Closed-Loop Resistance Welding Control: WIFM?

What’s the fastest and easiest way to improve your manufacturing welding processes? That’s simple: use a closed-loop resistance welding power supply! And you’re thinking “okaaay…what’s ‘closed-loop’ and why do I want to use it? I know why you want to sell it – it’s a higher end power supply that costs more money, but exactly how will that help me in my process?” Well, I’m going to tell you.

What is closed loop? At a high level, closed-loop resistance welding power supplies use current and voltage feedback sensors to precisely control the energy delivered to the parts. This ability to accurately control weld energy is a key factor in overcoming problems associated with process variation and the rapid changes in resistance that happen during the weld.

WIFM (what’s in it for me)? Here’s my take on the five benefits of using closed-loop technologies for the resistance welding process:

1. Repeatable output– Resistance values can shift from weld to weld due to normal variations in the welding process. Process issues such as electrode wear and part positioning challenges can lead to poor weld quality when using open-loop power supplies. Closed-loop power supplies respond to these changes every 10-250 microseconds, keeping the programmed parameter (current, voltage or power) constant, leading to more consistent welds.
2. Upslope control – If too much energy is applied before the electrodes have a chance to seat properly, contact areas can overheat, resulting in expulsion, electrode sticking, and weak welds. Closed-loop power supplies allow you to program a very precise upslope at the beginning of the weld pulse, which reduces the initial contact resistance and focuses weld heat into the parts. Be sure to use a long upslope for very hard or resistive parts.
3. Feedback modes to meet specific situations – Closed loop technologies can deliver energy in the form of constant current, constant voltage, or constant power. The appropriate feedback mode to use depends on the part and process challenges associated with the application. Constant current is great for welding flat parts where the part-to-part and electrode-to-part contact is controlled and consistent. I recommend using constant voltage mode for welding non-flat parts and wires. The constant power mode is especially useful for breaking through surface oxides and plating.
4. Built-in monitoring – The voltage and current feedback sensors used for closed-loop control provide really useful process monitoring data. Weld current, voltage, power, and work piece resistance readings can be used for process development and statistical process control. Graphic waveform traces provide simple, dynamic weld information for process understanding and diagnostics. Upper and lower reject limits can signal operators and automation via programmable relay outputs.
5. Process tools – Advanced monitoring features can be used to reduce or eliminate inconsistent welds. For example, the pre-weld check and energy limit functions are used to detect missing or misaligned parts. The Active Part Conditioner (APC) feature helps cope with varying oxide levels on part surfaces. Process tools make it much easier to implement successful process control programs, so important to meeting demanding production and quality requirements.

So what is in it for you? Better control. More consistent welds. Less scrap. All of which equal more profit for your company!

For more information read Fundamentals of Resistance Welding and The Benefits of Closed-loop Control for the Resistance Welding Process.