Jumpy and Erratic Trackpads

Apple has gone through great lengths to be at the cutting edge in terms of input devices, particularly with its laptops. With the MacBook, Apple introduced and evolved the multitouch capabilities and has invested heavily in research and development to ens

Apple has gone through great lengths to be at the cutting edge in terms of input devices, particularly with its laptops. With the MacBook, Apple introduced and evolved the multitouch capabilities and has invested heavily in research and development to ensure the feel and functionality are as flawless as possible. For the majority of people, this effort has paid off; however, for some, there reports or erratic trackpad behavior.

Apple Discussion poster "Jackosh" writes:

"I notice that my trackpad is completely erratic. It will work normally for a short while, but then it will move VERY fast for a quick second, or just jump to a different part of the screen. I've had this machine since November 2007 and have never had anything like this happen before."

Apple's trackpads use a technology that tracks capacitance changes in a grid of circuits as finger passes over the grid. Apple has released a few knowledgebase documents on this technology, and the following excerpt describes the technology in detail:


The technology works on a principal called coupling capacitance. As your finger moves over the surface, the trackpad evaluates the change in capacitance between two layers of measurement electrodes built into the surface of the trackpad. The electrodes are arranged in a checkerboard configuration, which creates a series of intersections, or couplings, where the capacitance is measured by the trackpad. The horizontal electrodes send out a test pattern signal which the vertical electrodes sense. As a finger approaches the trackpad surface, its presence lowers the capacitance (coupling coefficient) at the closest electrode intersections and modifies the signal received by the sense electrodes.


By monitoring this change in capacitance, the trackpad is able to first locate and then compare the current location of low capacitance (where the finger is touching) to the previous location, and then moves the cursor accordingly. This cycle of sending out a test pattern, sensing the change in capacitance, and moving the cursor is constantly repeated. The high resolution of the trackpad (387 dots per inch in the horizontal and vertical directions) allows precise control over cursor movement down to the pixel level. The Trackpad control panel provides  adjustments over a full range of acceleration to ensure precise control.


While the tracking technology has been refined over the years, there still are problems that can arise where the trackpad behaves erratically. When this happens, the cursor will sometimes be jumpy, or sometimes not move at all and take several swipes before the tracking is recognized and translated into movement. This can be caused by a variety of reasons, including faulty hardware settings, and both electrical and physical trackpad electrode interference.


Possible Fixes:

1. Reset the PRAM

Improper settings in the computer's parameter memory (which contains some mouse and trackpad settings), may  lead to problematic trackpad behavior. To reset the PRAM, hold the options-command-P-R keys at start-up, and wait for the computer to reset and chime several times before releasing the keys and allowing it to boot normally.


2. Clean the trackpad

The trackpad needs to have a clean interaction with the skin on your finger. If the trackpad has grime on it or if your hands are otherwise not making direct contact with the pad, the pad will not work properly. Spray some glass cleaner on a cloth (not directly on the pad) and gently wipe until the pad is clean.


3. Clean and dry your hands.

Skin lotions and oils can cause enough interference to prevent the trackpad from working. In addition, excess wetness and sweat on your fingers can cause capacitance problems, and you should be sure your hands are and the trackpad are dry.


4. Ground the computer and your hand.

Static electricity may interfere with the trackpad, and prevent the capacitance in the electrodes from being detected properly. Be sure static buildup in the computer is discharged by grounding it, which can best be done by plugging in the power adaptor. Then be sure you are also grounded by touching the computer's case while it is grounded. Additionally, while using the trackpad, try keeping one finger such as a pinky or thumb touching the case to the side or front of the trackpad. This will prevent static charge differences between your hand and the trackpad from developing during use.


5. Remove jewelry

In some instances, jewelry may influence the capacitances of the trackpad electrodes, and removing it may help. This will probably me more of a factor for individuals with many bracelets and rings, though any jewelry can potentially interfere with its function.

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