Measuring hit location and velocity

Hi there,

I want to make a project where I have a grid comprised of some number of of 6" by 6" squares, where a ball is thrown at it and it detects (1) which square was hit and (2) how fast the ball was thrown.

The receiving device needs to be able to take a beating. I want to throw a baseball at it at typical baseball pitch speeds and have it not fall apart. For clarity, that means:

  • A projectile that is 72.64–74.68 mm in diameter (2.86–2.94 in) with a mass of 141.75 to 148.83 g (5.00 to 5.25 ounces)

  • Thrown at speeds up to 44 m/s (about 99 mph)

In order to make this cheaply, I was thinking of using a piezoelectric impact sensor attached to the back of each square. However, as I'm new at this I had some questions I was hoping somebody here could answer:

  • Would piezoelectric film or the little enclosed disc "elements" (as seen on adafruit) be better to use?

  • I'm hoping to be able to make each square out of leather. Can I simply attach the piezoelectric sensor to the leather directly, whether stitched or adhered in some other way? I'm planning to put the sensor on the back of the square so it's not being struck directly.

  • Would the sensors in this case be durable enough?

  • Can I derive speed accurately based on the force of the impact?

  • Anything else I should be aware of, or alternative suggestions?

An alternative I was thinking of was to use a 2D grid of IR sensors (a horizontal set and a vertical set) along the inner borders of a square to be able to measure the location of a passing ball, and I figured it might be easier to measure velocity this way as well based on how long the IR sensor beam detected an obstacle. However, this seems like a significantly more expensive option, with each sensor at $15 instead of like $1 for each Piezo element.

Whew, hopefully that's enough to go on! I'd really appreciate any help or advice. I've been watching videos and looking at tutorials but while I feel I can code the thing, I'm wondering about the practical implementation aspects of it.

Thanks in advance!!

I'd go for the light barrier approach, with iR remote control sensors of 2$/doz. The mechanical sensor readings depend on the square material, its bearing, position and angle of the hit. Either solution requires mechanical engineering.

Just SWAG'ing.

  1. a. Suspend a piece of fabric with load cells. Ball impact will deflect fabric, pulling on load cells in proportion to energy imparted.

b. Since load cells don't play well with shock loads maybe rig up some mechanism attached to the fabric which deflects on impact. Measure deflection with array of IR sensors.

  1. build an X-Y matrix of IR sensors mounted behind the fabric. Ball impact will create a temporary bump which blocks certain sensors - think keyboard matrix. Decode the sensors to determine impact location.

.02

I see 38 kHz TSOP sensors on Taobao going for ~$6-12 (RMB 5-10) a piece, shops on aliexpress are usually a bit higher in price. Add an array of directional LEDs opposite, modulate them (can be done using a single pin - just add appropriate drivers), and you have your grid for really cheap, and you can measure your location. Add a second grid and you have your speed.

Having a sensor take the impact of such a high speed ball is a bad idea. Mechanically very hard to pull off (keeping your sensor in one piece and the wires from breaking while still allowing it to measure the actual hit).

Wow, thanks for the great responses. Sounds like using a matrix of sensors is a better idea than trying to use an impact sensor. Sounds good to me, and thanks for the suggestions on keeping the cost down too :slight_smile:

TSOP sensors are designed for intermittend modulated IR (remote controls).
TSSP sensors are for continious modulated IR.
Narrow beam IR LEDs might be needed.
Leo..

If the entire area is flooded by IR, generated from the same signal source, it's sufficient to restrict the receivers' beams.

I had a work colleague that was involved in some NATO projects 20+ years ago.

For the trajectory/speed data*, they used phased-array radar* off to the side of the range, and could recreate visual imagery of the projectile during flight - but didn't worry about capturing the impact energy, because that could be calculated by knowing the mass and speed at the moment of impact.

Apparently it worked well...

  • nothing says the radar has to be RF based... depending on the physics involved, optical / IR / laser scanning could be used.

A radar speedometer works in direction of the flight, from front or back of the missile. The Doppler effect modifies the frequency of the beam, and that difference is measured. Not easy with IR or RF signals, perhaps possible with ultrasonic and suitable circuitry.