Novel Intelligent and Sensorless Proportional Valve Control with ...

Research ArticleNovel Intelligent and Sensorless Proportional Valve Controlwith Self-Learning Ability

Bayram Akdemir

Electrical and Electronics Engineering, Engineering Faculty, Selcuk University, Campus, Selcuklu, 42075 Konya, Turkey

Correspondence should be addressed to Bayram Akdemir; [email protected]

Received 16 January 2016; Accepted 8 June 2016

Academic Editor: Chi Chiu Chan

Copyright © 2016 Bayram Akdemir. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Linear control is widely used for any fluid or air flows inmany automobile, robotics, and hydraulics applications. According to signallevel, valve can be controlled linearly. But, for many valves, hydraulics or air is not easy to control proportionally because of flowsdynamics. As a conventional solution, electronic driver has up and down limits. Aftermanually settling up and down limits, controlunit has proportional blind behavior between two points. This study offers a novel valve control method merging pulse width andamplitudemodulation in the same structure. Proposedmethod uses low voltageAC signal to understand the valve position and usespulsewidthmodulation for power transfer to coil. DC level leads to controlling the valve andAC signal gives feedback related to coremoving. Any amplitude demodulator gives core position as voltage. Control unit makes reconstruction using start and end pointsto obtain linearization at zero control signal and maximum control signal matched to minimum demodulated amplitude level.Proposed method includes self-learning abilities to keep controlling in hard environmental conditions such as dust, temperature,and corrosion. Thus, self-learning helps to provide precision control for hard conditions.

1. Introduction

Proportional valves in hydraulics and pneumatics are widelyused inmany applications and areas such as robotics, presses,and machines. Air or fluid flowing can be controlled easilyaccording to requests as on or off. For many applications,only on and off are not enough for comfortable controlling.Although automatic control strategy is very complicated,proportional control ability opens a gate with many abil-ities [1–4]. Valves for hydraulics and air flows have moreimportance to control for many kinds of machines such asautomobile, robotics, or planes in order to improve controlability and robustness; proportional control is widely used forflows controlling. But, especially in case of air, flow controlis not easy because of gas features. Proportional pneumaticvalves can have hysteresis levels as high as 15%, which canwreak havoc on closed-loop control systems [5, 6]. Anotherproblemwith proportional valves is that they often vary fromone to another in terms of maximum flow and gain [7].Gain is the rate of flow change to valve input current, andengineers require consistency and linearity for tight control.Two conventional ways, constant current or pulse with

modulation (PWM) techniques, lead to simplifying controlof valves. PWMand constant current control techniques havesome weaknesses [8, 9]. The proposed method has feedbackabilities that come from its own structure so it leads to moreprecision control compared to conventional two ways. Theproposed method has PWM structure for power transferringand alternative current (AC) low voltage modulated signalsuperimposed to PWM signal to obtain core moving volume.In order to obtain robust control ability through the lifespan,self-learning ability algorithm is supported and discussed.Novel method measures core distance at zero control signaland maximum control signal at each running.

2. Conventional Methods forValve Controlling

Two conventional methods, PWM and current source tech-niques, are simple control methods for proportional valvecontrols and both control circuits are so simple to apply.On the contrary, these methods have some weaknessesbesides their simplicity. Pulse width modulation, which isa commonly used technique to control systems, is easily

Hindawi Publishing CorporationJournal of SensorsVolume 2016, Article ID 8141720, 6 pageshttp://dx.doi.org/10.1155/2016/8141720

2 Journal of Sensors

realizable with semiconductor switches. It has been alreadystated above that almost no power is dissipated by the switchin either on or off state. PWMmakes inertial electrical devicescontrolling simple via modern electronic power switches.PWM can be used to adjust the total amount of powerdelivered to a load without losses normally incurred when apower transfer is limited by resistive means. The drawback isthe pulsations defined by the duty cycle, switching frequency,and properties of the load. With a sufficiently high switchingfrequency and, when necessary, using additional passiveelectronic filters, the pulse train can be smoothed and averageanalog waveform recovered [10]. Figure 1 shows (a) PWMstructure and (b) constant current driver structure. Thereare only two possible situations for the switching element.In both states, practically, power losses can be negligible[11, 12]. Generated PWMsignal occurs on the load as the samecharacteristics. In general, frequency is constant and pulsewidth is variable on time axis. Equation (1) shows outputaverage voltage related to pulse width:

𝑉average = 𝑉cc × (pulse widthpulse length

) . (1)

From the equation, generated (direct current) DC voltagecan be varied using PWMmodulation easily. On the contrary,output voltage not only is related to pulse width but also isrelated to 𝑉cc (power supply). Using this structure any valvecan be controlled via varying voltage. In addition to powersupply level, valve current cannot be controlled if it is notmeasured. These two weak points are difficulties of PWM.

Current source is a very old and robust model to avoidPWM weakness and electromagnetic interference. It is sosimple to implement. Current flow is directly related tocontrol voltage (𝑉

𝑥) and cannot be changed even if 𝑉cc

changes in determined limits [13–15]. Changing power supplylevel does not affect the current level. Output current (𝐼

𝑐) only

depends on𝑉𝑥input signal. Equation (2) shows calculation of

current flow. As soon as 𝑅1and 𝑉

𝑥are constant, there is no

way to change the current:

𝐼𝑐=(𝑉𝑥− 𝑉𝐵𝐸)

𝑅1

. (2)

On the contrary, current source can have high power losson the controlling element [16, 17]. Power dissipationmust becalculated carefully to protect the device in order to carry onthe running.

3. System Details

The proposed method uses PWM structure, amplitude mod-ulation, and current source in the same structure for preci-sion control. PWM structure is used for controlling powertransfer. Amplitude modulation at medium frequency givesinformation about the core deep and core moving on the rail.In the valve, electromagnetic force and spring force are faceto face with each other and core moving depends on electro-magnetic force, so it is directly related to controlling current.So core traveling on the rail can be changed by adjusting

the current level. If there could be a feedback from the coredepth such as linear variable differential transformer (LVDT),it is possible to hold the core on the constant depth. Whenthe core moves incorrectly, amplitude modulated feedbacksignal replies with a correction. An inductor is a passiveelectrical component used to store energy in a magneticfield. Inductor has a virtual resistance against frequency. Insome determined conditions, inductance changes linearly.Inductance value can be calculated from

𝐿 =𝜇0𝜇𝑟𝑁2

𝐴

𝑙, (3)

where 𝐿 is inductance in henries, 𝜇0is permeability of free

space,𝑁 is number of turns, 𝐴 is area of cross section of thecore in square meters, 𝑙 is length of core in meters, and 𝐾is Nagaoka coefficient [18]. Inductor has a virtual resistance,namely, impedance, versus the frequency. Equation (4) showsinductor impedance:

𝑍 = 2𝜋𝑓𝐿. (4)

Any change at the inductance affects the AC current flowsthrough the core. Changed inductor helps us to evaluatethe core distance given ℎ via amplitude modulation. For theproportional valve structure, there are three inductors to beexplained. One of them is air core inductor, second one ismagnetic metal envelope inductor, and core inductor comesfrom moving related to 𝑙. During the core moving, summedthree inductors can be evaluated. 𝐿

0is air core inductor, 𝐿

𝑒

is envelope metal inductor, and 𝐿𝑐is moving core inductor.

Total 𝐿 is sum of the three inductors. Number of turns and allinductors can be explained as a function of ℎmoved distancethrough the hole. When 𝑙 equals ℎ, 𝐿

0and 𝐿

𝑒disappear and

only 𝐿𝑐occurs in total inductance. The starting point is at

ℎ = 0; total inductance can be shown only by 𝐿0and𝐿

𝑒.Three

kinds of inductors can be explained as a function of ℎdistanceand 𝑙 as shown in Figure 2.

Through ℎ and 𝑙, number of turns can be spread linearly.From this point,𝑁 is a linear function of ℎ and 𝑙 as shown by

𝑁ℎ=𝑁 × ℎ

𝑙. (5)

Total inductance is sum of three inductor structuresdefined as 𝐿

0, 𝐿𝑒, and 𝐿

𝑐. Total inductance is given by

𝐿𝑇= 𝐿0+ 𝐿𝑒+ 𝐿𝑐. (6)

Radii are different for all inductors. For 𝐿0radius can

be written as 𝑎, for 𝐿𝑐it can be written as 𝑏, and difference

between 𝑎 and 𝑏 is for 𝐿𝑒as shown in Figure 2. From (5), for

every separated inductor, they can be expressed by (7), (8),and (9) for 𝐿

0, 𝐿𝑒, and 𝐿

𝑐, respectively:

𝐿0=𝜇0𝐾 (𝑁 (𝑙 − ℎ) /𝑙)

2

𝜋𝑎2

𝑙, (7)

𝐿𝑒=𝜇0𝜇𝑟𝐾 (𝑁 (𝑙 − ℎ) /𝑙)

2

𝜋 (𝑏 − 𝑎)2

𝑙, (8)

𝐿𝑐=𝜇0𝜇𝑟𝐾 (𝑁ℎ/𝑙)

2

𝜋𝑏2

𝑙. (9)

Journal of Sensors 3

T

D

t

u

Isense

GND

Load Vavg =

T

D

Vcc

Vcc

(a)GNDGND

Q1

ICC

Vx

VBE

E

B

R1

Load

Vcc

(b)

Figure 1: Proportional valve driving techniques: (a) PWM based driver; (b) constant current driver structure.

l

Spring Core Coil

l

h

aa bb

L0 Le Lc

+ +=

a − b

(a) (b) (c) (d) (e)

Figure 2: Valve is separated inductor structure: (a) side view; (b)vertical view; (c) 𝐿

0simulates air core; (d) 𝐿

𝑒is constant core; (e) 𝐿

𝑐

is variable inductor related to movable core.

From (5)–(9), total inductance can be expressed by

𝐿𝑇=𝜋𝜇0𝐾

𝑙3(𝑁2

(𝑙 − ℎ)2

𝑎2

)

+ (𝜇𝑟𝑁2

(𝑙 − ℎ)2

(𝑏 − 𝑎)2

) + (𝜇𝑟𝑁2

ℎ2

𝑏2

) .

(10)

From the equations, inductance characteristic is a variablethrough ℎ and has virtual impedance which can be expressedas 𝑍 in (4). 𝑍 varies linearly related to inductance at constantfrequency. Current has two parts and one of the parts is lowvoltage AC signal which is superimposed on DC runningcurrent. Current flows through the inductance, switchingelement, and current sense (resistor based). Low voltageDC current, superimposed with AC current, makes the coremove. According to moving distance or moving depth, totalinductance changes and changed inductance affects only ACcurrent level. AC current level is a measurement of thecore depth. Although 𝐿

𝑒impedance, 𝐿

𝑐impedance, and 𝐿

0

impedance are linearly through the frequency, 𝐿𝑇is not

linear. At any instant, 𝑉𝑎can be calculated by (11) and AC

current is related to total impedance including core resistanceand 𝑅

1and current equation is given by (12) as follows:

𝑉𝑎AC= 𝐼AC × 𝑅1, (11)

𝐼𝑎AC=

𝑉AC(𝑅1+ 𝑅𝐿) + 𝑗2𝜋𝑓 (𝐿

0+ 𝐿𝑒+ 𝐿𝑐)1

, (12)

where 𝑅1≪ 𝑅

𝐿and 𝑅

𝐿≪ 𝑗2𝜋𝑓𝐿

𝑇at AC conditions.

For constant frequency, impedance has linear correlationwith inductance. Output voltage, 𝑉

𝐵, is related to inductance

as nonlinear characteristics. 𝐿𝑇behaves as an amplitude

modulator and modulation characteristic is related to acts.Amplitude modulation is a simple modulation technique,which superimposes carrier signal and data signal [19, 20].For proposed method, only carrier signal was scaled to useinductance. 𝐶

1separates DC current and AC modulated sig-

nal from each other and leads AC signal to pass demodulatorstage. Amplitude demodulation [21, 22] simply has one diode,resistor, and capacitor, as shown in Figure 3.

DC and AC signal have to move through the valveand DC current does not encounter any virtual impedancewhich is caused by frequency. AC signal has to overcomevirtual impedance which is changing by core volume insideof the valve rail to occur in the outside as 𝑉

𝐵. Output AC

signal amplitude is at a different level according to ℎ depth.Demodulated signal over 𝐶

1, 𝐷 passes through the low pass

filter 𝐶2and 𝑅

2. Figure 4 shows 𝑉

0output voltage. As much

as valve core moves, the obtained 𝑉0output voltage varies

according to total 𝐿𝑇. Amplitude modulated frequency was

chosen as 100 kHz in order to avoid interference betweenPWM frequency and AC modulating signal. PWM runningfrequency was used as 10 kHz. Obtained feedback signal hasnegligible ripple at least as long as two times greater thanPWM frequency [23]. In application 10 times greater ACsignal is superimposed to PWM signal.

Although inductances of 𝐿0, 𝐿𝑐, and 𝐿

𝑒have linear char-

acteristics independently, at any time, three inductances arevalid as sumof the inductances as 𝐿

𝑇. So total characteristic is

not linear and has to be corrected by the control unit. Table 1is calculated virtually, and, from Table 1, 𝐿

𝑇is not linear

through ℎ distance. Total nonlinearity must be correctedusing correction factor.

Figure 5 shows feedback signal related to ℎ distanceand correction curve in the same graph. After correctionon feedback signal, core moves behavior linearly. Learningmethod is based on inductance level at zero control voltageand stepped values span with 10% PWM levels. If the movingdepths are linear through the PWM, microcontroller basedcontrol unit obtains function in red line as a feedback


C2

GND GND

VDC + VAC

(IDC + IAC)

R1

C1

V0

Vb

Va

RL +

)

Amplitude demodulator

Dideal

RT)

R2

Figure 3: Amplitude demodulator to obtain feedback signal as 𝑉0.

t

V

V0

Vb

Related to inductance

Amplitude Actual voltage

Figure 4: Demodulated AC signal from PWM as feedback signal.

0

0.16

0.66

0.5

0.33

0.83

1

0.16 0.33 0.5 0.66 0.83 1

(Vol

ts)

Increase h and inductance (mm, Henry)

Values are normalized

Output-correction signal and inductance

Correction signalOutput signal

Feedback signal

Figure 5: Output, feedback, and correction signal.

Table 1: Inductance changing through the distance.

Inductance changing through the core movingℎ 𝐿

0𝐿𝑐

𝐿𝑒

𝐿𝑇

Correction0 0.1 0.8 0 0.9 01 0.081 0.648 0.018 0.747 1.3386882 0.064 0.512 0.072 0.648 3.086423 0.049 0.392 0.162 0.603 4.9751244 0.036 0.288 0.288 0.612 6.5359485 0.025 0.2 0.45 0.675 7.4074076 0.016 0.128 0.648 0.792 7.5757587 0.009 0.072 0.882 0.963 7.2689518 0.004 0.032 1.152 1.188 6.7340079 0.001 0.008 1.458 1.467 6.13496910 0 0 1.8 1.8 5.555556

signal and calculates function in pink line to control valveproportionally and precisely.

Microcontroller based control techniques are widely usedin literature [24, 25]. Microcontroller connections are shownin Figure 6. For every start, microcontroller scans valvebehavior to compare the memorized one and present one.Then it refreshes the old information with present one to cre-ate new control characterization to minimize environmentaleffects such as temperature and dust. Possible changing oncore inductance due to temperature and control characteristicis renewed for robustness.

Microcontroller unit records feedback signal at zerocontrol voltage and saves this value as 𝐿

0+ 𝐿𝑐. According

to initial inductance, obtained feedback signal starts frommaximum level and approaches zero when the core reachesend point. During the moving, feedback signal follows asecond-order equation. FromTable 1 𝐿

0and 𝐿

𝑐start from 0.1

and 0.8, respectively, and both end with zero. If core materialhas high 𝜇

𝑟, effects of 𝐿

0are so low andmay be negligible. For

real world application, 𝐿0has negligible effects on 𝐿

𝑐and 𝐿

𝑒

and therefore 𝐿𝑇.

Microcontroller has two control strategies for self-learning. First way is very simple to apply and is based onblind learning. Microcontroller unit just saves feedback sig-nals and control signals level as PWM level. End of the PWMspans from 0% to 100% with small steps; microcontrollerconnects the dots with lines and obtains control strategy fromthe curve line. Second way requires more math calculationsthan the first way. Microcontroller unit calculates a trajectoryas a second-order equation and subtracts the core constantbehavior characteristics during the run time. The rest of thefeedback signal can be evaluated as moving depth by controlunit. Both methods can be applicable and the first one is verysimple learning way for robotics application avoiding backkinematics system behavior calculation.

4. Implemented Circuit Results

In this study, a linear valve behaviorwas used asmeasurementof the distance of coil. Thus, displacement of the coil can

Journal of Sensors 5

GNDGND

Amplifier

Analog to digital

converter

MOSFETdriver

Mic

roco

ntro

ller

Feedback unit

Control unit

Proportionalcontrolinput

Load

R2 R1C2

C1Dideal

Vcc

Figure 6: Block diagram of proposed method.

Obtained AC signalDisplacement

Linear valve

Figure 7: AC signal behavior related to displacement of the core.

Regulated air output Air input

Movable core Coil

Figure 8: Linear valve (Opel Vectra) for sample application.

be calculated coil of the linear valve without using externalmeasurement device. Table 2 shows the displacement andobtained AC signal due to moving. Figure 7 shows theimplementation of the circuit.

According to study, inductance must be changed due tocoil moving. Thus changed inductance affects the impedanceof the coil under AC guide signal to measure the displace-ment. Table 2 showsmeasured real values due to coil moving.In order to implement the proposedmethod, air valve regula-tor for Opel Vectra was used. All of the car manufacturers usean air regulator to regulate the air input under idle running.Some of them prefer motor based regulator circuits; some ofthem use linear valve based solutions. Opel uses linear valveto regulate the air intake for idle running. Figure 8 showstested linear valve for proposed method outcomes.

Preferred linear valve is highly robust and is designed forautomobile sector. For the valve, total displacement is 5.9mm

Table 2: Measured and obtained values for the sample applicationof linear valve.

Min. Max.Inductance (mH) 17.168mH 17.558mHCurrent (A) 0 1.2 AAC signal 𝑉RMS (volt) 0.332 0.382Position (mm)∗ 0 11.25∗Note: be aware that position has offset value with different displacement.

and DC power supply necessary to run is 0–12 Volts. Table 2shows the obtained results after trying the proposed method.The obtained values from the application are 0.39mH and50mV related to displacement.

From Table 2, changing inductance of the coil affectsthe impedance. Changed impedance only could affect theobtained AC signal. On the contrary, DC current cannot beaffected by AC signal. Only DC current makes coil move andonly AC signal gives response related to displacement. Totaldisplacement is 5.9mm. AC signal varies from 0.332 to 0.382.According to obtained values, a constant can be calculatedusing

𝑘 =Δ (𝑉AC)

Δ (mm)=𝑉rmsmm,

𝑘 =0.382 − 0.332

5.9=8.47𝑚𝑉rms

mm.

(13)


From (13), 𝑘, a constant related to moving core displacement,is calculated.

5. Conclusion and Future Work

In this study, novel approach is discussed to control any valveproportionally. The aim is to control the linear componentwithout any external sensor to improve control ability. PWMand current mode driving techniques were merged andamplitude modulation was used for feedback to obtain coreposition information. This study involves not only propor-tional control but also sensorless position control for anyelectromagnetic core based valve. In particular, air valve con-trolling is not easy because of air behavior. This study helpseliminate some drawbacks such as temperature, hysteresis,and friction losses at any time. Air intake controlling for thecars could be robust in case of using the proposed method.Novel method makes robust control strategy in case of hardenvironmental conditions such as dust and temperature,because of the microcontroller scanning the valve at everystarting. On the contrary, the proposed method creates afeedback related to displacement without external measure-ment device. The proposed method may offer open frameair valve controller to carry on control abilities continuouslywithout any weakness through the lifespan. With learningability, self-test strategies can be applied and service periodcan be reduced by self-learning.

Competing Interests

The author declares that there are no competing interests.

Acknowledgments

This study was supported by EEM Company which works inthe sector of vertical transporting industries.

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