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Process monitoring is indispensable

Capacitor discharge machines (CD machines) are predominantly used in series production, in which many hundreds of parts are welded per shift. No process for the online testing of the welded connections (similar to the previously applied ultrasonic test methods for resistance spot welding) is known and non-destructive testing is rarely used. Therefore, cyclically produced parts are withdrawn from the production process and subjected to destructive testing in accordance with the QA specifications. In order to ensure that these random samples are representative of the connections welded in the interim period, effective process monitoring is essential.

The machines in the PrimKoM product range and the PrimKoS current sources are equipped with the PrimusKE welding control, which offers numerous measuring channels through which to measure the relevant process variables with a scanning frequency of 20 kHz.

The current measurement input 1 (terminal 1X3) is intended for the connection of a Rogowski belt (with 150 mV/kA), which is installed as standard; an associated integrator is integrated in the control. Also intended as standard is the measurement of the electrode voltage 2 (terminal 1X13). Depending on the customer’s wishes, it is possible to connect two incremental transducers 3 and/or up to four freely definable analogue measuring signals 4 (0 to 10 V or 4 to 20 mA). The two connections for the proportional valves 5 (PV1 and PV2) are equipped with an analogue input, via which the actual signal (insofar as the proportional valve has a feedback) is also recorded.

The measured signals are fundamentally all (i.e. a total of 8 curves) stored with 500 points/curve in the data records for the welds. Numerous relevant operands and internal measured values – so-called META data – and the complete welding program (not only its name) supplement each data record, of which the last 100,000 are stored directly in the welding control. PrimusKE deliberately dispenses with an external PC, in order to safeguard the recording of the data. However, it is naturally also possible to store all data cyclically or in parallel to production on a separate PC as a backup.

The extraordinarily broad data basis forms a secure foundation for archiving the relevant process parameters. With this, it is also possible to analyse the individual welding processes in detail years later, or to verify their implementation with all interrelations.

Monitoring methods:

Just as important as archiving the process data is monitoring each individual welding process, in order to automatically identify deviations from the target values. PrimusKE offers several possibilities here:

  • Firstly, it is possible to graphically present individual or arbitrary combinations of the following values in a progression chart for the last 100 welds: total energy target/actual; peak current; current-time integral, effective current; current increase time, welding time and current flow time; momentary resistance; application and yield path for channels A and B; force and force slump for channels A and B. This facilitates a check of the immediate past at a glance.
  • It is possible to check the charging voltage to ensure attainment of the target value before triggering the current flow. If this is not attained, an error message appears and the welding process is prevented. However, a data record is generated so that all data from this cancellation are archived.
  • It is possible to release the following program steps with external sensors via three free digital inputs. The current flow is only released as standard if the electrode force has developed and the control recognises the input “electrode force reached”.
  • It is possible to program safety times for the end of the charging and welding process, and when waiting for an event (e.g. setting of the input “electrode force reached”). Exceeding these times also leads to cancellation with error message and data archiving.
  • The process variables effective current, welding energy, application position, yield path, residual thickness, electrode force and reduction in force can be monitored individually or in combination, to determine whether a minimum limit has been undershot or a maximum value exceeded.
  • All error messages are reported in plain text. With process cancellation, it is optionally possible to release the welded part only after acknowledging the error.