposed digital

hydraulic unit produces smooth movement when there is variation in switching delay values.

There are many ways to increase the smoothness of motion. The correct way is to utilize valves

provided with small switching delays. Pressure peaks depends also on applying coding

techniques. Theoretically, we can come to know that PNM technique produces less peaks, Pulse

code Modulation gives high peaks and Fibonacci coding technique gives medium peaks.

7. CONCLUSIONS

In our project, we performed two control methods based on the steady-state characteristics of

the cylinder. First one is feed-forwarding method and second one is cost function method.

Feeding the combination to the programming, which is obtained from the velocity matric gives

the desired velocity control. If you see the velocity matrix, nearly 4-5 different combinations

can be possible for same velocity. This is one of the advantages for DFCU. On the other hand,

we can reduce the error using the cost-based function. Here we can select the combination

which minimizes the cost-function value. So from this method, we can feed to the programming

the exact combination with minimum error. To enhance the behavior of the cost-function based

controller, we should repeat the algorithm with more reference velocities and pressures with

the help of proper logic in the coding part. Finally we want to conclude that the trend has been

changing to Digitalization in all fields. So we can also change in our Hydraulic applications.

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