Microcomputer control for an industrial sewing machine

Microcomputer control for an industrial sewing machine The application of a 4-bit microcomputer to an automatic thread trimming sewing machine is described by Katsuhiro Fujikawa

The application of the microcomputer to various industrial machinery or equipment is expanding. The sewing industry is no exception to this trend, positively employing microcomputers for more advanced automation, labour saving and further industrialization. To satisfy recent needs, a sewing machine motor equipped with eddy current variablespeed clutch, LIMI-STOP Z, was developed. The L I M I STOP Z features, besides a friction-free low inertial clutch used instead o f the conventional friction type clutch, the use o f a 4-bit microcomputer in the control system. This paper describes the principle, composition and functions o f the LIMI-STOP Z.

The LIMI-STOP Z consists mainly of the following three parts: • motor with built-in electromagnetic clutch and brake • synchronizer that is directly coupled to the sewing machine shaft and detects shaft position and needle stop position • control box necessary to control the motor and the se,wing machine motion in response to operator's pedalling operation The overall composition of LIMI-STOP Z is shown in Figure 1. The industrial automatic thread trimming sewing machine where the LIMI-STOP Z is installed is shown in Figure 2 in its marketed appearance and its construction is schematically shown in Figure 3.

Figure 1. Overall view of LIMIT-STOP Z clutch is demagnetized and the electromagnetic brake is excited for a while. With a stop signal from the synchronizer, the motor is stopped at the predetermined position.

SEWING MACHINE An induction motor running at a constant speed is used to drive the sewing machine. When the pedal of the sewing machine is depressed to start the sewing machine, the electromagnetic clutch is excited to let the motor shaft engage with the sewing machine shaft, thus permitting the sewing machine to run. A synchronizer senses current speed and feeds back speed detective signals. Therefore, the speed control is based on a closed-loop and speed is variable continuously. When the sewing machine is stopped, the electromagnetic

Sewing Machine Manufacturing Division, Nagoya Works, Mitsubishi Electric Corporation, No 1-14 Yada-Minami 5-Chorea, Higashioku, Nagoya, Japan

vol 5 no 8 october 1981

Figure 2. Photograph o f the automatic thread trimming sewing machine

0141-9331/81/080351-06 $02.00 © 1981 IPC Business Press

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Figure 3. Construction of the sewing machine

SEWING MACHINE OPERATION AND FUNCTIONS The sewing machine operation and functions include the following:

Pedalling operation The pedal is depressed by toe to start the sewing machine. The sewing speed can be controlled from the minimum speed to the maximum speed within a range of I - 2 5 by degree of pedalling. The sewing machine stops when the pedal is returned to the neutral position. The synchronizer develops a 'needle down signal' and thus the needle stops at the down position. By kicking down the pedal by heel, the sewing machine works momentarily until the needle assumes the up position and stops with a 'needle up signal'. In association with such a needle movement, the thread trimmer, wiper and presser foot are also automatically controlled. Thus, only operating the pedal is necessary for the operator to accomplish sewing work and skill is no longer necessary to achieve high productivity.

Operation of external switches There are three external switches which are usually provided in the modern industrial sewing machine: •

a 'manual backtacking switch', which is used to reverse the stitching direction at any point in the sewing operation • a 'presser-foot lifter switch', which is used to lift the presser-foot when the sewing machine is at a standstill • a 'needle up stop switch', which allows the needle to be relocated and stopped at the up position, if the needle has stopped in any other position than the up position In addition to these switches, additional switches such as an emergency stop switch are particularly useful. 'Needle down priority' and 'needle up priority' switches are also available.

Sewing pattern select function By operating the switches on the control panel, under the machine table, diversified patterns can be made. The sew-

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Figure 4. Photograph of the control pane/ ing pattern control is based on the control of a number of switches and backtack, which in turn is controlled by the backtack solenoid. Figure 4 shows the control panel. As is apparent from' the Figure, the desired pattern can be directly selected by a single operation of the selector switch. The present number of stitches and the selected pattern become effective at the time of the first pedalling after the presetting and selection, and automatically reset when the selected pattern is completed, or the thread is trimmed. A fixed stitch number of repetitive straight stitchin function is newly incorporated to facilitate label sewing up and pleating in the latest model of the LIMI-STOP Z.

CONTROL BOX The control box may be divided into the following functional sections: •

pedal sensor section that develops pedal toe down and pedal heel down signal, as well as output voltage magnitude of which corresponds to the degree of pedal depressed • speed control section that controls the sewing machine operation in general (stop, start and speed) • auxiliary control section that controls needle up stop, needle down stop, automatic thread trimming and backtrack for sewing pattern selected on the external control panel Among them, a microcomputer is employed for the auxiliary control. Figure 5 shows the circuit board of the microcomputer. The following description is concerned with the control system and the microcomputer itself.

Speed control The speed control system is of digital servo type in which a pulse generator with 31 poles is incorporated in the synchronizer on the machine shaft to feed back the information of shaft rpm to the speed control system. In order to improve the rise time characteristics of the motor, the clutch is devised so that it is driven for a certain time on over-voltage source in response to pedal toe down signal ($1) from the pedal sensor and, after the short over-voltage time, the current is restricted to suppress heat rise of the clutch coil and the clutch drive transistor.

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Figure 6 shows the block diagram of the speed control circuit. IMLO (medium speed lock), IMCO (medium speed command), LLKO (low speed lock), SRT (operation command) all of which are input into the V/F converter and BK (brake command) input to the brake drive circuit are those output from the microcomputer in the auxiliary control circuit. The power source provides stabilized power supply of +10 V DC for logic circuit as well as unstabilized power supply of +30 V DC. A u x i l i a r y control c i r c u i t The auxiliary control circuit consists of a single.hip microcomputer and eight CMOS chips. The circuit diagram is shown in Figure 7. The following description is mainly concerned with the hardware and software of the microcomputer.

Functions o f the microcomputer The functions of the microcomputer used in the newly developed auxiliary control system include the following: Interlocking function for input switch setting The presetting of the input switches is sequentially executed in association with signals from the pedal sensor under the predeter mined interlocking conditions. Sewing pattern control function This function includes pattern selection, counting of number of stitches, control of backtrack solenoid and control of steady speed command signals (IMLO, IMCO, LLKO, etc) that are independent of the pedalling operation, and ensures exact pattern sewing.

Figure 5. Control circuit PCB LLKO IMLO

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Figure 6. Block diagram o f speed control circuit

vol 5 no 8 october 1981

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Figure 7. Microcomputer and interface. Note : a, middle speed release switch; b, 1 or 2 position switch: c, one shot switch

Timing control Timing of each solenoid operation is shown in Figure 7 is controlled in response to pedalling operation, presetting of input switches and selected sewing pattern. For the timing control, clock function of the microcomputer is fully made use of on software

Hardware In order to assure fast response, the microcomputer (with 2 k ROM, 48 of RAM, 35 I/O lines and an instruction cycle of 10/Is) itself is used only to control the auxiliary functions of the sewing machine and does not include the speed controlcircUit. A diode matrix is used so that the signals are only effective when the pedal is depressed by toe and signals. The response of this is not so important, but the signals must be successively verified (pedal toe/heel down signals, needle down/up signals, for example). Thus the number of input ports is largely minimized. The I/O lines and these interface circuits are as follows:

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Input ports PA0 PA3, PBo -PB3 The input ports are used to lead various data from input switches, the control pane, etc into the microcomputer and consists of a diode matrix. A filter circuit is incorporated in the circuit to prevent malfunction and damage to the microcomputer, due to external noises and surge vol tage. Pea -PC2, PD1, PD2 Since the needle down signal, needle up signal, pedal toe down signal and pedal heel down signal are frequently developed in the automatic thread trimming sewing machine, these signals are directly input through the interface circuit to shorten the execution time.

Output ports PEo-PE3, PFo-PFI

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Scanning signals for each input, PAo-PA3, PB0-PB3 are output from these ports.

is largely dependent on, besides the rational arrangement of the memory map, the effective use of available memory area. Figure 8 shows the flowchart of the newly developed programming. The details of the software are as follows:

PGo-PG3, PHo-PH3, PI2 These output ports are for solenoid drive output and speed control command output. Of the many solenoids, the trimmer solenoid is effective within the interval from the needle down posffion to th~ needle up position. The wiper solenoid is driven for about 70 ms after each completion of the thread trimmer solenoid motion. The backtack solenoid is energized during the sewing machine operation and the presser-foot lifter solenoid is energized only when the sewing machine is at a standstill, according to the setting of control switches and the sewing pattern selected. Software The sewing machine control program mainly comprises reading and selection of input data, counting the number of stitches and sequence control including automatic backtack control, necessary at sewing start and end, thread trimming control, wiper control operation and presserfoot lifting control. Although the motion of the sewing machine itself is simple, input conditions for its control are numerous, requiring the use of many flags. In fact, most of the RAM is used for the flags. Since directly accessible memory area in the RAM is restricted, the success in programming

Interlock for input signals For safety operation of the sewing machine, the input signals or data are conditioned by the following interlocks - a--e and the program is made up to satisfy these requirements. • pedal down signals are ignored when the power supply is turned on • high speed operation is inhibited, or pedal operation remains ineffective during thread trimming • the manual backtack switch is ineffective during the standstill of the sewing machine, or during control of number of stitches • number of stitches cannot be changed during operation of the sewing machine • while the sewing machine goes on automatic backtacking at the start or end of sewing, sewing speed is kept at medium speed, independent of the pedalling operation Control of number of stitches (stitch counter) The number of stitches present on the digital switches on the control panel is stored in the memory. The content, i.e. the number of stitches, stored in the RAM is decremented at every needle up signal. When the count reaches the predetermined value, a speed command signal is developed to decelerate speed, and to complete the programmed stitching.

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1 Figure 8. General flow chart. Note : 51, pedal toe down switch; 52, pedal heel down switch," 53, manual backtracking switch," USW, needle up switch. FSW, foot lifting switch

vol 5 no 8 october 1981

355

Pattern sewing control Six sewing patterns shown in Figure 4 are realized in response to the pattern selection on the control panel and the present number of stitches.

a friction-free clutch in the motor, reliability is largely improved. The new LIMI STOP Z featuring largely improved performance, reliability and productivity could meet users' recent needs for modernization and labour saving in sewing work.

CONCLUSIONS In place of 50 ICs (CMOS) used in the sewing machines which used to control sewing operation, a 4-bit single-chip microcomputer is introduced in the latest model of the industrial automatic thread trimming sewing machine. With the microcomputer, besides the reduction of number of ICs from 50 chips to eight chips, the applicable range to automatic sewing system~ is increased and, with the use of

REFERENCE Kobayashi et al Eddy-current variable speed clutch motor LIMI-- STOP Z Mitsubishi Electric Bulletin Vol 54 No 10 (1980)

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