This problem shows up both as noise and as a change in calibration of the Dual-Range Force Sensor when the Motion Detector is used at the same time with a LabPro interface. The problem does not happen with all Dual-Range Force Sensors, and it does not happen with LabQuest. It is only when the 2008 version of the Dual-Range Force Sensor is used with a LabPro and a Motion Detector.
If you are not sure which version of the force sensor you have, check here: TIL 3658: Which version Dual-Range Force Sensor do I have?
When used in combination with a Motion Detector and a LabPro, and the data rate is chosen so that the Motion Detector is undersampling, then there is noise in the Dual-Range Force Sensor readings of about 0.3 N magnitude.
Undersampling refers to a data-collection mode where the Motion Detector samples at a rate lower than that of other sensors, leaving some blank rows in the data table. The default data rate on detecting a Motion Detector and a Force Sensor is 50 Hz, which will result in 25 Hz data collection with the motion sensor. In other words, the noise is present under default data-collection conditions.
This noise is due to variations in the power available to the Dual-Range Force Sensor when the Motion Detector is firing, as opposed to when it is not firing.
If undersampling is not used, there is little noise, but the calibration is incorrect.
In tests, the calibration of the Dual-Range Force Sensor on the 10-N range changes by about 4% when a Motion Detector is used at the same time.
1) Reduce the data rate so that undersampling is not active. 30 Hz or slower will turn off undersampling. This reduced data rate will make collision data for very short events impossible to collect. Use magnetic bumpers or something else to extend the duration of the collision. This eliminates the noise problem, but not the calibration change.
2) Use LabQuest, which rarely experiences this issue.
3) Use the newer 2009 version of the Dual-Range Force Sensor.
In 2009 we redesigned the circuitry in the Dual-Range Force Sensor to avoid these problems. The newer units work well and the calibration does not change when a Motion Detector is added. They use a 2.5 volt voltage reference, so changes in the power line when the Motion Detector fires do not cause problems. You can identify the newer units from the older units by the font of the silk screening of the sensor name. The newer, improved 2009 versions have the sensor name in ALL CAPS.