A SOFTWARE-BASED FREQUENCY LOCKED LOOP DC MOTOR DRIVE SYSTEM

A SOFTWARE-BASED FREQUENCY LOCKED LOOP DC MOTOR DRIVE SYSTEM

ABSTRACT

The paper considers controlling DC motor speed using LabVIEW-based frequency locked loop control algorithm which consists of a set of virtual instruments. Power interface circuit is required to supply the motor. In order to realize frequency-lock; a software digital integrator is used by implementing up-down counter. Proportional control term is included in order to control the depth of integral action. The result of the experiment shows acceptable start-stop time characteristics and that motor speed is hold constant independent of motor load changes. 

Keywords: LabView, Phase-Lock, Digital Integration, DAC, Counter, Optical Encoder.

INTRODUCTION

DC motors have been widely used in robotics and in industrial variable speed applications because of their desirable speed-torque characteristics and simplicity of control. Feedback motor drive is required in order to achieve high accuracy and better performance. Conventionally, this is achieved by analog servo feedback system in which any change in speed is sensed by tachometer and compared with a fixed reference voltage to generate a correction signal. However, this analog feedback system is not satisfactory where excellent speed regulation and fast dynamic response are required. These features can be achieved by using a phase-locked loop control system. In the phase locked loop configuration, motor speed is converted to a digital pulse train, which is synchronized with a reference digital pulse train. In this way, by looking onto a reference frequency, precise control of motor speed is achieved. Reference frequency can be derived from a precision crystal controlled source or any frequency source with the required stability and accuracy. Motor speed is sensed by either a Hall Effect devices or an optical encoder. The Phase Locked Loop (PLL) may be considered as a servo system, which controls the phase of its output signal such a way that the phase error between the output phase and the reference phase reduces to a minimum. Traditionally, the most basic functional block diagram of a PLL is given as in figure 1. This diagram shows the components that every PLL must have, namely: a Phase Detector (PD), a voltage controlled oscillator (VCO), a Loop Filter (LF) and a feedback interconnection. The PD is a nonlinear device whose output contains the phase difference between the two oscillating input signals. The VCO is another nonlinear device which produces an oscillation whose frequency is controlled by a lower frequency input voltage. Concerning the LF, it is a low pass filter used to suppress the noise and high frequency signal components from the PD, and provides a DC controlled signal for VCO. If the PD is linear and the PLL in lock, then the filter output is proportional to a phase error. In general this block contains the required gain and filtering to set the loop‟s overall bandwidth and meet the necessary stability criteria.

STRUCTURAL ANALYSIS OF MOTOR DRIVE PLL

Hereinafter we shall consider three important issues, namely: the nature of VCO in motor drive loops, the extent of adequacy of existing (PDs) for motor Drives and the need of a phase-frequency or a frequency-phase detecting procedure. There are substantial differences between the PLL used for communication loops and those used for motor speed control. The VCO in conventional communication loop changes frequency very rapidly on command, while there is no real VCO in motor drive PLL. But at first glance, it seems that the motor, motor drive circuit, and the feedback speed encoder have simply replaced the VCO in the classic PLL, and they may be considered as a virtual VCO. In fact this virtual VCO is a little more complicated. The mechanical and electrical time constants of the motor more than just a voltage-in, frequency-out block. Another important issue is the nonlinear characteristics of the different types of detectors. The two most common methods of implementing PDs are (1) analog mixer and (2) digital PDs, which are implemented by sequential logic circuits. The product detectors may be considered as useless in motor drives because they produce no DC for acceleration or deceleration when out of lock, further more lockup may occur on harmonics, thereby yielding incorrect shaft speeds. Analysis of digital phase detectors shows that while the exact behavior of these digital phase detectors is necessarily nonlinear, the low frequency behavior is often linear. A good example for that is AD9901 PD. Add to that, that the characteristics of the square signal (PDs) are linear over different detection intervals. For triangle (PD) it is (-p/2, +p/2), for sawtooth PD it is (-p, +p), and for sequential phase frequency (PFD) it is (-2p, +2p). Thus, although there is a variety of phase detectors, no one (PD) is optimal or even applicable in each situation. Their usefulness depends greatly on the type of PLL they will be used in, and on the input signals that they will be encountering.

The third issue is detecting sequence. In motor drives the explicit variable is speed and not phase. PDs are used to recover the information contained in the phase of the reference signal, where in motor drive there is no information in the phase of the reference signal. Consequently, motor speed controls don not require phase lock, but only integration of the frequency error (w):

Where w=2pf.

Taking in consideration the above mentioned facts it is advisable to eliminate the frequency error at first, and then to realize phase correspondence by auxiliary means as required.

THE BLOCK DIAGRAM OF MOTOR SPEED USING PLL

Now it is obvious that we do need a frequency motor drive loop instead of a phase motor drive loop. Frequency motor drive loop must have a pure integration in the signal path in order to reduce the error between wr and wf to zero. This coincides with the fact where the traditional (PD) has a transfer function equal to Kf/S. Because of the digital nature of the feedback, digital integration is recommended. Digital integration may be realized by using a digital up down counter, with a subsequent Digital to Analog (D/A) conversion. D/A is carried out by using Frequency to Voltage (F/V) converter or DAC. In order to facilitate better dynamic performance to the PLL, it is imperative to include a proportional control component in addition to the integral action. This gives the designer possibility to soften the severity of the integral action as required. For the circuit to the work in a phase lock mode an additional signal path is required.

 

Figure 1. PLL block diagram

This path starts at the LSB of the counter and includes a differential operational amplifier. The amplifier compares the DC component of the counter LSB output to Vcc/2. Any difference in integrated and causes a change in the speed. Only when the DC component of LSB output is exactly equal to Vcc/2 (i.e. 50% duty cycle) will integrator amplifier output causes to change, and the phases of the speed encoder and reference frequencies are 180’ apart. The two signals are actually phase locked at 180’. The block diagram is shown in figure No(1),where

Km- monostable “gain”

G1- error amplifier gain (dimensionless)

ʈM- monostable filter time constant

Ki- integration “gain” (V/count)

ʈ1, ʈ2- op amp time constants (sec)

Kf- phase gain (dimensionless)

A1- transconductance (A/V)

ʈf –carrier filter time constant (sec)

N- number of lines on the disc(dimensionless)

KT- motor torque constant (oz-in./A)

J- total inertia (oz-in.sec2)

KD- damping constant (oz-in./1000rpm)

PRACTICAL INVESTIGATION OF FREQUENCY LOCKED DC MOTOR DRIVE

The frequency looked motor drive controller consists of a set of virtual instruments (VIs), which are interconnected to each other in order to realize the required control algorithm. A detailed description for the VIs is given in appendix No(1). The required VIs are:

- Analog input acquire waveform VI

- The digital Integrator VI

- Digital to analog Converter VI

- Analog output VI

The main advantage of using a software-based controller [8,9] is the gained flexibility in selection of controller parameters, tuning by that the controller in right direction. The output of the Data Acquisition (DAQ) board (PCI-MIO-16E-1) is fed to a power interface circuit, which drive the motor. The motor is PM, Dc type with the following parameters: supply voltage 24 V, In=0.6 mA, P=15 W, Ra=6 and the nominal speed is 1500 rpm. The feedback incremental transmissive optical encoder was assembled at the laboratory of process control. The disk of the encoder has 300 windows. The output of the encoder is connected to a Schmitt trigger IC (CD40105DE) in order to square the encoder output pulses. Schmitt trigger provides standard CMOS output levels. The output of Schmitt trigger is connected to two decade frequency dividers (CD4017B). The motor and power interface are parts of training board type (DELLREZO-DL-2314) which exists at the laboratory of process control- Mechatronics Engineering Department. The feedback signal is fed to a counter on the DAQ board. The frequency is measured using software VI(period Icon VI), and the speed is measured using the formula  

........................(2) 

The complete experimental setup is given in figure 2.

 

Figure 2. The experimental setup

RESULTS

Figure 3 shows no-load motor starting and stopping time characteristics.

 

Figure 3. No -load motor starting and stopping time characteristics

At the beginning the reference frequency makes the counter count to increase because it is large than the tachometric frequency which makes the counter count to decrease. As a result of that the counter increments up rapidly trying to reach a steady state quickly, where as each reference rising edge generates one step more and on the other side each tachometric rising edge makes one step down. Rising time is about 0.56 s. when the tachometric frequency equals to the reference frequency the counter count is still constant. When the motor is turned off, tachometric frequency is higher than the reference, the counter count starts to decrease till reaching zero condition or reset state. Figure 4 shows that the rise time is decreased when the reference frequency was increases from 41 Hz to 200 Hz.

 

Figure 4. Effect of frequency change on rising time

CONCLUSIONS

1- The feasibility of using LabVIEW –based frequency locked motor drive system is demonstrated. Precise speed regulation, stabilization is achieved and the system has the ability to recover to the rated speed as a result of load change.

2- The proposed system offers many advantages such as accurate speed control and digital control possibility.

3- Realizing a software-based controller, makes it possible to tune controller parameters to match the controlled object performance criteria, without having a rigorous identification of the controlled process.

APPENDIX 1.

LabVeiw VIs used in research.

Analog input Acquire WaveForm VI

In order to control the motor speed, it is necessary to get a feedback signal which represents the instantaneous value of the motor speed. This feedback signal shall be subtracted from a reference speed value and as a result of that speed error will be evaluated. Feedback signal is introduced to the DAQ board using Analog input Acquire WaveForm VI. the front panel and the block daigrm of Analog input Acquire Waveform VI are shown in figures 5 and 6.

 

Figure 5. Front panel of Analog Input Acquire Waveform VI

 

Figure 6. Block diagram of Analog Input Acquire Waveform VI

Function: This VI must be supported by a data acquisition device (DAQ) board to acquire one hundred samples from the feedback (tachometer) signal at the analog input channel with sample rate of one thousand samples /second. Notice that the feedback input signal must range between a high limit of (+10) and a low limit of (–10) volts to be acquiring correctly. These two limits is assembled into a cluster by Bundle VI, then we use the Build Array VI to get an array of clusters which represent the input limits of Analog Input Acquire Waveform VI. AI Acquire Waveform VI can configure its output to Scaled Array or Waveform data type, in our application we select the Scaled Array option, then the array passes to Index Array VI to get the feedback signal and show it using the feedback (tachometer) signal waveform graph. If an error occurred, the General Error Handler VI returns a description of the error and optionally displays a dialog box. This error is transmitted to the Unbundle Status VI to use with the stop button (when it is pressed) to give the option of stopping the VI. At the end of the acquiring process, the feedback (tachometer) signal passes to the following VIs.

Monostable VI

This VI generates a train of pulses as a response to an input signal. The generated pulses are with a predetermined duration no matter what the duration of the input pulses is. Figure 7 shows the main function of the monostable VI.

 

Figure 7. Function of Monostable IC

It can be notified from the figure above that changing the frequency of the input signal changes the duration (therefore the average) of the produced signal. The duration of the produced signal can be adjusted by changing the value of the duration control in the VI. For our application the input signal represents the feedback signal from the encoder (speed signal), while the output train of pulses, which are produced by PC, are fed to the motor. The PC (Lab VIEW and DAQ) stands for the monostable action Due to the inaccurate speed-measuring device, and because of the quick variation of speed-reading, we can’t apply the feedback signal as the input of monostable VI. Instead of that we extract the parameters of feedback (Amplitude, Frequency and pulse duration), after that we used them to generate a train of pulses that represent the actual feedback signal. The Front panel and block diagram of Monostable VI are shown in figures 8 and 9.

 

Figure 8. Front panel of Monostable VI

 

Figure 9. Block diagram of Monostable VI

Function:

Monostable VI consists of three stages which are included in one while loop .This loop keeps executing till stop button is pressed or an error occurred during VI execution.

Through first stage of the monostable VI we will receive the feedback (tachometer) signal, which was acquired in the first VI by using Analog Input Acquire Waveform VI.

Digital Integrator VI

This VI (figure 10) represents full simulated image for digital integrator which is derived basically from accumulated summation concept, and generates an output signal proportional to the time integration process to the frequency difference between feedback signal (optical tachometer) and reference software generated signal. This integration forces the steady state error between the tachometer and reference frequencies to vanish, according to the following equation:

 

Where:

ki =integrator gain

C = constant

The digital integrator consists of two major parts:

1- Digital up/down counter.

2- Digital to analogue converter.

A leading (rise or fall) edge of the reference frequency signal (fref) increments the digital counter, and a leading (rise or fall) edge of the feedback (tachometer) frequency signal (fref) decrements the digital counter down, thus giving an implicit value that partially represents the controlled speed of the motor and at which mode its working, in other words if the motor should get slowed or fasted. If the tow edges occur synchronously a special arrangement must be

applied to avoid count ambiguities, the up/down counting output connected to software DAC for both positive and negative values that produces an analogue output directly proportional to the stored binary number. Assuming that the motor starts from rest, at the beginning the reference frequency drives the counter contents up very rapidly, because very few count down edges are produced by the slowly rotating motor. As the counter contents increase, an increasing error voltage is developed at the output of the D/A converter. This error voltage is fed to the motor, which speeds up, thereby causing more count down edges. As ftach approaches fref, the counter contents in-creases slowly. At the instant, when ftach = fref the counter contents remains stationary. The stored count develops just enough output to drive the motor towards equilibrium by keeping fref = ftach, thereby producing a “frequency locked” condition. Note that this is not true phase lock, because the phases are not detected yet.

 

Figure 10. Front panel of Digital Integrator

VI Function Digital Integrator VI consists of four stages, as in the monostable vi these stages are included in a while loop with stop button connected to it’s conditional terminal, therefore, the VI keeps executing until the stop button is pressed or an error occurred during the vi execution. It is possible to control the delay time (in milliseconds) between the loop iterations by manipulating the value Millisecond Multiple. The block diagram of the first stage digital integrator is shown in figure 11. In the first stage we generate the reference signal inside a continuous while loop. Using a Square waveform VI, that generates a square waveform with adjustable Frequency, Amplitude, and Phase parameters, then we get the reference signal by converting this square waveform to a pulse (1 and 0 amplitude) by using Square To Pulse VI, after that this reference signal is fed to the third stage immediately.

 

Figure 11. The block diagram of first stage digital integrator VI

In digital integrator VI second stage(figure 12) we will receive the feedback (tachometer) signal, which was acquired in the analog input Acquire Waveform VI, with the help of data acquisition device (DAQ).

Due to the inaccurate speed-measuring device, we can’t apply the feedback signal as the input of digital integrator VI. Instead of that we extract the feedback signal parameters (Amplitude, Phase, and frequency) by using the Extract Single Tone Information VI, and Pulse Measurements VI, that described in details in the previous VI.

After that we use the extracted parameters to generate a train of pulses that represent the real feedback signal using a Square waveform VI, that generates a square waveform with adjustable Frequency, Amplitude, and Phase parameters, then we get the Feedback (Tachometer) Signal by converting this square waveform to a pulse (1 and 0 amplitude) by using Square To Pulse VI. After monitoring this Signal by using Feedback (Tachometer) Signal waveform graph, it is fed to the third stage immediately.

 

Figure 12. The second stage of block diagram of digital integrator VI

The most important stage in the digital integrator VI is the third one which is shown in figure 13. In this stage we detect the leading (rising) edges of both the Reference and the Feedback Signals by the assistance of two while loop‟s shift registers:

Reference shift register for reference signal and Feedback shift register for feedback signal, then we use the Get Y Value vi, that returns the amplitude value from the input signal specified in index which we connect to an 1024 times counter by Index shift register, the 1024 count is due to software and hardware capabilities.

In our case we need two shift register elements to remember the last two iteration values (the latest value at the top element), so the process of detecting the rising edge is done while the two-shift register elements transfer the signals amplitude values from one loop iteration to the next one, so we can compare these amplitudes by the use of Greater? VI that returns a Boolean indication when a rising edge has just occurred .In other words when the latest (top) amplitude value is greater than the previous one (below).

 

Figure 13. The third stage of block diagram of digital integrator VI

In the fourth stage (figure 14) we use Up\Down Count shift register (to round the Up\Down Count value between the while loop iterations) and three nested case structures (shown in figure 15) (controlled by the Boolean indication) to enable the reference signal's rising edges to increment the up\down count (this appears in the second true case), and to enable the feedback signal „s rising edges to decrement it (this appears in the third true case). In order to avoid count ambiguities If the two rising edges occurred synchronously a special case is added to ignore the two edges (this appears in the first true case, and the third false case).

Note: The shift register must be initialized by the zero initial value to reset the shift register for every execution for the while loop.

Due to the sign of the count’s (positive or negative), the count value is fed to the positive or to the negative 16-bit Digital To Analog Converter (DAC) VI (which will be described later) to produce an analogue output directly proportional to the stored binary number. This analog output is shown by using u\d count waveform graph, which is included in one time running For Loop, and the Positive and Negative least significant bits is monitored by using two Boolean indicators.

 

Figure 14. The fourth stage of the block diagram of digital integrator VI

 

Figure 15. The three cases of the block diagram of digital integrator VI

Digital To Analog Converter (DAC) VI

The digital to analog converter, better known as the DAC, is a major interface circuit that forms the bridge between the analog and digital worlds. DACs are the core of many circuits and in displays, and many computers – controlled devices.

A DAC is an electronic component that converts digital logic levels (bits) into an analog voltage. The output of a DAC is the arithmetic sum of all the input bits weighted in a particular manner:

Where:

Wi : The weighting factor

Bi : The bit value (1 or 0)

i : The index of the bit number, In the case of a binary weighting Wi = 2i

The front panel of 16-bit DAC VI is shown in figure 16.

 

Figure 16. The front panel 16 –bit DAC VI

Figures 17 and 18 show the block diagram for positive and negative 16 –bit DAC.

 

Figure 17. The block diagram for positive 16 –bit DAC

 

Figure 18. The block diagram for negative 16 –bit DAC VI

Function:

In both positive or negative 16-bit DAC we use an array of Boolean controls to represent the digital input, and by using Index Array VI which returns the value (0 or 1) of the bits of Boolean array at index of the digital input, then we multiply these values by the outputs of 16 digital amplifiers, weighted with 32768, 16384, 8129, 4096, 2048, 1024,512, 256,128, 64, 32, 16, 8, 4,2and 1.

The complete expression for positive or negative 16-bit DAC is given by the expression:

*Positive analog output=32768b15-16384b14-8129b13-4096b12-2048b11-1024b10-512b9-256b8-128b7-64b6-32b5-16b4-8b3-4b2-2b1-1b0

*Negative analog output=32768b15-16384b14-8129b13-4096b12-2048b11-1024b10-512b9-256b8-128b7-64b6-32b5-16b4-8b3-4b2-2b1-1b0

Note: how each bit circuitry is similar to all other bits differing only by weighting factor. The summing of the outputs of the 16 digital amplifiers is representing the analog output, which is displayed by a Double-precision floating-point number. So that any number between -32768 and +32768 can be represented by a16- bit binary number.

AO Waveform Generation VI

After generation and compounding the correction signal from the outputs of the monostable VI and Digital Integrator VI, We need to send this control signal to the motor to achieve zero speed error at the motor shaft, in order to stabilize the speed of the motor.

In our application we use the data acquisition device and lab view Analog Waveform Generation VI (figures 19 and 20) to send the control signal to the motor.

 

Figure 19. Front panel of Analog output Waveform Generation VI

 

Figure 20. Block diagram of Analog output Waveform Generation VI

Function

This VI must be supported by a data acquisition device (DAQ) board to generate one thousand update /second from the control signal at the analog output channel.

Notice that the feedback input signal must range between a high limit of (+10) and a low limit of (–10) volts to be generated correctly. These two limits is assembled into a cluster by Bundle VI, then we use the Build Array VI to get an array of clusters which represent the input limits of Analog Output Waveform Generation VI.

Analog output Waveform Generation VI can configure its input to Scaled Array or Waveform data type, in our application we select the Scaled Array option, so we use the build Array VI to get the control Waveform ,and show it using the control signal waveform graph.

If an error occurred, the General Error Handler VI returns a description of the error and optionally displays a dialog box. This error is transmitted to the Unbundle Status VI to use with the stop button (when it is pressed) to give the option of stopping the VI.

At the end of the generation process, the control waveform signal passes to the motor.

REFERENCES

[1] Phase lock loops and frequency synthesis , Venceslav F. Kroupa, John Wiley and Sons LTD, 2003. Chichester UK. ISBN 0-470-84866-9.

[2] Ultrahigh speed phase/frequency discriminator . Analog devices, Inc,1999, AD9901, Noorwood, Ma02062-9106, USA.

[3] Phase Locked Loop techniques – A survey Guan-Chyun, James C. Hung, IEEE transaction on Industrial Electronics Vol 43, N 6, December 1996.

[4] Design Notes on precision phase locked speed control for DC motors, Unitrode Corporation- 1999, 7 continental BLVD, Merrimack, NH 030 USA.

[5] Phase locked loops: A control Centric Tutorial. Daniel Abramovitch, Agilent Albratories, communication and optics research Lab, 3500 DeeCreek Road, M/S:250-9 Palo Alto, CA 94304, USA.

[6] Speed Control DC Motor under varying load using phase-locked loop systems, Wisnu Djatmiko, Bamang Sutopo, Proceeding of international conference on electrical, electronics, communication and information. CECI.2001, March 7-8, Jakarta.

[7] Phase lock Loops for DC Motor speed control , Dana F. Geiger, Johan Wiley and Sons, 1981, New York, USA.

[8] Phase-Locked Loops, Design, Simulation and applications, Rolan E. Best, sixth edition, Mc Graw Hill, 2007 , New York, USA.

[9] Phase Locked Loop Circuit Design, Worceste Polytechnic Institute, Prentice Hall, 1991, New Jersey, USA, ISBN 0-13-662743-9.

[10] Phase Lock Basic, William F. Egan, PhD Conta Clara University John Wiley and Soms, INC. Publication, 2n edition,2007, USA. 

ROBOTIC CABLE INSPECTION SYSTEM USING MICROCONTROLLER AND GPS TRACKER

ROBOTIC CABLE INSPECTION SYSTEM USING MICROCONTROLLER AND GPS TRACKER

ABSTRACT

This paper presents the model of a mobile robot that can inspect the conditions in an underground cable tunnel even in dangerous environmental conditions where human presence is harmful. Hence, this robot substitutes the inspection of a human in the underground cable tunnel. The robot does online inspection through the tunnel travelling from one end to other end and all the information about the underground tunnel with respect to temperature, presence of harmful gases, obstacles, fire accidents, failure in supply etc. is transmitted wirelessly to a device on the ground. Hence this knowledge of situation at tunnel sent by the robot enables us to estimate the danger level and accordingly plan the measures to address the problem in a fastest way possible. Though there were several approaches made to enable this online monitoring system, this paper presents the functioning of robot with a GPS tracker which gives the co-ordinates of the fault location and this helps us to locate the exact fault location reducing the time and work to replace the faulty part. This enables zero downtime of supply, interruption free supply.

KEYWORDS- GPS tracker, online inspection, robot, underground cable, zero downtime.

INTRODUCTION

Apart from generation, transmission of electrical energy stood as a challenging task due to various constraints since the inception of using electrical energy. It is common to have transmission lines held along the road side being mounted on poles. But the idea of having an underground cable to transmit power is also another alternative. Overhead lines are vulnerable to lightning strikes which can cause service interruption. Overhead lines use bare conductors and can cause damage if they break. They are considered to be unsightly as they mar the scenery of the landscape.

The maintenance cost of overhead lines is more and the voltage drop in overhead lines is more Hence, underground cable for power transmission stood as second thought apart from the overhead lines. There are several challenges to be addressed in implementing this technology. To make the tunnel environment safe enough for functioning and maintaining zero downtime of supply we need to check and correct the faults taking place in the underground cable. However, it is a difficult task to locate the fault in underground cable when compared to overhead lines. This mobile inspection robot is one endeavor to check online the condition of tunnel using mobile inspecting robot. This approach gained momentum and there are many experiments being conducted and tested for the online monitoring.

EXPERIMENTAL DETAILS

A. Equipment’s Used

1) Temperature sensor - LM35: The LM35 [4, 3] series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage over linear temperature sensors calibrated in ° Kelvin, as the user is not required to subtract a large constant voltage from its output to obtain convenient Centigrade scaling.

2) IR Sensor: The IR Sensor is a general purpose proximity sensor. Here we use it for collision detection. The module consists of an IR emitterand IR receiver pair as shown if Fig 1. The high precision IR receiver always detects an IR signal. The module consists of a 358 comparator IC. The output of sensor is high whenever it is in IR frequency and low otherwise. The on-board LED indicator helps user to check status of the sensor without using any additional hardware. The power consumption of this module is low. It gives a digital output.

 

Figure1: The IR Sensor Schematic

3) Gas sensor: Semiconductor type combustible gas sensor MQ2 is used to detect the presence of smoke and other harmful gases like methane. It can detect the concentrations in the range 300 – 10000 PPM. VH is the heater voltage and VC is the test voltage. VH supplies the working temperature to the sensor, while VC is used to detect voltage on VRL on load resistance.

 

Figure2: This is the basic Circuit of GAS Sensor

4) Discharge Sensor: The Fig 3 is the discharge sensor circuit which can detect the invisible fields of voltage which surrounds all electrified objects. It acts as an electronic "electroscope. “Regular foil-leaf electroscopes deal with electrostatic potentials in the range of many hundreds or thousands of volts. Its sensitivity is very high. Since "static electricity" in our environment is actually a matter of high voltage, this device can sense those high-voltage electrically charged objects at a great distance. If a metal object is lifted up upon a non-conductive support and touched against the sensor wire, the sensor can detect whether that object has an electrostatic potential of as little as one volt.

 

Figure3: The Discharge Sensor Schematic

5) Metal Sensor: Inductive proximity sensors operate under the electrical principle of inductance. To amplify a device’s inductance effect wire is twisted into a tight coil. This inductive proximity Sensor M12PNP has four components; the coil, oscillator, detection circuit and output circuit. The oscillator generates a fluctuating magnetic field the shape of a doughnut around the winding of the coil that is located in the device’s sensing face which is shown in Fig 4. When a metal object moves into the inductive proximity sensor’s field of detection, Eddy circuits build up in the metallic object, magnetically push back, and finally reduce the Inductive sensor’s own oscillation field. The sensor’s detection circuit monitors the oscillator’s strength and triggers an output from the output circuitry when the oscillator becomes reduced to a sufficient level.

 

Figure4: Inductive Sensor Schematic

6) Hall Sensor: The hall voltage produced is due to the creation of electric field by the separation of electric charges in an external magnetic field. That means whenever electric charges on this Hall sensor W49E are in the external magnetic field due to the current flowing through the cable they experience a force and get aligned on either sides creating a magnetic field and hence the hall voltage.

V = R (I * B)/t

V is the hall voltage, I is the current through the hall sensor, B the external perpendicular magnetic field and t the thickness.

 

Figure5: Basic Hall Effect Sensor

7) Wireless transceiver (RF CC2500 Wireless module): This High Speed CC2500 Based Wireless module is a plug and play replacement for the wired Serial Port (UART) supporting baud rates up to 38400.Commercially available Remote Control (R/C) units use small microcontrollers in the transmitter and receiver to send, receive and interpret data sent via radio frequency (RF). The receiver box has a PCB (printed circuit board) which comprises the receiving unit and a small servo motor controller. RF communication requires either a transmitter matched/paired with a receiver, or a transceiver (which can both send and receive data). RF does not require line of sight and can also offer significant range (transmission distance). Standard radio frequency devices can transfer data between devices as far away as several kilometers and there is seemingly no limit to the range for more professional RF units. The robot used is made semi-autonomous robot with RF capability since it allows the robot to be as autonomous as possible, provide feedback to a user and still give the user some control over some of its functions. Range 60+ meters, Line of Sight 30 meters range indoors. Direct Replacement for wired Serial Cable for and serial communication can be achieved.

8) H-bridge: While controlling the speed of a DC motor with a single transistor has many advantages it also has one main disadvantage, the direction of rotation is always the same, it’s a "Uni-directional" circuit. H-bridge circuit arrangement and this type of circuit will give us "Bi-directional" DC motor control as shown below.

 

Figure6: H-bridge circuit diagram

The Transistor or MOSFET H-bridge as shown in Fig 6 is probably one of the most commonly used type of bidirectional DC motor control circuits which uses both NPN and PNP in each branch with the transistors being switched together in pairs to control the motor. Control input A operates the motor in one direction i.e., Forward rotation and input B operates the motor in the other direction (Reverse rotation). Then by switching the transistors "ON" or "OFF" in their "diagonal pairs" we can achieve directional control of the motor.

For example, when transistor TR1 is "ON" and transistor TR2 is "OFF", point A is connected to the supply voltage (+Vcc) and if transistor TR3 is "OFF" and transistor TR4 is "ON" point B is connected to 0 volts (GND). Then the motor will rotate in one direction. If the switching states are reversed so that TR1 will be "OFF", TR2 will be "ON", TR3 is "ON" and TR4 is "OFF", the motor current will now flow in the opposite direction causing the motor to rotate in the opposite direction. Then, by applying opposite logic levels "1" or "0" to the inputs A and B the motors rotational direction can be controlled as follows.

9) DC motor: 60RPM 12V DC geared motors for robotics applications. It gives a massive torque of 38Kgcm. The motor comes with metal gearbox and off-centered shaft Featuresof the motor are -60RPM 12V DC motors with Metal Gearbox and Metal Gears, 18000 RPM base motor, 6mm diameter shaft with M3 thread hole, Gearbox diameter 37 mm., Shaft length 15mm,180gm weight, 38kgcm torque, No-load current = 800 mA, Load current =up to 7.5 A(Max)

10) Video& Audio (JPEG Color Camera Serial UART Interface): New generation serial port camera module can capture high resolution pictures using the serial port. It is a modular design that outputs JPEG images through UART, and can be easily integrated into existing design. The infrared feature has a built-in sensor to sense the ambient light and will automatically turn on the infrared LED.

 

Figure7: Camera used.

11) Navigation GPS (Tracking Module): The Fastrax UP501 is a GPS receiver module with embedded antenna and tiny form factor 22.0 x 22.0mm x 8mm. The Fastrax UP501 receiver provides very fast enhanced navigation accuracy by utilizing WAAS/EGNOS corrections, which may be enabled via NMEA command. The Fastrax UP501 module provides complete signal processing from internal antenna to serial data output in NMEA messages. Fastrax UP501D.

The Dual-SAW filter is targeted for telematic applications where a radio transmitter is placed close to the GPS receiver. The dual filter design will provide higher attenuation outside of the GPS band and it helps to reduce the risk of EMC issues that are sometimes present when high-gain systems (GPS receiver) that are in strong signal field. National Marine Electronics Association NMEA is a standard protocol, use by GPS receivers to transmit data. NMEA 0183 sentences are all ASCII.

12) Microcontroller: The board is based on the ATmega320.Microcontroller it has 54 digital input/output pins (of which 14 can be used as PWM outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started.

13) Programming: Program is developed to make the data obtained from the controller into the computer to be user friendly. This program is built on java language and it converts the raw data into graphs, tables, danger indicating marks etc. This helps us to understand the data in a very easy and a fast way.

B. Construction and Functioning:

The First step of this project is constructing a platform to carry all the above discussed sensors integrated into circuits into the tunnel. A 9mm plastic sheet is cut into a hexagonal shaped platform.

 

Figure8: Hexagonal base plane of Robot

Two 10 RPM motors are fixed at the back end of the platform to drive the robot Chains are installed onto these wheels to enable the even distribution of robot’s weight and to reduce the slip value All the sensors, transceiver, GPS tracker, H-bridge and power supplying battery are connected to the microcontroller according to the pin diagram. Each sensor is tested by a sample program to make sure of its functioning. Then the GPS module is connected to the transmission and receiving pins of the microcontroller and the tracker is tested. The presence of multisensory and additional devices may lead to drain voltage conduction. To protect the micro controller from this, external voltage is supplied besides which an external hardware Arduino Sensor Shield V.5 is used.

 

Figure9 Block Diagram

The microcontroller has only two pins for external communication (TX and RX) which are to be accessed by both the GPS tracker and the wireless transceiver. To attend this need the serial port software is used to convert the digital pins (PWM pins) into communication pins (TX, RX) which are connected to transceiver. Now the transceivers are tested.

They are programmed for a single channel number and device ID and then loaded. These are run and checked on run mode configuration.

All the above circuit elements are integrated into the circuit to their final positions and after testing the whole circuit is placed on the robotic base. Final testing of all the setup is done. The robotic platform is free to move in any of the direction and can move inside the tunnel with ease. This is placed in the virtually created underground cable environment in our college laboratory and the robot is passed through this passage.

The information regarding the environment around the robot is transmitted wirelessly to the wireless receiver and transmitter (transceiver) which is connected to a microcontroller. The data is thus transmitted by the transceiver on the robot to the transceiver connected to the microcontroller near the control unit. Microcontroller when interfaces to a computer the data can be seen on the computer screen in the form of graphs and tables.

 

Figure10: Operator Control

The robot can be controlled from the operator end when the robot cannot make a decision regarding the next step to be taken. It can thus be halted and brought back, when it is not safe for the robot to navigate in the tunnel. The signal from the transceiver near the operator is sent to the transceiver on the robot to control the robot functioning.

RESULTS

The results below are taken when the environment around the cable is safe and when artificially created faults are present their respective results are obtained. The result in Fig 11 is shown when the environment is safe and hence when all the sensors are in off state.

 

Figure11: All sensors are off

 

Figure12: Two faults occur at same time

 

Figure13: Four sensors are active due to faults

 

Figure14: Red color on the screen here represents the intensity of the fault and all the faults occur at same time

 

Figure 15: Output taken from the microcontroller before processing the data. This is the actual data from the micro controller which is taken as input to the computer and converted into graphs and pictorial representation for easy understanding using the processing software.

CONCLUSION

The aim of the project is realized by testing the inspection of a mobile robot in a virtual environment conducive in producing real time operating atmosphere of an underground cable which can accurately spot the fault point and can report the co-ordinates of the fault point, which is novel attempt by using a GPS tracker.

 

Figure 16: Robot carrying the Circuit Elements

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