RU2164845C1 - Method of control cycle of article rotation surfacing - Google Patents

Abstract

FIELD: mechanical engineering; rotation machining of articles. SUBSTANCE: method relates to surfacing of exhaust valves of internal combustion engines by powder alloys. Method provides measuring of active values of processing parameters, comparing the parameters with rated values and, in case of their differences, adequately correcting process of surfacing. In process of surfacing, parameter characterizing heat conditions is calculated and is used for correction using ratio of heat introduced into article within preset time to angular displacement of article within this period as correction parameter. EFFECT: improved stability of process of article surfacing. 2 dwg

Description

 The invention relates to the field of plasma surfacing (welding) and can be used for machining rotation parts, mainly the seating surface of the chamfer of exhaust valves of internal combustion engines with powder alloys with desired properties in the conditions of machine-building and repair enterprises of various industries.

 Achieving a high degree of reproducibility of the process parameters without operator intervention is extremely necessary, since a number of destabilizing factors affect the surfacing process: a large level of signal ripples in the control and measurement circuits, a change in the network voltage, a change in the length of the arc during the cycle and from cycle to cycle due to inaccuracies in the manufacture and basing of the workpiece, etc. When the parameters fluctuate, the arc power and, accordingly, the amount of heat transferred to the part change, which negatively affects the weld formation, process stability and reproducibility of the results. To obtain high quality of the formed layer during surfacing, stable dimensions of the deposited bead and the zone of thermal influence, minimal heating of the base metal, it is necessary to change and precisely control the process parameters during the surfacing cycle.

 To solve such a problem in real time is possible only with the use of automation based on modern computer technology.

 The use of a computer in an automated control system (ACS) for a plasma-powder surfacing cycle allows the use of universal equipment with such software that is as close as possible to the psychology of a process engineer, facilitates the selection of optimal modes and makes it possible to carry out operational control of process parameters.

 ACS can "continuously" control the output parameters (current, arc voltage, welding speed, filler material consumption), providing the necessary thermal power released on the arc, taking into account the amount of molten material, the speed of rotation of the part, its size and gradual heat saturation. This is especially important when welding annular welds onto small workpieces, since the process proceeds quickly in time and does not always manage to reach a quasistationary state. During the surfacing cycle, the unevenness of heating is enhanced by the application of thermal fields from the action of the heating source from the beginning of the cycle to its completion.

 Modes for various workpiece sizes can be stored in the computer’s memory, with various heating programs, including the start of surfacing, controlling the amount of heat in the surfacing process depending on the size of the workpiece, the thermophysical properties of the materials used and the surfacing conditions, etc.

 A known control system for the welding cycle, including monitoring the state of the process by measuring the welding current and arc voltage, comparing the values of the welding current, arc voltage and welding speed with their corresponding threshold values, establishing the presence of deviations of these values from the norm during the welding process, giving a signal about malfunctions, if at least one of these deviations exceeds a threshold value that controls the amount of energy on the arc (current, voltage) and continuously records the magnitude of the current, voltage and welding speed [1].

 The disadvantage of this method is that it evaluates the individual electrical parameters characterizing the thermal characteristics of the process. This increases the error in estimating the thermal parameters of the process, complicates the work of the operator, since the control is carried out separately for each parameter.

 A known method of controlling a plasma-arc surfacing, involving applying to the surfaced product at least one weld seam (roller) from the surfacing material using a plasma arc with the plasma moving relative to the product at a speed of one and a half to fifty meters per minute, while the plasma arc power choose one that ensures penetration of the surface of the product to a depth of not more than 0.5 mm, and grain growth in the melting zone does not exceed 10% without cooling the product during surfacing and [2].

 In this method, the criteria for assessing the permissible degree of heating of the workpiece are defined and the dependencies that allow calculating the surfacing modes are described, however, the method for monitoring the thermal state of the product during surfacing and the possibility of uniform heating of small-sized parts are not defined.

 A known method for controlling the surfacing cycle, taken as a prototype, involves measuring the actual value of the output power, comparing this value with the reference value of the power read from the memory, adjusting the reference signal to match the actual value of the power and the reference value of the power, and controlling the output power of the surfacing using the adjusted reference signal [3].

 In this method, if the correction value exceeds the set value, an alarm is issued and a command to suspend the surfacing process is issued, and when comparing the actual value of the output power and the set power, the reference signal is stepwise adjusted at constant time intervals for each set value.

 Evaluation and adjustment of power during the surfacing process allows you to more accurately and efficiently estimate the amount of heat entering the billet, compared with the method [1], but this method is convenient only when surfacing with a constant speed. When surfacing with a variable speed during a cycle, it is necessary to take into account the ratio of power to surfacing speed, since changing the surfacing speed requires adjusting the power, and knowing the ratio of the thermal power to the welding speed (surfacing) - linear energy, expands the possibilities of evaluating the thermal state of the workpiece and allows more accurately predict the shape of the weld bead.

 The problem to which the proposed method is aimed is to ensure the stability of the plasma surfacing process of the exhaust valves by increasing the reproducibility of the parameters characterizing the thermal characteristics of the process and facilitating the work of the operator.

 It is solved by the fact that in the method for controlling plasma surfacing of rotation parts with parameters varying during the cycle (arc current, arc voltage and speed of movement of the part), a parameter characterizing the thermal regime is calculated and used for correction. In this case, the specific amount of heat per unit of the angular displacement of the part is used as this parameter - an analog of the linear energy for the parts of rotation, and to determine it, the ratio of the amount of heat received in the part over the time interval to the angular displacement of the part over the same interval is calculated.

 The time interval for which the surfacing parameters are calculated and corrected is chosen as a compromise between the smoothness and speed of the control system on the one hand and the instability of the electrical parameters of the surfacing and interference in the measurement system on the other hand. If the surfacing parameters (arc current, arc voltage) would not have ripples, and the signals from the sensors would not have interference, then the time interval could be reduced to the time of passing one measurement and control cycle in the computer program. In practice, pulsations of the surfacing parameters themselves (currents and voltages) and pulsations in the measuring sensors reach tens of percent. If network (50 Hz) power supplies are used without intermediate frequency conversion, then in this case it is convenient to use intervals that are multiples of the network frequency, i.e. 20 ms, 40 ms, etc. as a time interval. Since the average value of interference in this case is equal to zero, they do not “swing” the control system.

 The conversion of information on input parameters into information on a parameter characterizing the thermal regime of the surfacing process allows the transition from three parameters: current, voltage and speed of rotation of the workpiece, to one characterizing the thermal regime. This allows to increase the accuracy of estimating the amount of heat transferred to the workpiece, and to facilitate the work of the operator during operational control.

 The proposed method is illustrated by the drawings shown in FIG. 1 and 2.

In FIG. 1 shows a functional diagram of a device that implements a method for controlling a surfacing cycle;
in FIG. 2 is a diagram of stresses at points 1 and 2 indicated in FIG. 1.

 The device contains a first digital-to-analog converter (DAC) 1, specifying the amount of deposited powder, a second DAC 2, controlling the speed of rotation of the workpiece, a third DAC 3, setting the current of the main arc, a fourth DAC 4, setting the current of the auxiliary arc, the first analog-to-digital converter ( ADC) 5, measuring the value of the main arc current, the second ADC 6, measuring the value of the auxiliary arc current, the third ADC 7, measuring the voltage value of the main arc, the fourth ADC 8, measuring the value of the auxiliary arc voltage, sensor 9 angles of rotation of the motor shaft, a generator of 10 clock pulses, a first counter of 11 pulses, a second counter of 12 pulses, a digital input / output device 13 for ISA bus, a computer 14 PC, a third counter 15 with a division ratio of 2, an inverter 16, a fourth counter 17, fifth counter 18.

 As the angle sensor 9, you can use, for example, an optical sensor angle code BE-178A5 Z = 1000, which generates 1000 pulses per revolution. This sensor has a separate output giving a zero mark (one pulse per revolution), which is connected to the digital input of the PCL-720 board. Before starting work, the computer rotates the workpiece by one revolution and constantly monitors this digital input, the appearance of a pulse at this input is used for the initial binding of the angle of rotation of the workpiece.

 The generator 10 clock pulses is a generator with a quartz resonator, and counters 11, 12, 17 and 18 are programmable timers type 580VI53.

 The digital input / output device 13 is located inside the computer and is installed in the ISA slot. The remaining devices can be located both inside the computer (on the circuit board of the device 13 digital input / output), and outside it, depending on their design. As the device 13, for example, a PCL-720 digital input / output board can be used, which in addition to the digital inputs and outputs also includes an address decoder and a buffered bus that allows you to connect various digital devices with an 8-bit data bus. The board also has an empty mounting field that allows you to install (solder) various microcircuits.

 The method is as follows. Before starting work, the technologist creates a table-file in which the first column is the value of the angle of rotation of the workpiece at the approximation points, and the other columns are the values of the controlled parameters at the nodal points - the task of the powder flow rate, rotation speed, thermal power and auxiliary arc current. When the control program starts, data from the file - table are approximated and coefficients of approximating expressions are calculated in advance. To reduce the influence of interference on the measurement error, the ADC conversion time of 5, 6, 7, and 8 is selected as a multiple of the frequency of the network interference, and in the case of using 50 Hz network power supplies without intermediate frequency conversion, it is chosen equal to 20 ms. The counter 12 is programmed into the frequency divider mode, and its division coefficient is set so that pulses are generated at its output, the frequency of which exceeds the frequency of the pulses from the angle sensor 9. Given that the surfacing speed does not exceed 1 revolution per second and the number of pulses of the sensor 9 does not exceed 1000 per revolution, the pulse frequency at the output of the counter 12 can be chosen equal to 1 kHz. For example, if the frequency of the generator is 10-1 MHz, then the division ratio of the counter 12 is set equal to 1000, which leads to the generation of interruptions with a frequency of 1 kHz in the computer. The interrupt number is selected based on the configuration of the computer and the resources installed in it.

 At the output of the sensor 9, during rotation of the workpiece, pulses are generated (see Fig. 2, diagram 1), the duty cycle of which may not be exactly 2, so these pulses are sent to the input of the binary counter 15 (microcircuit 561ie10), from which pulses are already taken with a duty cycle strictly equal to 2 (see figure 2, plot 2). If there is a logical unit at the output of the binary counter 15, the counter 17 will work, and if there is a logical 0, the output of the inverter 16 will be the logical unit and the counter 18 will work. If you wait for the state of the signal at the output 15 to change, you can read in one of the counters 17 or 18 is a number proportional to the duration of the pulses from the sensor 9, and, knowing the frequency of the generator 10, calculate the duration of one period from the sensor 9, that is, the current (within one pulse of the sensor 9) speed of rotation of the workpiece.

In the process of surfacing, after 1 ms, the interrupt processing routine is called in the computer in which it is carried out:
- reading the contents of the counter 11 and calculating according to its contents the current angle of rotation of the workpiece;
- reading the output status of the counter 15 and, if its contents have changed compared to the previous interrupt from 1 to 0, then the data is read from the counter 17, when the state changes from 1 to 0, the data is read from the counter 18. The current rotation speed is calculated. If the change of state does not occur, then only its state is remembered until the next interruption;
- checking the readiness of the data in the ADC 5,6,7,8 (whether it took 20 ms after the start of the ADC) and reading, when the data is ready, the contents of the ADC 5,6,7 and 8. Next, the current values of the corresponding surfacing parameters are calculated, including effective heat output and the next launch of the ADC;
- Calculation of new values of currents, speed of rotation and amount of powder using pre-calculated approximating expressions for a new value of the angle of rotation;
- entering the appropriate codes in the DACs 1,2,3 and 4;
- checking for compliance of the measured values with the values calculated for a given current angle, and in the case of an unacceptably large deviation, a message about this in the background program.

 Thermal power is controlled by setting the arc current. In advance, for a particular equipment, a relationship is determined, for example, in the form of a polynomial of the second degree, between the power released on the arc (electric, effective or thermal) and the specified arc current. In the surfacing process, the obtained dependence is used to embed the required amount of heat into the part by setting the arc current and, in addition, the real pairs of the set current and the measured power are stored. After the next surfacing is completed, the dependence of power on current is recalculated based on new data. Thus, possible sensitivity deviations of the DACs, power supplies, changes in the surfacing conditions (arc length, electrode wear, etc.) are automatically taken into account. This allows iteratively approaching the given dependences of the surfacing parameters even under the conditions of destabilizing factors.

 In the background program, if necessary, the current measured values are written to a file, the emergency digital inputs are monitored (such as an accident in the power supply, lack of burner cooling), the deposition process is visualized on the display in the form of graphs of the set and measured values. The deviation of the measured values from the set values, at which surfacing is stopped, is set separately for each parameter before surfacing in the configuration file.

 When the specified rotation angle is reached, the surfacing is stopped and the obtained real dependencies of the specified parameters are analyzed.

 Comparison of the value of the thermal parameter with the nominal one allows one to stabilize the quality of the deposited layer due to more accurate dosing of heat input, since those that are specified by calculation and (or) experimentally, i.e., reflecting a specific statistical sample associated with a specific surfacing process.

 The implementation, in case of deviation of the thermal parameter from the nominal, process correction in the next surfacing cycle avoids mass rejection during surfacing in large-scale production.

Sources:
1. Japanese application N 2-148483, CL B 23 K 9/095, d.p. 02/12/92

 2. French patent N 2698572, cl. B 23K 10/02, d.p. 06.03.94 g.

 3. Japan Patent N 6000273 B4, CL B 23 K 9/095, d.p. 01/05/94

Claims (1)

 A method for controlling the surfacing cycle of rotational parts, including measuring the effective values of process parameters, comparing these process parameters with nominal values and, if their values deviate, adjusting the surfacing process, moreover, during the surfacing process, a parameter characterizing the thermal regime is calculated and used to adjust process, characterized in that as a parameter characterizing the thermal regime, use the ratio of the amount of heat received in the part for a given interval Al time, to the angular movement of the part for the same interval.

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