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Veritas et Visus Touch Panel November 2005
31
Palm rejection on resistive touchscreens
by Geoff Walker
Have you ever used a device such as a webpad and had the cursor jump around or disappear under your hand as you
tried to write something on the screen? If so, you’ve experienced a lack of palm rejection.
Palm rejection allows a user to rest their hand or other object on a touchscreen without activating it. Palm rejection
is usually significant only in devices with screens larger than about four inches, since with smaller screens the
user’s hand rests mostly on the screen bezel. It’s also usually significant only in devices that are intended to be
used with a pen or stylus, since if a device is meant to be used only with pure finger-touch, there’s no reason for the
user to rest their hand on the screen.
Palm rejection isn’t really about your palm; it’s about the part of your hand that pokes the writing surface when you
hold a pen. For some people, it’s the skin protrusion resulting from the creases on the edge of your hand; for
others, it’s the knuckles on your little finger. Both of these are relatively sharp points that act like a finger-touch.
Resistive touchscreens can’t handle multiple touches, so the cursor goes somewhere other than where it should be.
How common is poor palm rejection? Unfortunately, it’s very common. It seems as though almost every pen
tablet created in the last five years has had poor palm rejection (Tablet PCs don’t count, since they use active
electromagnetic digitizers rather than touchscreens). Hardware OEMs just don’t seem to think about this issue
during design, and the touchscreen vendors contribute by minimizing the issue or appearing uninterested in doing
the required customization.
Poor palm rejection can kill a product. Here’s an example from my consulting experience. A few years ago a large
hardware OEM bought into the Microsoft hype and created a very nice “Smart Display” (originally known as a
“Mira” device). Unfortunately, since Microsoft’s basic concept was seriously flawed, the product didn’t sell very
well. The OEM tried re-purposing the product as a vertical-market data collection tablet (i.e., a wireless thin
client), at which point they discovered that none of their enterprise prospects were willing to buy the device.
Prospects found that having to hold their hand above the screen while using the product (due to lack of palm
rejection) was just too uncomfortable to tolerate for any length of time. That OEM’s Smart Display is still a
product in search of a market.
How Palm Rejection Works: Figure 1 shows the basic components of a typical resistive touchscreen. A
touchscreen contains two layers of transparent conductor (ITO, indium tin oxide), one on top of the glass substrate
and one on the underside of the PET (polyethylene terephthalate) top layer. The two layers are kept apart by
transparent spacer dots. When a finger or pen
presses on the top surface, the transparent
conductors make contact and the touchscreen
controller determines the touch position.
The key to palm rejection on a resistive
touchscreen is the spacing of the spacer dots.
Most touchscreen vendors use spacing (pitch) of
around 3.5 mm and a dot size of around 0.1 mm.
This is a compromise that allows the touchscreen
to work both with large, blunt objects such as a
finger and with small, sharp objects such as a pen.
Figure 1: Basic components of a typical resistive touchscreen
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- z080535202012-06-08就一页简单的pdf介绍,知识量有限
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