Programmed control of mechatronical system for drills testing
VLADAS VEKTERIS, MINDAUGAS JUREVICHIUS, ALGIS DAKTARIUNAS
Department of MachineBuilding
VilniusGediminasTechnicalUniversity
Basanaviciaus 28a, Vilnius LT2009,
LITHUANIA
Department of Biochemistry and Biophysics
Vilnius University
Ciurlionio 21, Vilnius LT2009
LITHUANIA
Abstract: In this paper we present the mechatronical system for the drills testing. Both, apparatus and algorithmic parts of the system for control of hydro-mechanical actuators and for data acquisition from sensors and transducers are analyzed. Programmable adjustment of system parameters for obtaining more precise measuring results of a drill is explained. The testing results of the drills of different diameter that have been registered in the process of manufacturing are included.
Keywords: Mechatronical system, drill testing, algorithm, computer based data acquisition and control.
1Introduction
Problems of programmable control of industrial machines and estimation of quality of products in the process of manufacturing of industrial machines and tools are in the center of attention of engineers and scientists [1, 20]. The testing systems need to include flexible and programmable equipment [18] which lets to implement precise control of mechanical, hydro-mechanical actuators and supply correct data from various sensors or transducers. The equipment must be universal enough for reorganization of the system when investigation of additional parameters of the product or controlling of additional actuators are necessary. Producers in European countries to enhance the quality of drills use different test methods and equipment [2]. Quality of the drills can be enhanced using test systems, which evaluates static and dynamic parameters of strength in stage of manufacturing.
2 System for drills testing
The mechatronical system for drills testing consists of three main modules: a hydro-mechanical module, a module of electronics and a software module. Hydro-mechanical module (Fig.1) includes a hydraulic station 1, a protective hydro-valve 2, a transfusion valve 3, a pressure regulator with proportional control 4 , a hydraulic distributor 5, throttle-valves 6, a hydro-cylinder 7, a slide-block 8 and a drill breaking tip 9 (Fig.1). Electronics module (Fig.2) includes a force sensor 10, a displacement transducer 11, computer 12 with a printer 15, multifunction data acquisition card (MFDAQ) 13 and a card of power amplifiers and event counters (PAEC) 14. Software module include special program "Graztai" for control of all devices of the system and for representation of parameters of the drills as graphics and for saving experimental data into the file.
2.1 Electronics module of data acquisition and control
Hydro-mechanical devices of the system are controlled by electrical signals applied from control module. Programmable electronic devices such as multifunction a data acquisition and control (MFDAQ) card, the power amplifiers and event counter (PAEC) card are installed inside the computer. Block scheme of electronics module of a system is shown in Fig.2.
A multifunction data acquisition card is used for collecting of the data as analog and digital signals and also for generating of electrical signals by using a computer. The card implements these functions:
Fig.1. Simplified scheme of mechatronical system for drill testing.
1.Parallel, programmable input/output of digital information;
2. Programmable timer;
3. Conversion of digital code to analog voltage;
4. Conversion of analog signal to digital code;
5. Analog signal amplification by using programmable gain amplifier.
Block scheme of multifunction data acquisition card [9, 12, 16] is shown in the Fig.3. The card includes an address selection block, buffers for data and address buses, which provide compatibility of data, address and control buses of computer with internal bus of the card; a control register, which is used to write information about data acquisition card configuration, status register, which indicates a work of a timer and analog-digital converter (ADC), a quartz oscillator, which generates an exact signal of frequency of 2 MHz for using by timer; the timer [3, 15] includes three programmable counters of 16 bits each; a module of analog-digital conversion (ADC), which consists of first and second multiplexer [12], differential and programmable amplifier, control chain and analog-digital converter (ADC) [11]; a module of digital-analog conversion (DAC) [13], which consists two digital-analog converters (DAC); a programmable device for parallel input/output of digital information [4, 15] and a circuit to generate signal of interrupt .
The main electronic part of the testing system is multifunction data acquisition and control (MFDAQ) card which synchronizes all devices and processes during system work. Devices of the card [18] are used for adaptive amplification and registration of analogue signal which is proportional to a force [8] applied to the break tip, generate analogue signal of low power for control of pressure regulator (for the valve of proportional control). The card can be used to generate digital control signals of low power for hydraulic-mechanic valves (this function depend on selection in the card of power amplifiers and event counter).
Fig.2. Block scheme of electronics module for drill testing.
A card of power amplifiers and events counters (PAEC) implements specific functions for industrial applications, such as follows:
1. Amplification of low level signal and generating of precise excitation voltage for sensors with sensitive elements of type of Wheatston bridge;
2.Amplification of power of analog signal [6], maximum to 120 W;
3.Amplification of power of digital signals;
4.Counting of impulses (events).
PAEC card includes a block of address selection, which forms signal about selection one of device of the card, a data buffer, a control register, which is used for internal control of digital signal amplifiers, programmable counters, an amplifier of low level signal which coefficient of the gain is selectable byswitch, which generates excitation voltage, selectable by switch, a power amplifier of analog signal for amplification of power of the signal from digital-analog converter of MFDAQ card, a digital signal amplifiers with optically isolated inputs and relay in outputs (Imax= 5A) which are used to switch electrically controlled valves of distributor, a logic circuit for input of differential digital signals.
The amplifier of low level signal [7] gets input from load cell and intensifies the signal magnitude by defined coefficient (10,100, 200, 500). Output signal of the amplifier is passed to analog input of MFDAQ card , but in noisy environment it can be processed by filter of low frequency. Amplifier generates excitation voltage, which can be selected by switches as 10V, 5V, 2,5V or 1,25V, for excitation of circuits the load cell.
As source of energy for power amplifier [6] is computer power supply block, so maximal amplifier output voltage is +10V, current - 3,3A. Using the power supply source of higher voltage the output current can be increased. Output signal of amplifier is used to control the pressure regulator.
Inputs of digital signal amplifiers with the relays DA1-DA4 in outputs are optically isolated with sources of the signal (with internal parallel register or MFDAQ digital input/output register). Outputs signals of relay are used to switch the hydraulic-mechanical valves according to the program.
Linear displacement transducer returns to the PAEC card the signals which represent a displacement of the tip of breaking , direction of movement and etc . These signals is processed in logic circuit and are passed to the input of event counter [3]. The content of event counter (32 bit) is a number of impulses which have been registered during movement of the tip of breaking, so production of the number of impulses to the constant of transducer of displacement indicates precise position of the tip.
3. Control of mechatronical drill testing system
The drill testing system can work in two modes: in mode of manual control and in mode of programmed (automatic) control.
3.1 Manual control
The valves of distributor and valve of transfusion which control the oil flow to hydraulic cylinder are switching by using buttons installed in the desk of the stand in manual control mode (see Fig. 1). This mode is implemented applying simple logic device mounted in the control module and is used for testing of the hydraulic-mechanic system. The pressure of the oil applied to hydraulic cylinder is constant and is defined by electric current supplied from control module.
3.2 Programmed control
Initially to explanation of programmed control mode, it is necessary to describe the algorithm of operation of the hydraulic- mechanical system (see Fig.1). A hydraulic station 1, if the transfusion valve 3 is closed, supplies the oil through pressure regulator 4, one of two valves ("UP" or "DOWN") of hydraulic distributor 5 to hydro-cylinder 7, which force the sliding block 8 to move. On the sliding block 8 is mounted a load cell 10, as a force sensor, a photoelectric displacement transducer 11 and a drill breaking tip 9. Procedure of drill testing can be described as follows: by applying oil pressure to input "UP" of hydraulic cylinder 7 the sliding block 8 is forced to move to upper initial position and then stops. Then program define initial pressure of the oil in pressure regulator 4 and the valve "DOWN" of distributor 5 is switched on. The oil flows into the inlet "DOWN" of hydraulic cylinder 7 and is moving sliding block 8 down. When the break tip 9 reach a drill, the electric current supplied to the pressure regulator 4 is increasing until the breaking of the drill occur (pressure in hydraulic cylinder need to be increasing). During procedure of drill breaking the force applied to the drill and the displacement of the break tip 9 are continuously measuring. When drill is broken, the valve "DOWN" of distributor 5 is switched off and the valve "UP" is switched on. Then the sliding block 8 is lifting to reach the initial position, transfusion valve 3 is switched on, pressure inside of hydraulic cylinder 7 decreases. The hydraulic station 1 returns on into the idle mode.
The hydraulic-mechanic system for drills testing is controlled by the program "Graztai" [17, 19]. The program consists of three subprograms which are used for system control and measurement, data visualization and analysis and for adjusting parameters of testing.
By selecting subprogram "Measuring" the system is controlled according parameters defined by operator in initial tests. These parameters are loading from appropriate file before drill testing .
Subprogram allows to choose one of three regimes- "Control testing", "Measurement", "Finish". The first regime is useful for testing of electronics when hydraulic-mechanical device is controlled by computer with commands chosen manually, so we can test hydraulic- mechanic valves and all electronics, except the data registration part. The main is regime "Measurement", which is used for full programmed control of hydraulic mechanical devices, data acquisition, visualization of drill testing procedure and data saving. Let we explain actions of the system in procedure of testing (see Fig. 3). After starting the regime "Measurement" the system is setting into initial stage: valves "UP" and "DOWN" of distributor are switched off, valve of transfusion is switched on, the electric current in coil of pressure regulator is low (DAC code is zero, power amplifier voltage is zero), a gain coefficient of MDAQ programmable amplifier is set to 1.
Fig.3. Diagrams of control signals and signals from a load cell and in outputs of the displacement transducer in programmed control mode: a, b –in valves of distributor, c- in transfusion valve, d- voltage in DAC output, e- pressure in hydraulic cylinder, minus sign mean that oil is passed into inlet of cylinder which moves sliding block up; f- load cell output signal, g- contents of the event counter.
The program loads parameters file and, according values defined in the file, sets up initial parameters for procedure of testing. During this setup procedure first is setting initial codes of a scale and the codes of digital to analog converters (DAC), which define an initial pressure. We used 6.25 MPa initial pressure (DAC code 80), when scale code 127, but the same initial pressure can be achieved by using other combination values of scale code and DAC code. This is useful when necessary to obtain more accurate step for pressure increasing . After setting of electric current for pressure regulator, at a moment t0 is switching of a transfusion valve and is switching on a valve "DOWN" of the distributor (see Fig.6a) . The sliding block as a result of action of constant initial pressure of the oil in the hydraulic cylinder, begans to move down and procedure of initial data acquisition begans. Analog signal from output of a load cell is amplify by instrumental amplifier of PAEC card and, depending on a coefficient of a gain, which was defined in the file of parameters, is amplify by programmable amplifier in MFDAQ card additionally. Additional amplification is necessary to evaluate measurement of the force with maximum accuracy for drills of different diameters and to adapt the system to perform the test using a full scale of the analog-digital converter (ADC). The speed of measurements is defined by the sampling interval which was written in the registers of programmable timer from the file of parameters. Setting of this value is not critical, because during the test can be made more than 30,000 of samples, for example, if sampling interval 1 millisecond, then drill breaking process recording procedure can last 30 seconds. Before the break tip reach surface of the drill, the program periodically, with a frequency defined by time interval, performs measurement of voltage from load cell and calculations of average value of the voltage. Average value (UAVE ) is used to define of zero level until the force is not applied to a drill. Threshold value is set as 0.01 part of full scale of measurement (UFS ) and procedure of data registration will be begin when UADCUAVE + 0.01UFS. Calculation of average value of output signal from the load cell when the force is not applied enables to adapt the measurement procedures to the signal level shift which can occur by changing signal gain coefficient or as result of other factors. In addition, necessary to estimate that maximal output signal of a load cell is 21.12 mV for full load of 2500 kN only [8], so in experiments where applied force is small, the high coefficient of amplification is necessary. The scale of force measurement is defined by gain coefficients of the instrumental and programmable amplifiers by using calibration curve for the load cell. During the test procedure the scale is changing by setting the gain coefficient of programmable amplifier. In situation when the drill was not inserted or it is necessary to stop the test, operator can use "ESC" button or the program will be stoped automatically, when contents of event counter, which is connected to displacement transducer, reach given number, which means that distance to the drill is too long. The main measurement loop will began when the break tip will touch the drill and voltage from load cell will reach threshold (Fig.3f). From moment t1 analog voltage from the load cell and content of event counter are reading and displayed into the graphic. At the same time periodical increasing of code of the DAC begans (Fig. 3d). Output voltage via a power amplifier control a pressure regulator and increase the oil pressure in the hydraulic cylinder (Fig.3e). It is possible to change a speed of increment of the force applied to the drill by changing a number of samples which will be omitted before increasing DAC code. Procedures of incrementation of applied force and measuring lasts until drill is breaking of (Fig.3f, Fig.3g) or until the force stay in maximal value during defined number of time intervals. Such situation can occur when diameter of the drill which is testing exceeds the defined maximum. System is simply transformed into more powerful, it is necessary to increase a power of hydraulic station and change the load cell with appropriate nominal range. After drill is breaking of, the voltage in output of the load cell is decreasing and the content of event counter is increasing (Fig.3f, moment t2 ) . If the voltage drop down more than 50 % of maximal value or a content of event counter exceedes defined limit, then registration is finished. At moment t3 (Fig.3e, Fig.3f) the procedure of data registration is finished, the valve "DOWN" of the distributor is switched of and valve "UP" is switched on (Fig.3a, Fig.3b). The program changes the code of the DAC to the initial value (Fig.3d) so that a pressure (Fig.3e) in the hydraulic cylinder be enough to move a slide block up to initial position only. Experimentally had been set DAC code equal to 100 (7.8 MPa) for reverse moving of sliding block in to the initial position. During tests have been obtained that 3 seconds of time are enough for sliding block to reach initial position. Then (t4) the valve of distributor is switched of, the DAC code is set to zero, a transfusion valve is switched on (Fig.3b, Fig.3c, Fig.3e) and hydraulic system is set into the idle mode. New test can be performed after operator will change the drill.
3.3 Adjusting of parameters for system control
According description above for programmed control are used the number of parameters that are saved in the parameters file. We had associated the file names with a diameter of the drills , so by defining drills diameter in main program, an operator chooses corresponding parameters for control and measuring. We need explain meanings and possible ranges of parameters:
Time interval- define sampling frequency, and is used for control of all devices of the system;
Number of time intervals defines duration of measurement procedure as a number of time intervals and show maximal number of samples of measurement which will be made during drill testing procedure;
Number of channels defines number of used channels in testing procedure. In experiments described below is used one analogue and one digital channel, but without modification of the system a number of channels can be increased;
Coefficient of a gain of programmable amplifier which is implemented in MFDAQ defines range a of a signal from load cell. By changing this parameter the range of force measurement is adjusting to achieve maximum accuracy for the drills of different diameter (Fig.4 c). It is possible to select- 1, 2, 4, 8, 16, 32, 128, 1024.
DAC scale define maximal possible current applied to pressure regulator or maximal pressure (if DAC_scale code127, in hydraulic cylinder can be reached 40 MPa maximal pressure) (Fig.4 a),