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Nonconventional Technologies Review 2012 Romanian Association of Nonconventional Technologies Romania, June, 2012

SOME WAY TO MONITORING THE CONSUMED ENERGY, LOCAL AND GLOBAL, DURING THE ELECTRICAL DISCHARGE MACHINING PROCESS

Emanoil Toma1, Carmen Simion2, Ioan P. Mihu3 and Mihail Aurel Ţîţu4 1 “Lucian Blaga” University of Sibiu-Romania, Department of Computers and Electrical Engineering, emanoil.toma@ulbsibiu.ro

2 “Lucian Blaga” University of Sibiu, Department of Industrial Engineering and Management, carmen.simion@ulbsibiu.ro 3 “Lucian Blaga” University of Sibiu-Romania, Department of Computers and Electrical Engineering, ioan.p.mihu@ulbsibiu.ro

4 “Lucian Blaga” University of Sibiu, Department of Industrial Engineering and Management, mihail.titu@ulbsibiu.ro

ABSTRACT:. The consumed energy is determined by sensing and processing the voltage and the current in front of the principal consumers : the power supply of pulse generator, the displacement system of the electrode-tool, the x-y or other axis displacement systems, the circulating system of dielectric liquid, eventually in front of the auxiliary activating systems (magnetic, ultrasonic). An electronic circuit was conceived, based on the high performance microcontroller and dedicated Analogue Front End Device, in two variants. The first variant is very simple; microcontroller receives one pulse at every watt-hour of consumed energy, accumulates it, and communicates via CAN-bus. The second variant is more complicated in the software area; it can measure all power parameters: W, VA, VAR, power factor, RMS voltage and current, total harmonic distortion. It can be a veritable energy analyzer if the software is very elaborated. The main system can communicate with each measuring system and can offer a detailed evolution of consumed energy for analysis. KEY WORDS: Active Power, Energy, Analogue Front End Device, Microcontroller, Controller Area Network (CAN).

1. INTRODUCTION For reducing the consumed energy in Electrical Discharge Machining (EDM) is necessary to monitor the consumed energy for the entire machine tool and for the local consumers: the power supply of pulse generator, the displacement system of the electrode-tool, the x-y or other axis displacement systems, the circulating system of dielectric liquid, eventually in front of the auxiliary activating systems (magnetic, ultrasonic). The system can be applied for other dimensional machining process.

1.1. Energy measurement principle. Electrical energy is defined by equation (1), referred to the figure 1, where i is the instantaneous current, u is the instantaneous voltage across the circuit and p is the instantaneous power.

∫∫ ⋅⋅=⋅= dtiudtpte )( (1)

u

i

CIRCUIT

ELECTRICAL

Figure 1. The energy defining schema

If the power and the energy have positive sign the circuit is Load and consumes energy. If the power and the energy have negative sign the circuit is Source and delivery energy. The measuring circuit is must to sensing the current and the voltage. The conditioning signals are must be multiplied. The results is must be integrated.

Measurement principle is illustrated in figure 2.

METER

LINE_OUT

U

LINE_IN

NULL_OUT

COUNTER

VOLTAGE

I

N=prop.W

CONVERTER

ELECTRONIC

NULL_IN

POWER TO-FREQUENCY

Figure 2. The principle of measuring schema

The block diagram has a power meter, and the counter share having a voltage to frequency converter and an electronic or electromechanical counter.

Energy measurement could be implemented in two principal ways: analogue and digital.

LINE_IN

MULTIPLIER

R_sense

PASSANALOGUE

TO-FREQUENCY

x

y

COUNTER

LOW

u

LINE_OUT

NULL_IN

R1

CONVERTER

R2

NULL_OUT

FILTER

N=prop.W

VOLTAGE

u

Figure 3. Analogue variant of energy measurement

In the analogue variant, presented in the figure 3, the power meter is based on an analogue multiplier.

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On the inputs of the multiplier come two voltages. One is proportional to the voltage line and the other is proportional to the current.

Between voltage and current there are phase shifting (φ).

tUtu ωsin2)( ⋅⋅= (2) )sin(2)( ϕω −⋅⋅= tIti (3)

U and I are the RMS (Root Mean Square) value of voltage and current.

The output of multiplier gives a voltage proportional to the instantaneous power. The expression of is:

)()()( titutp ⋅= (4) )2cos(cos)( ϕωϕ −⋅⋅−⋅⋅= tIUIUtp (5)

The instantaneous power has two spectral components:

The first term of (5) equation is average value, named real power. the second term is an 2f harmonic component

The low pass filter must reject the 2f component.

The voltage to frequency converter produces the output pulses proportional to the average real power.

These pulses will be accumulated in electronic / electromechanical counter.

The digital variant is presented in the figure 4:

S&H

TO-DIGITAL

LOW-PASS

u

DIGITAL

TO-DIGITAL

ANALOG

CONVERTER

ux

LINE_OUT

CONVERTER

COUNTER

R2

CONVERTER

NULL_IN

ANALOG

N=prop.W

R1

DIGITAL

R_sense

S&H

TO-FREQUENCY

LINE_IN

FILTER

y

NULL_OUT

Figure 4. Digital variant of energy measurement

The analogue signals ux and uy proportional of the voltage and the current must be applied to the inputs of the sample and hold dual unit. The two analog to digital converters furnishes simultaneously at its outputs the two signals in digital form.

The multiplier is hardware implemented for increase the speed. The digital low pass filter is usually a recursive filter based on the structure showed in figure 5 [4].

0

-1

x[ ]

-1

n

z

n-1

n

1

y[ ]

b

w[ ]

1

ny[ ]

b az

Figure 5. The diagram of digital Low Pass Filter (Infinite

Impulse Response Filter).

1.2. Actual state of energy measurements techniques.

Thanks to progress in technology of integrated circuits were achieved complex circuits having inside analogue and digital blocks like microcontrollers. Actually most measuring instruments are performed around a microcontroller. The importance of energy for human society imposed achievement of many instruments for measuring electrical energy consumed.

On the market there are portable measuring instruments named electrical power analyzers. These instruments offer a lot of information about the quality of consumed energy:

catch power quality events such as fast transients (impulses) to few µs, voltage sags (dips), and swells; track total harmonic distortion to the few tens harmonic; record energy use (kWh), and log all power parameters:

watts, va, var, power factor, etc.; data log RMS voltage and current over time

(days/weeks/months) with cycle-by-cycle resolution; display real time values in meter window – just like a

multimeter; display oscilloscope waveforms, harmonic spectra, phasors

via a PC.

These instruments also have one or more communications modules, even wireless.

Having current and voltage probes, these instruments can be easy connected (see figure 6).

ANALYZER

ENERGY

ELECTRICAL

MAINS

NL3L2

probe

L1

current

LOAD

Figure 6. The connection schema of portable energy analyzer

In the consumed energy monitoring we can also use the energy counters. These can be connected in circuit like in figure 7.

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L2_OUT

N_IN

L2

L3_IN

N_OUT

L1_OUTL1

L2_INCOUNTER

L1_IN

MAINS LOADNL3

ENERGY

L3_OUT

Figure 7. The connection schema of energy counter

It is obvious the necessity of inserting the energy counter in circuit between mains and consumer. Between line_in and line_out there are a shunt resistor or primary coil of current transformer. If the current transducer is based on Hall Effect, between mains and load there are only a wire [1], [2], [5].

2. MODULES OF ENERGY MEASURING BASED ON MICROCONTROLLER AND ANALOG FRONT END CIRCUIT

According to the necessity of reducing power consumed in electrical discharge machining, we conceived a low cost module, to be placed in front of each share part consumer of the machine tool (the power supply of pulse generator, the displacement system of the electrode-tool, the x-y displacement system, the circulating system of dielectric liquid, eventually in front of the auxiliary systems (magnetic, ultrasonic)).

The simplified electrical schema is showed in figure 8.

We used for current sensing a shunt resistor (R_sense) followed by a programmable amplifier contained in the Analog Front End (AFE) circuit MCP3905. The power supply for the AFE circuit, showed in figure 10, is referred to the LINE_IN point.

For voltage sensing we used a voltage divider (R1, R2). Maximum voltage applied to the input of amplifier is a few mV.

Programmable gain amplifier (PGA) can be programmed to have 1, 2, 4 or 16 gain factor, according to the value of R_sense. Analog to digital converter is 16 bits delta-sigma type. Two digital high-pass filters cancel the system offset on both channels such that the real-power calculation does not include any circuit or system offset.

After being high-pass filtered, the voltage and current signals are multiplied to give the instantaneous power signal. This signal does not contain the DC offset components, such that the averaging technique can be efficiently used to give the desired active-power output. The instantaneous power signal contains the real power information; it is the DC component of the instantaneous power. The averaging technique can be used with both sinusoidal and non-sinusoidal waveforms, as well as for all power factors. The instantaneous power is thus low-pass filtered in order to produce the instantaneous real-power signal.

A digital-to-frequency converter accumulates the instantaneous active real power information to produce output pulses with a frequency proportional to the average real power. The low-frequency pulses present at the FOUT0 and FOUT1 outputs are designed to drive electromechanical counters and two-phase stepper motors displaying the real-power energy consumed. Each pulse corresponds to a fixed quantity of real energy, selected by the F2, F1 and F0 logic settings.

The pulses are applied to timer input of microcontroller. The microprocessor substitutes the electronic/electromechanical counter and supplementary communicates on industrial CAN bus with other modules and/or PC.

Optionally the module can also communicate with PC via RS232 serial interface.

For three phase energy counter is necessary to have three analog front end circuits MCP3905, like is showed in figure 9.

Every AFE circuits have proper power supply referred to the own LINE_IN point and proper voltage and current sensing circuits also. On the other hand every AFE circuits have is followed by proper opto - coupler circuit to transmit pulses to microcontroller.

R4

VDD_AUX

PGA

A

ADC

ADC

-

-

+

+

ANALOG DIGITAL

CH0+

CH0-

CH1+

CH1-

AVDD DVDD

DVSSAVSS

HPF

HPF

LPF DFC

OUT0

OUT1

OUT

i

u

p

FFHF

MCP3905

NULL_IN

LINE_IN

R2

R1

NULL_OUT

LINE_OUTR_sense

R3U2

TCDT1101G

VDD1VDD1

TX

RX

CAN H

CAN L

CAN- TRANSCEIVER

1

2

3

4

5

6

7

8

9

DB9_F

123

VDD

IC2

C1RX

C1TX

VSS

U1RX

U1TX

16-Bit

MICROCONTROLLER

RS232

VDD_AUX

Figure 8. The simplified electrical schema of proposed energy measuring module

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R4

VDD_AUX

CH0+

CH0-

CH1+

CH1-

AVDD DVDD

DVSSAVSS

HFOUT

U1

MCP3905

NULL_IN

R_IN

R1_R

NULL_OUT

R_OUTR_sense_R

R3U4

VDD_R

TX

RX

CAN H

CAN L

CAN- TRANSCEIVER

1

2

3

4

5

6

7

8

9

DB9_F

123

VDD

IC1C1RX

C1TX

VSS

U1RX

U1TX

IC2

IC3

16-Bit

MICROCONTROLLER

RS232

VDD_AUXVDD_R

CH0+

CH0-

CH1+

CH1-

AVDD DVDD

DVSSAVSS

HFOUT

U2

MCP3905

S_IN

R2_S

R_sense_S

VDD_S VDD_S

S_OUT

CH0+

CH0-

CH1+

CH1-

AVDD DVDD

DVSSAVSS

HFOUT

U3

MCP3905

T_IN T_OUT

R2_T

R_sense_T

VDD_T VDD_T

VDD_AUX

R6

R5U5

VDD_AUX

R8

R7U6

R2_R

R1_S R1_T

Figure 9. The simplified electrical schema of proposed three

phase energy counter

R17

470/2W

C20

470nF/630V

D4 PL15Z

C21 470u/16V

1 26

43578

U7

78L05

C23 100nVDD1

D5

1N4007

RV2 275V

NULL_IN

LINE_IN

NULL_OUT

LINE_OUTR_sense

Figure 10. Electrical schema of AFE power supply

Power supply for microcontroller and CAN transceiver must be galvanic separated toward mains. We used an integrated DC to DC converter AM1D-1212 (figure 11). Both power supply offer a weak current. If the microcontroller is used to perform more calculus the consumed current increase. In this case another schema of power supply is necessary.

R15

470/2W

C16 470u/16V

RV1 275V

D3

1N4007

C15

470nF/630V

D2 PL15Z

1 2

4 3

U5

AM1D-1212

C18 10u/16V

C19 100n

VDD_AUX

1 26

43578

U6 78L05

0

C17 2U2

LINE_IN

NULL_IN

LINE_OUT

R_sense

NULL_OUT

Figure 11. Electrical schema of microcontroller and communication port power supply

Analog Front End device used in fig. 8 is MCP3905 and another version, like MCP3909, has supplementary a digital serial peripheral interface and can communicates with microcontroller.

The microcontroller read the value of instantaneous real power and calculates the energy.

The microcontroller can read the value of voltage and instantaneous current for calculating the RMS value, Fourier transforming, total harmonic distortions, etc.

CAN_ADAPTER

CAN_BUS

no_2

no_1

MODUL

no_kno_3

MODUL

MODUL

PC

MODUL

Figure 12. Connection diagram of the modules

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R1

NULL_IN

0

R2

0

RS232

+

-

+

-

VDD1

VSS1

VDD2

VSS2

123

CAN- TRANSCEIVER

TX

RX

CAN H

CAN L

NULL_OUT

DB9_F

1

2

3

4

5

6

7

8

9

16-Bit

MICROCONTROLLER

RE0

VDD

SDO

RE1

SCK

RB2

SDIC1RX

C1TX

VSS

U1RX

U1TX

OC1

RB3

RB4

PGA

PGA

ADC

SINC

FILTER

ADC

3

3

FILTER

SINC

PHASE

SHIFTER

DIGITAL

SPI

INTERFACE

-

-

+

+

CS

SCK

SDI

RESET

SDO

DR

ANALOG DIGITAL

CH0+

CH0-

CH1+

CH1-

AVDD DVDD

DVSSAVSS

OSC1

MDAT0

MDAT1

U4

+

-

+

-

VDD1

VSS1

VDD2

VSS2

R_senseLINE_IN

0

0

LINE_OUT

Figure 13. The simplified electrical schema of proposed energy analyzer module.

The Analog Front End device used in figure 13 communicates with microcontroller through the digital serial peripheral interface (SPI). AFE not includes digital multiplier and digital low pass-filter. It offers information about only instantaneous value of current and voltage.

The 24 bits delta-sigma ADC increases the accuracy of measurement. The microcontroller must achieve the functions

performed by precedent AFE: digital multiplication, the offset rejection, and low-pass filtering. Moreover, an adequate software implemented on microcontroller could offer the possibilities to measure another parameters like RMS value of voltage and current, active power, apparent power, power factor such as is showed in figure 14.

Figure 14. The diagram of power parameters digital calculation.

3. REFERENCES 1. King, C., IEC Compliant Active-Energy Meter Design

Using The MCP3905A/06A, Available from: http://ww1.microchip.com/downloads/en/AppNotes/00994b.pdf

2. King, C., Quiquempoix, V. Designing with the MCP3901 Dual Channel Analog-to-Digital Converters, Available from: http://ww1.microchip.com/downloads/en/AppNotes/01300a.pdf

3. Chlebis, P., Pumr, J., Simonik, P. Current sensor with the DSP, International Conference, Applied Electronics (AE), ISBN: 978-80-7043-865-7, (2010).

4. Mateescu, A., Dumitriu, N., Stanciu L. Semnale şi sisteme – Aplicaţii în filtrarea semnalelor, Editura Teora, ISBN 973-20-0666-8, Bucureşti, (2001).

5. Moulin, E. Measuring Harmonic Energy with a Solid State Energy Meter, Available from: http://www.analog.com/static/importedfiles/tech_articles/17011149511795ANALOG_PAGE_68-71.pdf

6. Toma, E., Mihu, P. I., Nanu, D. Some possibilities to monitoring the energy consumed in electrical discharge machining, Proceedings of the 15th International Conference, Modern Technology, Quality and Innovation, Vol. II, pp 1077-1080, Vadul lui Voda-Chisinau, Republic of Moldova, (2011).