Do Sensors Make Sense?
Adding Sensors To Weld Fixtures
The conversation about whether to put sensors in weld fixtures is one we are having more frequently as manufacturers continue to invest in automating weld processes. There are several types and options when it comes to sensing. But before getting into details, the first question to be asked is “what problems are you trying to solve?” Adding sensors can help control quality and manufacturing issues, but it can also add unnecessary complexity and maintenance costs. Before assuming it is the magic bullet for solving quality and manufacturing issues, make sure you are aware of the downsides of adding sensors to your tooling as well.
Below we’ll outline some applications and options for incorporating sensors, along with some of the pros and cons with sensing in weld tooling. However, please note that incorporating sensors into weld fixtures has countless nuances and variables that can’t be covered in a blog article. This is just a basic overview to explain some of your options. If you would like to learn more about leveraging sensors for your next fixture project, we encourage you to reach out to one of our project managers to review your specific application.
Weld Fixtures and Proximity Sensors 101
Proximity sensors are one of the most common ways to incorporate sensing into weld fixtures. These sensors can be used in a variety of applications including part presence, poka-yoke, or even position verification.
Proximity sensors are categorized two ways – normally open or normally closed. Another way to think about this is the sensor verifying something is either present or not present. Both applications are used in weld fixtures. Other proximity sensors including capacitive proximity sensors, photoelectric sensors, and inductive proximity sensors. The materials and applications determine the type of sensor required. Proximity sensors also have a variety of field or beam specifications.
Typically, sensors are tied into an I/O (input/output) block or stand-alone PLC (Programmable Logic Controller) on the fixture. If they are tied into an I/O block, you can connect the sensors on the fixture to your robot. This allows robot programming to incorporate logic into the weld sequence like, “don’t start welding if all the parts are not present” or “make sure this hole is in the right spot before welding” or “open this clamp once this weld is complete”.
If the sensors are tied into a PLC on the fixture there is usually an HMI (Human-Machine Interface) as well. You can simplify the logic by just having red light/green light indicators, but if you are investing in sensing it usually makes sense to have an HMI as well. An HMI will give you a wide range of control and numerous options for using the proximity sensors. We are not getting into that level of detail in this article, but if you would like to discuss the options and details our team would be happy to assist you.
Part Presence Sensors
Incorporating proximity sensors for part presence is one of the most common applications when adding sensors to your weld fixture. The sensor will sense whether a part present or not – pretty simple (in theory). The obvious benefit of part presence sensors is you can verify all the component parts are loaded before either clamping or starting the weld sequence. This keeps you from welding your fixture if something is missing, which is a good thing.
You don’t necessarily need to sense every component part in the fixture in every application. Sometimes there is only one or maybe just a few critical component parts that require part presence sensing. Complex and/or high cost weldments are typically good candidates for part presence sensors.
One major downside to adding part presence sensors to your weld fixture is restriction of weld access. Not only do you need space for the proximity sensors, but the components to hold the sensors, as well as weld and spatter protection for both the sensor and the cables. This adds potential maintenance issues and costs.
Poka-yoke is another common application for proximity sensors. The term comes from the Japanese phrase for helping a machine operator avoid mistakes (poka yokeru). Poka-yoke applications can verify the addition of left hand vs. right hand parts or indicate if a part is loaded backwards. Adding proximity sensors for poka-yoke applications can help reduce operator errors and decrease scrap rates.
Verifying secondary processes on incoming component parts, like checking for holes and location, is another common application. These applications keep the welder or robot from welding components with missing holes or secondary processes like forming, punching, etc. Of course, it’s better to check these component parts at the source, which will be another article. But whenever you catch them, it’s better to notice these types of mistakes before adding weld cost, having to grind welds off, or scrapping the parts all together. Anyone who has ever has to grind welds knows that should be avoided at all costs!
Position verification is another application for sensors. This is especially common when using multiple sub-assemblies and changeovers to accommodate similar but slightly different weldments using the same base fixture.
Options range from using proximity sensors to verify the correct sub-assembly is mounted to the base fixture in the correct spot to calling up the corresponding weld program according the relative sub-assembly. This can also be a great tool for verifying multiple length parts in an assembly or accommodating multiple lengths by using a linear rail. Sensors can verify the position of the linear rail quickly and reliably.
Of course, this can add complexity and cost to the fixture, so the cost benefits have to be weighed in each application.
Verifying clamp open, clamp closed, or both is also a very common sensing application. Again, these are usually tied into either an I/O block or PLC on the fixture.
This application is very common in robotic weld tooling. Verifying clamps are closed helps prevent the robot(s) from crashing into open clamps. Creating weld programs that avoid both open and closed clamps is an obvious work around, but this can add complexities and programming time, not to mention inefficient robot paths.
Another application for clamp closed sensing is when you are rotating the fixture upside down (usually for weld access on the backside). Having sensors for clamp closed adds a level of safety and prevents the operator or robot from rotating the fixture and having parts fall out. This can be a major safety concern, especially when dealing with heavy parts.
Clamp open sensing has many applications and benefits as well. Verifying clamp open is tremendously beneficial when incorporating part ejection, as you can imagine. Hard to eject a part when it is still clamped! It is also crucial when integrating any sort of automated part tending or load/unload automation.
Pros and Cons
While there can be a lot of benefits to using sensors in your weld fixtures, there are also added costs, the possibility of increased maintenance requirements, and other potential issues that can compound in the process. If you can clearly ask “What problems am I trying to solve?” and state a solution that incorporated sensors can solve, then yes, it’s a good idea to look at the options. However, in our experience, if you cannot definitively say WHY you need sensors, adding them can cause just as many – if not more – problems than it solves.
There are also other options for verifying and controlling processes that are more mechanical in nature, for instance, incorporating logic into your pneumatic or hydraulic systems. We do this routinely for many applications. Logic can verify parts are located and/or seated correctly, check to make sure a hole is present for a poka-yoke application, or ensure clamps are open before parts are ejected pneumatically. Stay tuned for a more in-depth discussion of this in a future blog post!
Again, incorporating sensors into weld fixtures has countless nuances and variables that can’t all be covered here. This is just a basic overview to know some of your options. If you would like to learn more about leveraging sensors for your next fixture project, I would highly encourage you to reach out to one of our project managers to review your specific application. We would be happy to help any way we can.