Pendulum Test Rig for a HailSens (10° Tilt) Using 2×2 Wood

Portable shop-build to deliver repeatable impacts to the center of a HailSens haptic sensor, with geometry options for “exactly over sensor” and “more perpendicular.”

Purpose

The HailSens is mounted with its face tilted about 10° down from vertical. We want a simple pendulum rig where the pivot sits on a wooden cantilever made from 2×2 lumber, positioned about 10 in (0.254 m) behind the sensor, so we can drop known masses from known angles.

We’ll also allow the cantilever to be mounted slightly shorter than 10 in so the pendulum motion lines up more nearly perpendicular to the tilted sensor — this removes the cos(10°) penalty and mirrors what the manufacturer accounts for when they specify energy on a 10°-tilted unit.

1. Materials (Printable List)

Lumber

  • Vertical support: 1 × 2×2 × 48 in (≈ 1.2 m)
  • Cantilever/boom: 1 × 2×2 × 14 in (we will drill 3 pivot holes at 7.5 in, 8.5 in, 10 in from the post)
  • Base piece (optional): 1 × 2×2 × 18 in or a 12 in × 18 in × ¾ in plywood board

Hardware

  • Wood screws #8 or #10, 1½–2 in, qty 10–12
  • 1 × ¼″ eye bolt (2–3 in long) + washers + nut (pendulum pivot)
  • 2 × small screw eyes / cup hooks (for cord)
  • 6–8 ft low-stretch cord (paracord is fine)

Impact Capsule

  • 1″ PVC end cap or small plastic cup with lid
  • 1 small machine screw + nut to make an attachment point in the lid
  • Soft pad (neoprene / felt / craft foam) ~1–2 in dia, qty 2
  • Printed or plastic protractor (tape to post)
  • Kitchen scale (to weigh the test masses)

2. Layout Figure

2×2 Vertical Post 2×2 Cantilever (above sensor) Pivot HailSens (10° down) 10 in (post → sensor)

Figure 1 — Side view. The cantilever sits above the HailSens; the pivot is directly over the sensor center; the pendulum swings right → left into the surface.

3. Build Steps

  1. Base & post: Screw the 2×2×48 in vertical to your plywood base or to a bench so it stands upright. Position the whole rig so the post is 10 in behind the HailSens center.
  2. Boom: At the height of the HailSens center, screw the 2×2×14 in boom to the front of the post so it sticks toward the sensor.
  3. Pivot holes: From the post face, measure and drill ¼ in vertical holes at 7.5 in, 8.5 in, and 10 in along the boom.
  4. Install eye bolt: Drop the ¼ in eye bolt through any one of the holes and secure it with washer + nut underneath. This is your pendulum pivot.
  5. String & capsule: Tie paracord to the eye; tie capsule to the lower end; add soft pad to capsule front. Adjust cord so the capsule front is exactly at the HailSens center.
  6. Protractor: Tape a protractor to the post so you can pull the pendulum to 15°, 30°, 45°, 60° and release without pushing.

4. Operating Modes

Mode A — Field-like (use cos 10°) Pivot at 10 in, pendulum swings basically straight down. This reproduces how the sensor is actually mounted (10° tilt). In your math, multiply the impact speed by cos 10°.

Mode B — More perpendicular (no cosine) Move pivot back to 8–8.5 in so the capsule arrives closer to the sensor normal. In the math, you can set the misalignment angle α → 0°, so cos α = 1.

5. Calculations

Use SI in the math: L in meters, g = 9.81 m/s², time in seconds. You can convert inches → meters with 1 in = 0.0254 m.

5.1 Pendulum speed

v = √( 2 · g · L · (1 − cos θ) )

θ is the release angle measured from vertical.

5.2 Normal component (for 10°-tilted sensor)

vₙ = v · cos α

α = 10° if the pendulum is swinging down and the sensor is tilted 10°. If you re-position the boom to make the hit perpendicular, set α = 0°.

5.3 Momentum per gram

If mass m = 1 g = 0.001 kg:

pper g = 0.001 · cos α · √( 2 · g · L · (1 − cos θ) )

5.4 Impact force per gram

Assume contact time τ (use 0.003 s = 3 ms if unknown):

Fper g(L, θ, α) = [ 0.001 · cos α / τ ] · √( 2 · g · L · (1 − cos θ) )

For an actual capsule of M grams:

F = M · Fper g

Example: L = 1.0 m, θ = 45°, α = 10°, τ = 0.003 s → Fper g ≈ 0.79 N/g · cos 10° ≈ 0.78 N/g.