Tech Note 548 Detail

Inaccurate Free Fall Time

Affected Products:

ME-9820 Discover Freefall System
ME-9889 Discover Freefall System

Problem/Symptom:
Inaccurate Free Fall Time

PASCO Solution:

Large (>10 ms) Error or No Time Recorded

The recommended “Time of Flight” option in DataStudio 1.9.8.x or SPARK system version 1.2.2 (or any previous versions) will return either incorrect or no results if the button is not released before the ball hits the pad. In addition, subsequent data trials within a run will be inaccurate.

On the GLX with firmware version 1.52, no data will be returned if the button is not released before the ball strikes the pad, but subsequent runs will remain accurate.

As a workaround, please do the following to obtain accurate results:

  1. Start a new run.
  2. Place the drop box at least 0.6 meter above the pad. (Greater height may be necessary depending on the reaction time of the operator.)
  3. Attach the ball to the drop box.
  4. Wait for the ready light to stop flashing.
  5. Press and quickly release the ball trigger button so that the button is released before the ball strikes the pad.

Small Delay (5-6 ms)

When using the ME-9889 and ME-9820 Discover Freefall Systems, there may be a delay in the system response. The ME-9889 has a delay of 5-6 ms, due to internal debounce circuitry in the device. (For the older ME-9820, the delay may be tens of milliseconds and may even appear to be negative. These delays in the ME-9820 result mainly from an unregulated pulsed magnetic field in the ME-9820, which could either push or pull the object under observation for tens of milliseconds after release. The ME-9889 has a much smaller uncompensated magnetic field due to feedback circuitry that )

These delays will not effect the accuracy of the calculation of the local gravitational acceleration g, provided that one determines g by the following method:

  1. Collect free fall times at least three times for each height. Take the mean of the results.
  2. Plot time of fall versus height for several heights.
  3. Fit the data using the following formula:

T =(2*x/g)^0.5+t0

, where T is the time of fall, x is the distance from the base of the ball to the top of the pad, and t0 is the offset time.

Inaccuracy in the Determination Acceleration of Gravity

The ME-6839 Drop Box is designed to reliably drop targets that are magnetically attached to the Drop Box permanent magnet by negating the magnetic field of the permanent magnet with the magnetic field of the current in a coil. By driving the coil so that its magnetic field is in opposition to the permanent magnet, the external field is reduced to a very low value for as long as current is available to effectively drive the coil. A magnetic feedback system is used to constantly adjust the coil current to precisely negate the field created by the permanent magnet. The capacitor bank stores enough energy to fully negate the field for about 20 ms and the microcontroller allows the coil to be driven for an additional 10 ms as needed.

Since there may be a slight uncompensated magnetic push or pull by the magnet/coil system on the object, it may improve the results to place adhesive tape on the stud to reduce the attractive force between the object and the magnetic stud of the Drop Box

Creation Date: 10/10/2005
Last Modified:
Mod Summary: