June 2006, Issue 191

Measurement System for Weight and Dimensions
Renesas M16C Platform Design Contest 2005

SIZE MEASUREMENTS

When I started this project, I first considered using a computer vision system for measuring the sizes of packages. I figured a box could be placed against a white background with a calibrated grid, and then a camera could process the image and determine the size from the remaining visible grid space. Such a system is feasible, but it isn’t cost-effective. Cameras and their associated interface electronics aren’t cheap, and I’d need a fair amount of processor and memory capability to process images. What’s worse, I’d have to devote a lot of space to the measuring device so the camera would be far enough away to capture an entire scene.

After considering all of the disadvantages associated with a camera-based measurement system, I set out to create a simpler, less expensive measurement scheme. Because I wanted to be able to measure cardboard boxes to the nearest inch, it seemed reasonable to use a sensor for each measurement point. I placed a row of inexpensive photocell light sensors 1² apart along the Weasure’s axis (see Photo 1b). This setup enables the system to measure a box’s height, width, and depth. When a box is placed on the platform, its size is determined by the photocells that aren’t illuminated. This scheme works particularly well with the M16C/62P microcontroller’s generous helping of parallel I/O. Even when using only leftover I/O ports that aren’t used by the Renesas SKP16C62P evaluation board or the other Weasure functions, there are 62 I/O lines left. This is enough to measure boxes up to 24² × 20² × 19² in size.

The photocells have a resistance of approximately 2 kW or less when well illuminated, and at least 20 kW or more when they aren’t. By connecting a photocell in series with a 10-kW pull-up, the photocell pulls the signal line to ground when it’s illuminated (generating a zero) and lets it go to one when the box covers it up. These signals are connected directly to the M16C/62P microcontroller’s input lines.