AMV-WELDCHECK - to monitor arc welding parameters and validate power sources

TWI Bulletin, May/June 1995

After employment at Leyland Motors for four years, he joined the then Process, Application and Control Department at The Welding Institute. After some 16 years in Arc Welding, he became Head of the Arc Welding Department in 1986. In 1987, he became the first research engineer at TWI to be awarded Doctor of Science for his contribution to arc welding and computer technology.

His research work has been largely devoted to process development and application studies in welding, cutting and surfacing. Over 60 of his papers have been published. He was awarded the Sir William Larke Medal of The Welding Institute in 1984, he became a Fellow of The Welding Institute and of The Institute of Metals in 1983 and a European Welding Engineer in 1993.

The new European standard Weld Quality Systems, EN 729-2, makes specific reference to the suitability of equipment for the application and monitoring of the essential parameters during welding ^[1] . Bill Lucas reports.

For over 10 years, TWI has been producing portable instruments for monitoring arc welding parameters, the specifications of which are given in Table 1 ^[4] . The common features of the instruments are that they can be used for monitoring with a sufficient level of accuracy and their calibration is traceable to a national standard.

Table 1 Comparison of validation accuracies and portable instruments measurement accuracies

QA-WELDCHECK is a simple robust instrument designed for monitoring welding current and arc voltage on the shop floor. PAMS V is a sophisticated, four- channel data logger which can also record wire feed speed, traverse speed and shielding gas flow.

In response to the demand for more advanced instruments for monitoring, calibration and validation of welding systems, Arc Monitor Validator or AMV- WELDCHECK is the latest instrument to be produced by TWI. The intention in this paper is to provide details of the instrument design and to describe its use for monitoring welding parameters and the validation of arc welding power sources.

Instrument design

AMV-WELDCHECK, Fig.1, is a microprocessor-based instrument which has been designed to meet the quality requirements of the new European standards, ie with an adequate level of accuracy for measuring arc welding parameters. The basic specification of the instrument (parameter range, resolution and accuracy) is given in Table 2. By using the latest microelectronic technology the following key features have been achieved:

• Light-weight and miniaturised
• Keypad entry of user/input data and parameter selection
• LCD display instead of separate meters
• User information can be displayed as text
• Memory for storing up to 12 hours of monitored parameters
• In addition to the normal welding parameters, temperature and gas flow can be recorded
• Line and air-borne HF protection.

The substantial reduction in size has been achieved by using a single liquid display (LCD) instead of separate analogue displays or meters for each parameter. All input data are entered via a 13-digit keypad, instead of with mechanical thumbwheel switches. The keypad entry and LCD have the advantage that the major parameters can be displayed simultaneously and the same display panel can be used for all versions of AMV-WELDCHECK.

Table 2 Specification of AMV-WELDCHECK

The heart of the instrument is a microprocessor which samples the data, performs calculations and stores the data in a non-volatile medium for subsequent retrieval. The results of calculations are formatted by the CPU for display or output to the printer. The CPU can communicate with other digital devices via a serial port using the industry standard RS 232 data link, a D-type connector is used to connect the instrument to an IBM PC or compatible computer.

Data from the various sensors are initially processed by an analogue circuit before being passed to the CPU as a continuous set of data. External instructions to the instrument are via the keypad and all visual instructions, and the input or processed data, are displayed on the 256 x 64 dot LCD, Fig.2a. All the signal processing electronics are on a single electronics card, the data processing is a separate card. A keypad is incorporated into the artwork membrane for the front panel, which has a thickness of no more than 2mm.

a) LCD display for the input and processed data;
b) Printout of the welding parameters and management information.

The operation of AMV-WELDCHECK is menu driven with user responding to questions and multiple choice options presented on the display. Because the data are held in memory, management information can be produced such as the average parameter values, arc time, sequence time, weld length, mean power, heat input, wire consumed and amount of gas consumed.

Memory capacity is sufficient to store approximately 10 hours of monitored weld parameters when logged at 30 second intervals, or approximately 20 minutes of data when logged at one-second intervals.

A hard copy of the parameters and management information is obtained from the built-in printer, Fig.2b. When printing data, the option is available for real-time continuous mode or a delayed mode where the data are printed some time after collection.

The instrument has an internal software driven calibration function which is simply effected by following the instructions provided via the display. As new calibration instructions are input via the keypad, minor adjustments to the relationships between the analogue inputs and the stored and displayed values can be readily altered. The ease with which calibration can be carried out greatly reduces any downtime.

Monitoring arc welding parameters

• Welding current
• Arc voltage
• Wire feed
• Traverse speed

Increasingly, the gas flow, the temperature of the workpiece and the arc energy, or heat input, must also be recorded to ensure compliance with the agreed welding procedure.

Welding current

The welding current waveform can be complex in that it may be either DC or AC and sharply fluctuating, for example, in the dip transfer and pulsed metal transfer modes. For DC, the mean/average current level is recorded using a Hall-effect device which is clipped on to the cable, Fig.3. The device should be attached to the return cable which is normally the most direct route between the workpiece and the power source. However, care should be taken to avoid leakage currents eg through safety earth connections attached to the workpiece, which will give a misleading (lower) reading.

AC can be measured as the root mean square (rms) or the mean value of the rectified waveform. As rms values are only correct for a truly sinusoidal waveform, AMV-WELDCHECK will record the mean (rectified) value using the Hall-effect device.

Arc voltage

The true arc voltage is the potential difference across the arc. Thus, the voltage should be measured by connecting the leads to the workpiece and the closest possible position to the electrode, ie at the back of the torch or gun, Fig.4. If the leads are connected at the welding power source, a higher voltage will be recorded due to additional potential drops at resistances such as cables connections.

Wire feed

The wire feed speed is recorded as the average or mean value. The transducer is a tachometer which is mounted directly on the wire, Fig.5.

Travel speed

The workpiece or welding head speed is similarly recorded using a tachogenerator which is driven by the traverse mechanism.

Shielding gas flow

The sensor for monitoring the gas flow is a heating element which is mounted in the gas line, Fig.6.

Workpiece temperature

The temperature of the workpiece is recorded by means of a thermocouple probe, Fig.7, which is brought into contact with the workpiece surface, or a permanently attached thermocouple.

Symbol.1.

Arc power

The true arc power is the product of the instantaneous arc voltage and the welding current. AMV-WELDCHECK will calculate the mean arc power from the instantaneous values,

Mean power, Pn = mean (v x i)n 

Arc energy

Arc energy, E = Σ Symbol.1. Pn x tn

where tn = duration of the nth simple.

Heat input

Heat input/unit length = E/I

where I = weld length

Other management information which can be derived includes sequence time, time since the start of weld, weld length and the amount of wire and gas consumed.

Power source validation

In power source validation, the accuracy of control must be determined, but the method will depend on the type of power source control ieconstant current or constant voltage. The power source is connected to a resistive load in order to record the current flowing in the circuit and the voltage at the output terminals for a range of current or voltage settings. The recorded values are compared with the manufacturer's specified values or the values obtained on the initial calibration/validation. BS 7570 specifies the conventional currents at conventional load voltages that a welding power source should be capable of supplying throughout its range of adjustment. For example, for a TIG power source, the formula for the conventional values, commonly referred to as the load line, is given by

U2 = (10 + 0.04I2)v

WELD VALIDATOR, a software program, has been produced to process the data produced by WELDCHECK and to provide a permanent record of the validation results for quality assurance purposes. The program which runs on a standard PC, will store the equipment settings or meter readings. Circuit current and load voltage values are transferred directly into the program via the RS 232 link. The software is capable of generating the power source static characteristics for adjustment settings from parameter data. By super-imposing the load line on these static characteristics, conventional circuit and load voltages can be determined and compared with specified values. A typical printout from the program is shown in Table 3 for the validation of a TIG power source.

Table 3 Validation certificate produced by WELD-VALIDATOR

References