Back Wall Sensor

The sensor must be able to detect when and where the balls impact as well as allow for the users to interact with the back wall.

This is essential to the design.

Options

• touchscreen technology

  • capacitive touch

  • resistive pressure sensing

  • IR Frame

• Alternative Sensors

  • Light curtains arrayed in a grid

  • LiDAR

Capacitive touch and pressure sensitive sensors, were then investigated. These posed a challenge as the ball is not in contact with the wall very long and does not impart much force on the wall due to the ball being under 50g. These two factors may cause the sensors not to generate a noticeable signal when struck by a ball.

Large scale touchscreens were investigated first, and it was found that infrared (IR) frames are commonly used for interactive displays of this scale. They are non-contact and can sense any object that breaks the IR beams. This technology also came recommended by Activate’s director of IT. Due to the large size (3mx2m) required for this project a custom IR frame would be required. A custom IR frame was quoted at over $7000, which exceeded the entire project budget.

Another non-contact sensor would be using multiple light curtains to create a pseudo IR frame with more granularity. A number of light curtains could be arrayed creating a 4x6 grid. However, light curtains are expensive (starting at over $500 per sender receiver pair [1]) and buying units, though less expensive than an IR screen would be over budget. However, these sensors are also very exp

Another option would be using a lidar to simulate a touchscreen. This would work well for user interaction, but not for ball detection. Calculations to find the LiDAR’s required speed are as follows. Assuming the ball does not slow down after coming in contact with the wall and using a ball speed of about 10m/s; if the lidar is offset from the wall by the diameter of the ball (63mm), the ball would then be in view of the lidar for the time it takes it to travel twice the diameter of the ball. This was calculated to be about 10ms. The lidar would require at least 3 measurements to ensure that a ball was measured and not just noise. This would require the lidar to spin at a minimum of 300 Hz. A supplier quoted that such a high-speed lidar would cost around $15,000, which is again beyond the budget of the project.

A summary of these finding is represented in this table.

An IR frame is an excellent option if it were not for the prohibitive cost. Because of this the team invested more time looking into different suppliers and found that we would be able to order a 3m x 2m frame directly from the manufacturer for about $800 CAD.

Considering this, the IR frame was the clear choice and ordered.