AVOID THE PITFALLS OF HOME MADE CENTER OF GRAVITY INSTRUMENTS

Making a center of gravity instrument does not seem insurmountable. Homemade solutions such as attaching a plate to 3 or 4 load cells can be done quite easily and at seemingly low cost. But...
   - What is the REAL cost?
   - What accuracy will you get?
   - What are the alternatives?

Hidden costs in making a CG instrument

How much does it cost to make a center of gravity instrument that gives you the accuracy you are looking for? Here are some estimates for a typical homemade CG instrument.

• Making the instrument (mechanical assembly) - You can probably buy the material to build a small center of gravity instrument (payloads under 50 lb) for less than $10,000. Add 2 man-weeks (80 hours) to put it together. At $50 an hour, you have already spent $14,000.

• Instrumentation - How will you do the data acquisition? How about the data display and wiring? These materials will cost you about $1,000 and two days to make it work.

• Software - Are you ready to write software for your instrument? Can you write the functional specifications and software internally, or will you have to find a vendor to write the software? All of this adds time (at least two man-weeks) at $75 an hour for the software programming required to use the instrument.

• Calibration - Once the instrument is assembled, you need to assess its accuracy. This process of trial-and-error requires constant iterations through small improvements until the instrument achieves your desired accuracy. This is a very lengthy process, and the more accuracy you are looking for, the more difficult it becomes. A typical center of gravity instrument will easily read to within +/- half an inch. Anything under a 1/4 inch becomes very difficult to make. It will take you a minimum of 3 weeks to make it work to your specifications.

• Total cost - The bottom line is that your instrument will end up costing at least $27,800, will require nearly 300 labor hours, and take at least 2 months to complete. A homemade machine requires significant investment in company resources and funding, yet it will still deliver questionable accuracy. Space Electronics' CG instruments provide a faster, more cost effective solution for obtaining center of gravity measurements.

The following table summarizes the main costs of making a homemade center of gravity instrument.

Hidden errors in CG uncertainty analysis

• How good is your instrument? When an instrument provides repeatable results, most users believe that the results are good. To be able to truly trust the data obtained from your instrument, you must be certain of the instrument's ability to perform in terms of:
      1. Accuracy - The differences between observed measurements and a standard.
      2. Repeatability - Variation when one person repeatedly measures the same unit with the same measuring equipment.
      3. Reproducibility - Variation when two or more people measure the same unit with the same measuring equipment.
      4. Stability - Variation obtained when the same person measures the same unit with the same equipment over an extended period of time.
      5. Linearity - The consistency of the measurement system across the entire range of the measurement system.

In reality, a repeatable instrument does not guarantee reliable measurements. Without a properly calibrated machine, the user will be unaware of the biases in their measurements.

• Instrument reference - How do you determine the point of application of the force on each load cell? Consider a pancake type load cell. Will the load be applied exactly at the center of the load cell? The answer is NO. Depending on the attachment of the load cell to the interface plate, the distortion of the interface plate, and the leveling of the instrument, the real point of application of the force can easily be 1/8 of an inch from the center of the load cell. A homemade CG instrument requires a minimum of three load cells, which means that the errors will add up.

• Weight uncertainty - How well do you know the weight of your payload? Read the load cell literature. It gives you a sensitivity number for the load cell, which is different from accuracy. You may have a very repeatable load cell (it always gives you the same result) but if the resolution on the load cell is not precise enough, your measurements might be repeatably wrong. Consider 1% error on a good load cell - in aggregate you can easily end up with 3% error on your total load.

Space Electronics center of gravity measurement equipment guarantees accuracy, repeatability, reproducibility, stability, and linearity performance that you can trust.

Alternatives to making your own instrument

Why not buy directly from professionals? For less cost, Space Electronics can deliver to you a CG instrument with 50 lb capacity and +/- 1 mm (0.040 inches) guaranteed accuracy over a wide payload range. Additionally, Space Electronics instruments come with a lot of features that are not available with a home made instrument, such as:

   • Direct CG display. Instead of having to recalculate the center of gravity location from the load cell output and the geometry of your design, our instruments come with a computer system and Windows software that will give you a direct reading of CG location in two axes.

   • Calibration software. Our instruments come with a calibration method. Our software will guide the operator through the calibration process. NIST certified calibration weights are also available if your company does not have a metrology department equipped with calibrated weights.

   • Guaranteed specifications. Our instruments have clear specifications. You can be certain of the accuracy of your measurements.

   • Step by step procedures. Our software guides the operator through the calibration and measurement process.

   • Tare out fixtures. Our software will allow you to account for fixtures used in the measurement. Taring out the fixture is done automatically.

   • Warranty. Our instruments are guaranteed for a period of one year.

So why hesitate? Read about our low cost center of gravity instruments or fill in the box below and ask for a personalized quote today.