|Title of Invention||
AN IMPROVED LOOP SENSOR FOR MEASURING THE LOOPER ANGLE FORTHE PURPOSE OF STRIP TENSION REGULATION IN A HOT STRIP MILL.
|Abstract||AN IMPROVEE LOOP SENSOR COMPROSINMG OF AN ABSOLUTE ENCODER GEAR BOX OPTOISOLATOR CARD MICROCONTROLLER BOARD USING 89C52, OFFSET ADJUSTMENT CARD CAN BE USED FOR PROCISE MEASURMENT OF LOOPER ANGLE FOR THE PURPOSE OF STRIP TENSION REGULATION IN A HOT STRIP MILL. THIS UNIT SHALL BVE USED TO PROVIDE FEEDBACK TO THE STAND SPEED REGULATION SYSTEM. IT IS THUS POSSIBLE BYU WAY FO THIS SENSOR OF INVENTION TO MEASURE LOOPER ANGLE AND PROVIDING GRAPHICAL OUTPUT TO OPERATOR ALONG WITH STND MOTOR CURRENT AND LOOPER MOTORE CURRENT FOR BETTER REGULATION OF THE PROCESS. FIG 1.|
|Full Text||FIELD OF INVENTION
The present invention relates to a development of an improved sensor for measuring the looper angle. More particularly the present invention relates to a development of an improved sensor for measuring the looper angle with an accuracy of at least + 0.35°, using digital system in a hot strip mill which would enable to regulate inter-stand tension by a microcontroller based system.
In a Hot Strip Mill looper is used in between two finishing stands to provide a desired tension in the strip. There are six finishing stands (F1 to F6) in the Hot Strip Mill of Rourkela Steel Plant. Looper 1 (L1) is between F1 and F2. Looper arm is driven by a 37 kW armature controlled DC motor through a two stage gear with gear ratio 78/30 in the first stage and 90/22 in the second stage. Looper motor is driven by a four quadrant thyristor converter.
Strip speed regulation scheme provides an additional reference to the stand drive to keep the strip loop length (61) between adjacent stand constant. If looper angle of looper-1 increases, the speed of F1 will decrease to maintain looper angle constant and thereby excess length (51) constant.
The looper motor provides a constant tension in the strip irrespective of looper angle. The ratio of ( motor torque / Strip tension) is function of looper angle. Motor torque is directly proportional to armature current and field current. Field current is kept at constant value. Therefore, motor torque is directly proportional to armature current. The current regulator works at its current limit. Varying the current limit in the drive, motor torque can be varied.
In the earlier system looper angle was measured by potentiometer / selsyn based analog sensor which suffers from drawbacks / disadvantages as hereunder:
A. Potentiometer / Selsyn based measurement is analog where precise and accurate measurement is not possible.
B. The output voltage from potentiometer based sensor depends on the contact pressure. Thus with axial thrust, the output voltage changes consequently an erroneous feedback is given to the stand speed regulation system.
C. Repeatability is not achieved with analog type of sensor.
D. Communication with other computer is difficult.
It is thus basic object of the present invention to overcome the drawback of prior art and provide a sensor for precise measurement of looper angle with an accuracy of at least + 0.35° for controlling inter-stand tension in strip between two stands.
Another object of the present invention is to provide a sensor of the kind stated above which with additionally have a facility to provide a graphical feedback of the looper angle and its variation, on-line, throughout the length of the strip, to the operator.
Yet another object of the present invention is to provide a sensor of the kind stated above which is a low cost solution in an hot strip mill to have a facility of looper angle measurement and communicating the looper angle, looper motor current (Torque developed by looper motor) and stand motor current to a computer system and which is simple to operate.
Thus, according to the present invention the improved loop sensor for hot strip mill comprising :
means for measuring rotation ; means for limiting rotation ;
means for reducing the encoder out put;
a signal processing means ;
means for setting the base valves and changing voltage levels.
According to a preferred aspect of the invention the loop sensor comprises :
means for measuring rotation being a 10 bit absolute encoder;
means for limiting rotation being a gear box unit to limit the rotation within 360°;
means for reducing the encoder output being an Opto-coupler unit to convert the
encoder output from 24 V to 5 V ;
a signal processing means being a microcontroller based signal processing board ;
means for setting base valves and changing loop voltages being an offset adjustment
card for setting the base value and changing the voltage level.
The above system of the invention basically comprises of the microcontroller board which is adapted to receive 10 bit digital inputs from an absolute encoder and generating an analog voltage corresponding to the looper angle. This analog voltage is applied to the stand speed regulation system to maintain a specific loop. The system also receives analog inputs corresponding to stand motor current and looper motor current. These values are transmitted to a computer system to represent the looper angle, looper motor current and stand motor current in graphical representation, which enables the operator to take corrective action.
The loop sensor further comprises a stand speed regulation system and to which the analog voltage is supplying to maintain a specific loop. The stand speed regulation system being adapted to receive analog inputs corresponding to stand motor current and looper motor current and it is adapted to transmit values representing said stand motor current and looper motor current to a computer system to represent the looper angle, looper motor current and stand motor current in graphical representation. Importantly, the above loop sensor is adopted for use in hot strip mills for the purpose of strip tension regulation.
The details of the invention, its objects and advantages are explained in greater detail in relation to non limiting exemplary embodiments of the loop sensor of the invention in relation to the accompanying figures wherein Fig 1 shows the schematic diagram of encoder based loop sensor.
Fig 2 is module inter connection diagram of the loop sensor.
Fig 3 is the wiring diagram of optical isolation card for encoder signal processing.
Fig 4 is the circuit diagram of offset adjustment card.
Fig 5 is the relation between looper angle and the output voltage.
Fig 6 is the characteristics of digital to analog converter.
Reference is first invited to Fig 1, where the scheme of encoder based loop sensor is shown. The figure shows looper L5 between two finishing stands F5 and F6. A typical hot strip mill consists of six finishing stands named as F1, F2, F3, F4, F5 and F6. Looper L1 exist between stand F1 and F2. The looper arm goes up and down by a looper motor (37 kW, armature controlled DC motor). The looper arm is coupled with the looper motor with a two stage gear of ratio 78/30 and 90/22. The equivalent gear ratio is 10.63. The purpose of the gear is to limit the degree of rotation of the looper arm. The motor is driven by a thyristor converter with associated control system named as Simadyne - C drive.
The connection of potentiometer (POT) in the earlier system is also shown.
The modified system consists of (i) a gear box of gear ratio 117/11 (equivalent to 78/30 X 90/22), (ii) An absolute encoder of 10 bit resolution, (iii) 10 channel opto-isolator card, (iv) Microcontroller board, (v) Offset adjustment card, (vi) DC power supply of 24 V DC and (vii) DC power supply of + 5 V, + 12 V DC.
The output of the offset adjustment card is connected with the Simadyme C drive. The microcontroller also generates a serial output (RS 232) which is connected to a computer system. In the computer system the looper angle, looper motor current and stand motor current can be seen graphically along with instantaneous values. Simadyme C is an analogue type regulator, which controls the looper motor. This is well known to a person skilled in the art. The input to Simadyme C is the looper angle in the scale of 0 to 65° corresponding to 0-10V DC. Its output is fed to a thyristor converter, which feeds DC power to the 37 kW armature controlled DC motor to run the motor forward to reverse as per requirement. The looper arm is coupled with the motor through a two-stage gear ratio. Movement of looper arm controls the position of the loop.
Fig 2 shows the module interconnection diagram of the loop sensor.
The system comprises of the following:
(i) A gear box of gear ratio 117/11 (equivalent to 78/30 X 90/22),
(ii) An absolute encoder of 10 bit resolution,
(iii) 10 channel opto-isolator card,
(iv) Microcontroller board,
(v) Offset adjustment card,
(vi) DC power supply of 24 V DC and
(vii) DC power supply of + 5 V, + 12 V DC.
The gear box (AG) is having a ratio 117/11 as shown in Fig 1 & 2. The gear box is connected in the non driving end of the looper motor. The purpose is that any axial thrust generated in the looper shaft will not be transmitted to the encoder. The looper shaft is driven by motor through a two stage gear (G) of ratio 78/30 and 90/22. In order to get the same rotation as looper shaft, a gear (AG) of same ratio (78/30 X 90/22 = 117/11) has been put in the non driving end. The gear (AG) is coupled with the motor shaft through a coupling.
This is a device which gives an electrical output correspond to angular position of the shaft. The absolute encoder is connected as shown in the figure with the gear of 117 teeth. The encoder power supply is 10 to 30 V DC. This power is provided by power supply PS 2. The output is 10 bit grey code. In one complete rotation of the shaft (360°) there will be 210 different outputs. Thus, resolution is 360°/ 210 = 360° / 1024 = 0.35 ° . The grey code output is connected with the opto-coupler card. The opto-coupler card gives 10 bit TTL output (5V level) which is connected with the microcontroller board.
The purpose of this card is to provide optical isolation between encoder output and Microcontroller input. The encoder output is 24 V DC. This card transform the voltage to 5 V DC. Schematic diagram of the Opto-isolator card is shown in Fig 3. LEDs are provided in the input side of the opto-coupler. Opto-coupler is realized by IC MCT2E. Whenever the encoder output bit is high (24 V), the output transistor of MCT2E will be in saturation. Voltage output at Pin-4 will be 4.3 V. This is connected with Microcontroller board. Whenever the encoder output bit is low (0 V), the output transistor of MCT2E will be in cutoff. Voltage output at Pin-4 will be 0 V. The input bit of the Microcontroller will be shorted with power supply common through 1 k? resistance. The encoder power is fed through 24 V DC power supply PS-2. Pin 5 of MCT2E is connected to 5 V power supply from PS-1. The opto-coupler is for 10 channel for 10 bit output from encoder.
The 10 bit output from opto-coupler card is connected with the Microcontroller board. The board has been designed across ATMEL make Microcontroller 89C52. The AT89C52 is a low-power, high-performance CMOS 8-bit microcomputer with 8 Kbytes of Flash programmable and erasable read only memory (PEROM). The device is manufactured using Atmel"s high density nonvolatile memory technology and is compatible with the industry standard 80C51 and 80C52 instruction set and pinout. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. The features of this Microcontroller is as follows:
• Compatible with MCS-51 TM Products
• 8 Kbytes of In-System Reprogrammable Flash Memory
• Endurance: 1,000 Write/Erase Cycles
• Fully Static Operation: 0 Hz to 24 MHz
• Three-Level Program Memory Lock
• 256 x 8-Bit Internal RAM
• 32 Programmable I/O Lines
• Three 16-Bit Timer/Counters
• Eight Interrupt Sources
• Programmable Serial Channel
• Low Power Idle and Power Down Modes
The following subsystems are available on the board:
• Crystal 11.0592 MHz
• Real Time Clock DS 1287
• 8 channel analog to digital converter, -5 V to +5 V DC, 12 bit, using AD574
• 3 channel digital to analog converter, -5 V to + 5 V DC, 12 bit, using AD 7521 JN
• 3 channel timer/counter, 16 bit, using 8253
• RS 232 serial output using MAX 232
• RS 485 serial output using 75176
When the processor is working, it communicates with a PC regarding all the calculations
performed within it. The microcontroller board is having one RS 232 output using IC MAX 232. The board communicates with a computer using RS 232 type communication. Microcontroller board is having one analog to digital converter (ADC) using Integrated Circuit (IC) AD 574 (Analog Device make). This IC is a 12 bit successive approximation type analog to digital converter. The board is having capacity of 8 channel analog input in the range of -5 V to + 5 V DC. The analog input is multiplexed and fed to AD 574. The purpose of selecting AD 574 is for its fast response time (in the order of 36 (µsec). Out of the 8 channel analog input only two channels are used. The looper motor current and Stand motor current is fed in these channels. The microcontroller board is having three independent Digital to Analog Converters (DAC). These DACs are of type AD 7521 A, make Analog Device and are of 12 bit resolution. Output voltage from the DACs are -5 V to +5 V DC. The digital input pins of AD 7521A are obtained from two numbers of programmable peripheral interface (PPI) of type 8255, INTEL make. The analog output corresponds to looper angle is fed to the offset adjustment card.
The processor is having a watch dog timer. As long as the CPU is running and executing the application program a pulse is available in port 1.7 (pin No 8) of the CPU. This pulse from port 1.7 resets a counter. As long as the counter is reseted cyclically, counter reading can not go high. If pulse is not available from port 1.7 then counter reading goes to high value and this high value resets the CPU 89C52. The reset pin of 89C52 processor is pin # 9. This pin is normally kept at low value by a pull down resistance of 8.2 K?. A high, Logic "1", on this pin resets the processor. While the application program is running, if CPU halts or if it is in an infinite loop then the processor gets reseted by a Logic "V pulse available from counter.
Offset adjustment card
The output from the Microcontroller is -5 V DC to + 5 V DC. The offset adjustment card converts this voltage to 0 to + 10V DC. Schematic diagram of the Offset adjustment card is shown in Fig 4. Output voltage from Microcontroller is added with -5 V (obtained from POT) and the output is inverted. Thus, 0 voltage output from Microcontroller will give +5 V from this card. The addition of voltage is done by OPAMP 741. This card converts +5V to -5V output from Microcontroller board to 0 to +10V DC. Voltage at pot is set to -5V. Offset voltage at 10° is to be adjusted by this pot. Table 2 shows the relation between the inputs and outputs of the offset adjustment card.
Power supplies PS1
The power supply PS 1 supplies DC regulated power to Microcontroller board. PS-1 generates + 5 V, +12 V and - 12 V DC. The CPU 89C52 operates on + 5 V DC supply. ADC and DAC requires + 12 V DC supply. PS 1 also feeds DC power to Offset adjustment card. Input to the power supply is 230 V AC, Single phase, 50 Hz.
Power supplies PS2
The power supply PS 2 supplies 24 V DC regulated power to Encoder. Power is fed through terminal J1-12 and J1-11 as shown in Fig 3. Input to the power supply is 230 V AC, Single phase, 50 Hz.
Algorithm of angle measurement and analog output generation
The following tasks are performed by software in the microcontroller unit.
¦ Reading the grey code from opto-isolator card.
¦ Converting the grey code to binary number
¦ Calculation of looper angle from the binary number
¦ Calculation of output voltage based on looper angle
¦ Analog output from DAC
¦ Reading looper motor current and stand motor current.
¦ Transmitting looper angle, looper motor current and stand motor current to computer.
Each module is described below: Reading Grey code:
Grey codes are read through port P1.0(LSB), P1.1, P1.2, P1.3, P1.4, P1.5, P1.6, P3.2, P3.3, P3.4 (MSB).
Converting the grey code to binary number
Encoder output is in Grey code and this output is processed by first converting it to binary. The logic of converting 10 bit Grey code to Binary code is explained in this section. In Grey code there is only one bit change between two successive number. The procedure for Grey to binary conversion is explained below:
Let us consider a 10 bit number (as obtained from encoder), which is to be converted to binary form.
MSB in Grey is same as MSB in Binary i.e. B9 will be same as G9.
B9 = G9
B8 = B9 XOR G8 ( XOR denotes logical EXCLUSIVE OR operation)
B7 = B8 XOR G7
Bn = Bn+i XOR Gn
Therefore, Grey 0110001101 is equal to 0100001001 in Binary
Calculation of looper angle
The binary number obtained in the above step is converted to decimal number (N). The decimal number (No) correspond to the base value is subtracted from N. Thus, (N- No) gives a value correspond to the angle. This number when multiplied by 0.35 gives the angle in degree.
Calculation of output voltage
The speed regulation system requires a specific voltage depending on the angle. The relation is shown in Fig 5. For 8° of looper angle output voltage required is 1.53 V DC and for 65° of looper angle output voltage required is 4.50 V DC. This voltage may be varied in different looper and depends on finishing stand speed regulation scheme. The voltage at 8° can be adjusted by the pot as shown in Fig 4. It is required to adjust the voltage at site and hence the provision of POT has been kept.
The microcontroller gives 12 bit (0 to 4096 in decimal) digital output to the Digital to Analog Converter (DAC). The transfer characteristics of DAC is shown in Fig 6. If the input number 0, the DAC outputs + 5 V DC. If the input number is 4096, DAC outputs -5V. When the input number is 2048, the DAC output is 0 V.
Reading Analog inputs:
The microcontroller board reads looper motor current, stand motor current in -5V to +5 V DC signal level. This corresponds to 0 to maximum value of current. These two signal is transmitted to a computer system with RS 232 serial link.
Recording data in a computer:
The looper angle, looper motor current, stand motor current is shown graphically to the operator for necessary corrective action.
Thus as detailed herein before the improved loop sensor of the invention basically utilizes (i) Absolute shaft encoder, (ii) Gear box of suitable ratio, (iii) Optical isolation card, (iv) Microcontroller board, (v) Offset adjustment card, to measure looper angle and providing feed back to the stand speed regulation system
The optical encoder comprises
(i) A 10 bit absolute encoder with Grey code output.
(ii) A gear box of suitable ratio to restrict the degree of rotation within 360°.
The opto-coupler board comprises:
(L) One 10 channel opto-coupler. Input signal is 24 V DC and output signal is 5VDC.
The Microcontroller board comprising
(i) Processor 89C52 with 12 MHz crystal, 8 channel Analog to digital converter, 3 channel digital to analog converter, RS 232 communication link.
(ii) Analog output of-5 V to+5 V DC
(iii) Watch dog timer for automatic reseting.
The offset adjustment module comprising
(i) Circuit to convert -5 V to + 5 V to 0 to 10 V DC signal, (ii) A pot to adjust the voltage at base value by + 0.5 V.
The loop sensor of the invention thus provides
(i) An analog output corresponds to the looper angle as per requirement of
the speed regulation scheme, (ii) Communication with a computer system through serial port for recording
looper angle, looper motor current and stand motor current.
It is thus possible by way of the above discussed system of the invention to precisely measure the looper angle for the purpose of strip tension in a hot strip mill.
Also the system provides information in a computer for better monitoring of the process. The system avoids sudden excess looping or sudden tension in the strip.
The system of the invention is thus a cost effective solution looper angle measurement is a hot strip mill.
1. An improved loop sensor for hot strip mill to regulate inter-stand tension comprising :
means for measuring rotation ; means for limiting rotation ; means for reducing the encoder out put; a signal processing means adapted to:
i) read signal generated by said means for reducing the encoder output;
ii) calculate looper angle from said signal;
iii) calculate output voltage based on said looper angle; s
iv) read looper angle, looper motor current and stand motor current; and
means for setting the base values and changing voltage levels based on said readings."
2. An improved loop sensor as claimed in claim 1, wherein the means for measuring rotation is a 10 bit absolute encoder.
3. An improved loop sensor as claimed in claim 1, wherein means for limiting rotation is a gear box unit adapted to limit the rotation within 360°.
4. An improved loop sensor as claimed in claim 1, wherein the means for reducing the encoder output is an Opto-coupler unit adapted to convert the encoder output from 24 V to 5 V.
5. An improved loop sensor as claimed in claim 1, wherein the signal processing means is a microcontroller based signal processing board.
6. An improved loop sensor as claimed in claim 1, wherein the means for setting base values and changing loop voltages is an offset adjustment card.
7. An improved loop sensor as claimed in any one of the preceding claims, wherein said microcontroller board being adapted to receive 10 bit digital inputs from an absolute encoder and to generate an analog voltage corresponding to the looper angle.
8. An improved loop sensor as claimed in any one of the preceding claims, optionally
comprising a stand speed regulation system and to which the analog voltage is supplying to maintain a specific loop.
9. An improved loop sensor as claimed in claim 8, wherein said stand speed regulation system being adapted to receive analog inputs corresponding to stand motor current and looper motor current.
10. An improved loop sensor as claimed in claim 9, wherein said stand speed regulation system being adapted to transmit values representing said stand motor current and looper motor current to a computer system to represent the looper angle, looper motor current and stand motor current in graphical representation.
An improved loop sensor comprising of an absolute encoder, gear box, optoisolator card, microcontroller board using 89C52, offset adjustment card can be
used for precise measurement of looper angle for the purpose of strip tension
regulation in a Hot Strip Mill. This unit shall be used to provide feedback to the
stand speed regulation system.
It is thus possible by way of this sensor of invention to measure looper angle and providing graphical output to operator along with stand motor current and looper
motor current for better regulation of the process.
|Indian Patent Application Number||135/CAL/2002|
|PG Journal Number||47/2007|
|Date of Filing||11-Mar-2002|
|Name of Patentee||STEEL AUTHORITY OF INDIA LTD.|
|Applicant Address||RESEARCH & DEVELOPMENT CENTRE FOR IRON &STEEL, DORANDA, RANCHI- 834002 JARKHAND.|
|PCT International Classification Number||G O 8 6 21/00|
|PCT International Application Number||N/A|
|PCT International Filing date|