Development of tonnage measuring instrument for forging equipment

Development of Tonnage Detection Instrument for Pressure Machine

YI Xianjun, YE Chunsheng

(State Key Lab. of Plastic Forming Simulation and Die & Mold, HUST,
Wuhan 430074, China)

Key words: tonnage measurement; low level signal amplification; peak detection

1 Functions of the measuring instrument
According to the process requirements of the forging equipment, the functions designed are as follows:
(1) There are two types of measurement modes: one is to track and measure the pressure value in real time under extrusion; the other is to measure the peak force of each blow under striking. When measuring in the latter, two situations should be distinguished: the striking operation of this shift starts from a new workpiece; the striking operation of this shift is the continuation of work on an unfinished workpiece.
(2) When the pressure value is tracked and measured in real time under extrusion, the measured pressure data is displayed in real time on the display screen; when the external force disappears, the displayed value disappears.
(3) When measuring under striking, the maximum value of the number of strikes and the peak value of the strikes in the current shift, the cumulative number of strikes and the maximum peak value of the strikes of the workpiece, and the cumulative value of the number of strikes recorded by the measuring instrument itself and the maximum value of the historical peak value of the strikes are recorded, and the peak values ​​of the last 20 hammers are recorded, and these data can be called out after the measurement is completed.
(4) The limit value of the detection force is set through the keyboard, and an alarm signal is issued when the measurement is overloaded.

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The hardware structure block diagram of the tonnage measuring instrument is shown in Figure 1. The travel switch mainly detects the rise and fall signal of the forging equipment's striking hammer, and sends the signal to the single-chip microcomputer after photoelectric isolation. The pressure detected by the pressure sensor enters the analog-to-digital converter TLC1543 with on-chip sampling and holding after signal amplification and peak value (maximum striking force) acquisition circuit processing, and the analog signal is converted and read under the control of the single-chip microcomputer AT89C52. The P11 pin of the single-chip microcomputer controls the peak acquisition circuit. When the instrument is set to measure the maximum striking force of each hammer in the striking mode, P11 outputs a low level, and the peak acquisition and holding circuit plays a peak holding role. The single-chip microcomputer samples the maximum force of each strike according to the signal measured by the travel switch; when the instrument performs real-time tracking and measurement of the pressure value, P11 outputs a high level, the peak holder function is bypassed, and the output voltage of the peak holding circuit closely follows its input voltage. X25045 is an integrated chip with 512 bytes of serial EEPROM, watchdog and voltage monitoring functions. It is used to record the number of blows, peak value of blows and force of each blow, and has an anti-interference effect on the operation of the instrument. When the pressure is overloaded, the microcontroller pin P12 sends a signal to control the bell to sound an alarm. The LCD Chinese display module and the coding keyboard realize human-machine operation and measurement display. The instrument has a printer interface, which can drive the micro printer TpuP40A to print out the recorded data; and communicate with the host computer through RS232.

3.1 Amplification of weak signal from sensor
The pressure sensor has a bridge circuit composed of 4 strain gauges. When external pressure is applied, the strain gauge deforms, and the corresponding resistance value changes, causing the bridge to lose balance; when the bridge is powered by an external power supply, a weak voltage output is generated. In order to ensure that the output voltage signal of the bridge can maintain a linear relationship with the pressure it bears, the power supply of the strain bridge is powered by a constant current source. As shown in Figure 2, the op amp uPC151C, the voltage regulator D1 and the precision resistor R3 form a constant current source of about 2mA, and the upper and lower ends of the pressure sensor have a voltage output of 0 to 15mV within the pressure range it bears. Since the pressure sensor used has no zero adjustment pin, a stable small DC voltage signal is taken from D1 by W1 to zero the bridge output.
Since the weak signal needs to be amplified by a high multiple, and the common mode voltage of the output signal of the sensor bridge is very high, a differential input dedicated instrument amplifier chip AD524 with a high common mode rejection ratio is used here. According to the connection method in the circuit of Figure 2, the circuit has an amplification factor of 1. The external gain adjustment resistor R4 has a great influence on the gain accuracy and temperature drift of the amplifier, so a high-precision resistor with a small temperature coefficient must be selected. W2 (connected between pins 4 and 5 of AD524) is the input bias adjustment resistor of the amplifier. The error caused by the input bias of AD524 is proportional to the gain of the amplifier. This bias adjustment resistor is indispensable in high-multiple amplification. The SEN pin of the instrumentation amplifier chip is the output reference terminal, which is grounded here. In actual applications, even if there is a small resistance value between the SEN pin and the ground, it will have a great influence on the common mode rejection ratio of the device, so it should be paid attention to when designing the printed circuit board of this part. The positive and negative power supply pins of AD524 are connected to filter capacitors C1 and C2 to eliminate the interference caused by the power supply.

Figure 3 is a circuit for collecting and holding the peak value of the striking force in striking mode, which is composed of a comparator LM311 and a sample-and-hold chip LF398. The LOGIC pin of LF398 is a sample/hold control pin. When the voltage of the LOGIC pin is greater than 1.4V, the output voltage of the chip tracks the input; when the voltage of the LOGIC pin is less than 1.4V, the output of the chip is maintained at the input voltage value when the LOGIC pin just exceeds 1.4V.

When the instrument is in continuous tracking measurement mode, the P11 pin of the single-chip microcomputer sends a '1' signal, so that LF398 is always in the tracking state, and the output Vout of the circuit is equal to the input Vin. 3.3 Software Design In the software design of the measuring instrument, a modular design method is adopted. The system software consists of the following modules: main program, keyboard interrupt service subroutine, data acquisition control subroutine, LCD display subroutine, printing program, serial communication subroutine, etc. The structure of the main program and keyboard interrupt service subroutine is shown in Figure 4 and Figure 5. The working mode flag, new workpiece flag, printer enable, measurement force overload threshold value and other data in the main program are set by keyboard interrupt. The keyboard is set with 5 keys, and the key values ​​are input to the CPU through 3 IO port lines after 8-3 encoding. The keyboard interrupt is set to the falling edge trigger mode, and a keyboard de-jitter circuit is designed in hardware. In the data acquisition control subroutine, filtering is performed when data sampling is performed. The program design adopts C language, and the program debugging work is completed in the KEIL C integrated environment.

The display uses a large-area LCD module with a 240×60 dot matrix. In the strike mode, the display can display historical data such as the number of strikes, strike peak value, and current strike value on the same screen. According to actual needs, large characters (20×40 dot matrix) are used to display the current measurement value, which is convenient for operators to observe real-time data from a distance.