Focusing on bent tubing
Making measurements using photogrammetry
June 29, 2009
Measuring the bends and straight sections of a bent tube can be tricky and time-consuming, especially if the tube has a large number of bends in several directions. Photogrammetry, also known as optical measurement, uses a booth equipped with several digital cameras to make a digital image of the part, allowing fast, easy measurements.
You have been bending tube and pipe for years, and while you're proficient at bending, you realize that you're spending too much time on quality control. Some of the parts are easy to handle and have just one or two bends, so measuring the straight sections and verifying the bend angles isn't too much trouble. On the other hand, you have some parts with more than a dozen bends that are long,
cumbersome, and consist of small-diameter tubing that flexes easily, so the straight sections aren't too straight unless supported. Some of your finished products, such as fuel lines and brake lines, have a multitude of bends in different directions, flared ends, and threaded fasteners, creating a significant measurement challenge.
Instead of taking manual measurements, wouldn't it be faster and easier to make virtual measurements? In other words, could you make a digital 3-D image of the finished piece and compare it to a digital 3-D reference? Optical measurement systems that use photogrammetry technology can do this for you.
What It Is, What It Does
Photogrammetry traditionally is accomplished by taking a series of digital images of an object from various views. The images are combined and compared to a reference that provides scale to create a digital model. This technology has been automated using a booth equipped with several digital cameras, a computer, and the necessary software. The cameras are spread out across the top of the
booth, and each is aimed at a slightly different angle to cover the entire measuring area. Such a system requires about 40 milliseconds to capture data from thousands of points along the straight sections and around the bends.
The software combines the images from all of the cameras to create a single 3-D model of the measured part, and compares that image with a reference model of the part. Based on this comparison, automated software provides a pass or fail indication which, as the term implies, lets you know if the part meets your specifications (see Figure 1). The analysis goes one step further and displays variance data on all sections of the tube,
including the bend areas. If the tube fails the inspection, this data enables you to make immediate changes to the bending process.
In other words, photogrammetry offers more than just quality control; it enhances your shop's process control. Because the information is digital, it can be used immediately to generate a report or to make process control decisions.
How It Works
After collecting images of the tube, the software uses the outside surface of the tube or pipe to determine its diameter. It then determines the location of the centerline and uses the rest of the data to construct a digital 3-D model, including bend points and the location of most external fittings. It compares this to your design data and provides the specific differences between the print
and the part and an indication of whether the part passes or fails the inspection.
Most systems are set up with an interface to several CNC benders. This provides immediate corrections for length, rotation, and bend angle for each machine, even if they are producing different tubes. If the system is not automatically connected, you can still use the data, but it will be a manual operation—the bender operator makes adjustments to the bender program.
Can a photogrammetric system measure
The possibilities for bent tube applications are limitless, which leads to a nearly limitless number of questions about photogrammetric systems. Among the most common are:
- Can it measure tube length? Yes. The overall length of the tube is measured at the same time as the bend points. If the ends happen to be in an orientation that is hard for the cameras to see, you can clip a small adapter onto the end (see Figure 2).
- How fast is it? Simple tubes with only a few bends can be measured in several seconds. Complex tube shapes involving 20 bends or more may take half a minute for the system to calculate all of the bend points and deviations. Usually the speed limitation is how fast you can load and unload the part.
- Can it measure several tubes at one time? The software is designed to provide data on only one tube at a time. Furthermore, these systems are intended for use in the bending area, where they can connect directly to the benders and provide bend correction information. Measuring a completed assembly of several tubes occurs too late in the production process to provide the
maximum impact on the bending operation.
If you need data on attachment points or flanges, you need to use adapters that are automatically recognized by the system.
- Can it handle a tube that is longer than the booth? Yes, it can. You place the tube in the booth so that one end is in the measurement area and the other end is sticking out of the booth. Take a measurement, then move the tube so that the second end is inside the booth and take a second measurement. The system stitches the two images together. This can be done automatically
with a single command from the operator.
- Can it detect or measure holes, notches, or end forms? It can measure these features if you can attach an adapter to the feature. Adapters typically are attached with a thumbscrew-type clamp or a magnet so they can be quickly attached and removed. New software developments can also identify the location of end form shapes if they approximate the shape of a geometric feature
such as a hemisphere, or if they have a change in diameter that can be identified by the cameras. You should ask the supplier about techniques that might be suitable for your specific situation.
- I have a part with tolerances that vary. Can a photogrammetric system handle this? It is possible to divide the model of the tube into sections that have different measurement parameters. These requirements are preprogrammed into the stored data for the specific tube being measured and can include different tolerances in different sections of the tube.
- It is also possible to place a greater weighting on the fit in certain parts of the tube. Constrained fits can be used to simulate the way the tube is used in its application.
- Some long, small-diameter parts, such as brake lines, sag because of gravity. Can this type of system compensate for this? Yes—the amount of sag due to gravity can be characterized for a particular tube and used to create a tolerance value. If the deviation in the actual tube exceeds the expected amount, the tube is rejected.
- Can it measure a tubular structure that branches, such as a Y-shaped workpiece or an exhaust manifold? That depends on how the manufacturer's software works. If the software is designed to find a centerline, it likely cannot follow two centerlines. However, this is an area where development is continuing.
- If the tube is a brake line and I want to measure it after it has the fasteners installed and the ends flared, will the fasteners disturb the measurements? Usually it is not a problem. In most cases the manufacturer can set up the program to skip over the section where a fastener is attached to the tube.
- Can I automate the material handling? Yes. A robot can grip the workpiece and position it in the booth. If the software can ignore a small feature such as a fastener, it can ignore a robot's end effector too.
- Who determines the reference point for the measurements? You do. You can designate one end of the tube to be the reference and base all the measurements on that end, or you could designate any other location of the tube to be the datum reference.
It's important to choose the right reference point for meeting your customer's specifications. A linear dimension that varies 4 mm end-to-end might vary 2 mm when measured from the midpoint to the end. This amount could mean the difference between passing or failing.
- What if I don't have a CAD model for my tube? In addition to tweaking bending programs, a photogrammetric system can create 3-D models of the bent tubes to create bending programs. Rather than have a bender operator spend several hours programming a complex brake line, a photogrammetric system can reduce it to a 15-minute exercise.
- Does a system like this have trouble with small-diameter tube or pipe? A variety of camera types are available for these systems. Small tubes typically are measured by smaller systems, which provide more data points in a smaller area (see Figure 3). The system may be equipped with high-resolution cameras if required. If the resolution is too low, the result would be just a few measurement points at the end of the tube. However, even in this case it is possible to get a good result if you insert an adapter into the tube end.
This allows the system to use much more robust data to calculate the end point.
- Who makes the adapters? Most tube and pipe fabricators make their own adapters. The system manufacturer could do it, but you are familiar with the specific tube shape, its application, and your customers' needs, especially as they concern restrictions or limits on how the tube is handled, installed, and used.
- What can I expect for accuracy? Check with the manufacturer. Some machines are precise enough to measure ±0.02 degree on bends and ± 0.1 mm on bend point locations. Smaller systems with high-resolution cameras are rated at ± 0.025 mm (25 microns) according to the manufacturer.
- How is the system calibrated? A calibration check is performed automatically when the system is started each day. Because this type of system has no moving parts, usually no change is observed.
If one of the cameras is out of position, it will be rejected. This type of system has sufficient redundancy so your work can continue. An annual check by the system manufacturer is a good idea. A traceable photogrammetry system is used to recheck the positions of all of the calibration targets.
- Where can I install this sort of system? Most fabricators install the system in the manufacturing area, where the action is. It does not require a lab setting, but common sense applies. For example, it would not make sense to put it next to grinding or milling operations that generate a lot of airborne particulates.
It should be installed near the benders for convenience.
Are You Taking Measurements or Enhancing Efficiency?
Because of their speed and ease of use, optical measuring systems are suitable for use on the production floor, directly integrated into the bending operation.
As more CNC benders are used in production, it makes sense to use digital reference data for measurement. This provides measurement information where it can have the biggest impact on the overall efficiency of the operation. It can reduce scrap, make adjustments to compensate for variances in the raw material's characteristics, and get your production operation online much faster after part