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How Do Laptop Touchpads Work?

10/02/2008 5:04 AM

Can anyone explain how laptop touchpads work, at the hardware level?

Thank you.

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#1

Re: Laptop touchpads...how they function.

10/02/2008 6:51 AM

I think they are capacitive sensor arrays. Your finger is effecively one plate of a capacitor connected to ground.

This article may help...

Del

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#2

Re: How Do Laptop Touchpads Work?

10/02/2008 11:58 PM

The mouse pad is touch sensitive and has coordinates (like a street map that has an index to find a street). It produces events and transmits coordinates for those events. A driver is installed in the operating system that translates events and coordinates into actions in the operating system. It is totally up to the driver as to what actions are performed for events. There are however some generally accepted event / action. For example, contact with the pad at any coordinate for far less than a second followed by no contact with the pad followed again by contact witht he pad at any coordinate for far less than a second is translated to a double click. Touching the pad at a coordinate and moving to another coordinate is considered to be a mouse movement. How far to move the mouse and the direction to move the mouse is up to the driver.

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#3

Re: How Do Laptop Touchpads Work?

10/03/2008 12:48 AM

Hello CowAnon:

I went to the site mentioned by Del the Cat and I have not found a more understandable explanation of this system......Thanks Del

I copied it for my own use and, pasted it here. Hope you do not mind Del.

This actually refers to Phones but the Touch-pad on a computer work the same way

Capacitive sensors can replace mechanical switches for touch control

Capacitive sensors offer a sleek interface for competitive consumer products.

By Wayne Palmer, Analog Devices Inc.


Mobile Handset DesignLine
(05/01/2006 5:00 AM EDT)

The success of the capacitive sensor scroll wheel used in devices such as the iPod has caused other consumer product development teams to evaluate capacitive sensors to enhance the user interface and aesthetic look of their products. Capacitive touch sensor interfaces aren't limited to MP3 players, but can be developed for any product currently using traditional mechanical switches, such as the menu control buttons found on today's latest mobile handsets. These advanced feature menu control switches could easily be restyled with a highly reliable, attractive, and cost-effective capacitive touch sensors.

Capacitive touch-sensor interfaces typically consist of a capacitive sensor, a capacitance-to-digital converter (CDC), and a host processor (Fig. 1). The sensors are manufactured using traces on a standard two- or four-layer pcb or flex circuit, and thus don't require any external components or materials.

Reliable sensors must be immune to the effects of environmental changes, and must maintain accurate sensitivity levels under all operating conditions. As the temperature or humidity changes, the characteristics of the pcb material will change. As a result, the output level of a printed-circuit capacitive sensor will drift. This could occur when a user goes from an air-conditioned automobile to a hot, humid environment, for example. To avoid intermittent contact errors, real-time drift compensation must be included in the CDC (Fig. 2).

The sensor's ambient value, measured by the CDC during the period when the user isn't making contact with the sensor, drifts as the environmental conditions change (e.g., increasing temperature or humidity). To compensate, the high and low threshold levels are dynamically changed to determine a valid sensor contact. The reference levels are rescaled at positions 2, 3, 5 and 6 to maintain optimal threshold reference levels, automatically tracking and compensating for drift errors.


1. A typical capacitive touch sensor interface is shown.


2. Pictured is the behavior of two sensors under changing environmental conditions with drift compensation enabled.

Printed circuit boards can be plagued by parasitic capacitances as large as 20 pF. This capacitance would offset the threshold at which the capacitive touch sensor is seen as pressed, thus changing its sensitivity. To compensate for the parasitic capacitance, DACs could be programmed to offset the input to the CDC. The parasitic capacitance is uniform from board to board, so a simple adjustment can be made at the time the boards are fabricated. This eliminates the need for external RC tuning components, thus minimizing the costs associated with material, assembly, and test. Individually adjusting the offset for each sensor lets designers take full advantage of the converter's resolution (Fig. 3).


3. Shown is an analog front end where DACs help eliminate the effects of the parasitic capacitances.

Traditional mechanical switches have a familiar sensitivity and tactile feedback. These parameters must also be considered and optimized for capacitive sensors. Different sensors will likely require unique sensitivities depending on the switch function or the switch's physical location on the product. Furthermore, one set of sensitivity settings may not be suitable for every user, so a feature that would allow the user to select different sensitivity settings—from a sensitivity control menu, for instance—would be ideal. The AD7142, for example, supports these sensitivity requirements by allowing an individual 16-bit sensitivity control register to be programmed for each sensor. These registers could also be embedded into the host firmware and provided in a menu display, allowing the user to select different sensitivity levels to meet their particular needs.

Battery power would be consumed unnecessarily if each sensor input was sampled during periods when the user wasn't making contact with a sensor. To maximize battery efficiency, the CDC should be able to detect when the user has stopped making contact with the sensors, and automatically switch to a low-power mode. When the sensor is re-contacted, the IC would automatically be reconfigured into the normal operating mode.

For additional power savings, a complete shutdown mode should be included. In this case, the entire IC would be shutdown whenever the sensors where disabled. Disabling the sensor switches is typically accomplished in portable products by setting a mechanical switch or selecting a blocking mode from a control menu.

An additional benefit of replacing traditional mechanical switches with capacitive sensors is a simpler manufacturing and assembly process. While traditional mechanical switches require manual insertion of each switch into dedicated holes in the plastic housing, a single capacitive sensor board consisting of all the switches can be placed under the plastic casing one step. A sensor board mounting cavity containing an alignment notch and some glue is all that's needed for the sensor board installation and alignment.

The host processor board will emit electromagnetic noise that can couple into the capacitive sensor and sensor traces, resulting in unpredictable sensor behavior. This could degrade performance, but simple methods help minimize the electromagnetic interference (EMI) effects on the sensors. First, the CDC should be mounted on the sensor board. This will minimize the sensor trace lengths, which reduces the chance of EMI being coupled onto the traces. Using a four-layer sensor board with a solid ground plane will provide additional EMI shielding to the sensors. If these two methods don't effectively isolate the EMI noise from the sensors, a grounded metal shield could be placed over the sensor board cavity (Fig. 4).


4. A grounded metal shield can improve the EMI rejection.

Capacitive sensor fields can couple to conductive surfaces such as the product's metal casing or conductive metal painted coatings, resulting in unpredictable sensor behavior. This places a mechanical constraint on how close the edge of a capacitive sensor can be located from the edge of a metal surface. Furthermore, the sensitivity of a capacitive sensor is also related to the thickness of the plastic located directly above the sensors. If the plastic is too thick, the flux field lines won't effectively pass through the plastic, making sensor performance unreliable. Typically, the distance from case to sensor should be greater than 1.0 mm, and the plastic thickness should be less than 4.0 mm to maintain proper sensitivity ranges.

About the author
Wayne Palmer is an applications engineer for the sensor products group at Analog Devices. Wayne received a BSEE from Northeastern University, Boston, MA. He can be reached at wayne.palmer@analog.com.

Take care..............

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#4
In reply to #3

Re: How Do Laptop Touchpads Work?

10/03/2008 4:11 AM

... no prob..it's not my writing...I just googggggled it up.

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#5
In reply to #4

Re: How Do Laptop Touchpads Work?

10/03/2008 4:20 AM

Yes first Google then ask

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#17
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Re: Search Perceptive or Just Google-Eyed?

10/05/2008 5:57 AM

"Yes first Google then ask"

Nicely convivial but...Nay! Sometimes the better part of seeking is perceiving in advance when "Googlets" found are likely only to talk around the answers being sought. (Sometimes better to...Cast the first stone , then Google, wouldn't you say?)

Guess Who??!

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#18
In reply to #17

Re: Search Perceptive or Just Google-Eyed?

10/05/2008 7:16 AM

I dunno.... cast is an anagram of CATS ... are you Boris Johnson? Natasha Kaplinski? Seven of Nine ... ok I give up.

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#19
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Re: Search Perceptive or Just Google-Eyed?

10/05/2008 8:52 PM

I find google very convinient and mostly it comes up with sites with an answer (if you use the correct search arguments).

First look up the answer then if you cannot find the answer, ask.

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#6
In reply to #4

Re: How Do Laptop Touchpads Work?

10/03/2008 4:26 AM

Hello DEl,

just did not want to tread on any toes, that's all. It is also respect! As you found the link.

I thank you back OK?.............

I do sometimes wonder why the posters do not try googling whatever they actually want, you know? There was at least three times in the last 12 hours where I googled exactly the request posted. And got loads of stuff back. I know some people want to 'do things correctly' and, they may have heard of, or read, that you should not put too many words in a search.? Well my way as is most I would think, is to search for what I want, and gradually remove a word until I get the results I was after. There is so many search engines which work in slightly different why to each other. But, if nothing is found, just go the Meta route? I have found you get more result on the subject you searched, is you search for 'singular', leaving the 'S' off the end...........

take care........

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#7
In reply to #6

Re: How Do Laptop Touchpads Work?

10/03/2008 4:44 AM

Yeh, the googling thing can be odd...I wouldn't ask here without at least trying google (other search engines also available) but some obviously don't try first. Maybe when a question is posted there could be a 'Yes I've tried Google' box to check!?

V1sor manged to find the data on my Stupid Switch in Fridge Freezer question, yet when I tried I just got jerked about...
I hate it when you get to a type in a manufacturer and model number but don't get to their website..and even when you do, they have some stupid search engine which doesn't even recognise their own model numbers.

I just happened to strike lucky with the touch pad one, I was interested to see if my understanding of 'em was correct (I thought they were worked by little mice running around inside)

Cheers

Del

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#10
In reply to #7

Re: How Do Laptop Touchpads Work?

10/03/2008 5:22 PM

Hello Del,

Maybe when a question is posted there could be a 'Yes I've tried Google' box to check!?

I have tried this route asking for an extra button on CR4 and it was not welcomed. So, I don't think that will work.

You horrible man, you have just burst my bubble........... Do you mean there isn't little tiny mices running about in there?. I park my 'mouse/mices' on a chunk of cheese, to take care of any problems. Now you say it don't work? How the hell does it work then, and, why is it called a mouse? Oooohhh, I just do not know what to believe in any more! ...........Just don't talk to me right!

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#11
In reply to #10

Re: How Do Laptop Touchpads Work?

10/04/2008 3:03 AM

you have just burst my bubble.

You need the KrisDelTM Bubble Repair kit.
We are hoping to sell a few to the financial institutions and esrate agents at the moment, so place your orders quick.

Del

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#12
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Re: How Do Laptop Touchpads Work?

10/04/2008 4:09 AM

.....and don't forget that the first 100 takers will have a unique opportunity to transfer funds to our new Tulip Investments hedge fund.....

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#14
In reply to #12

Re: How Do Laptop Touchpads Work?

10/04/2008 6:25 PM

Hello Kris,

I have posted Del and am now posting you............Can you tell me about the Tulip Investment hedge fund? Wouldn't these tulips be easier to see and be more colourful if they were not in a hedge? Problem being in a hedge.................

You can often end up with your B.......................................ulbs cut off!

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#16
In reply to #14

Re: How Do Laptop Touchpads Work?

10/05/2008 3:07 AM

Hi babybear,

It's really very simple to make money with KrisDelTM - You send us a large amount of money and we buy nice bulbs on your behalf. Because you are one of the lucky initial 100 investors to get in on the strictly limited offer, others will be keen to buy your stock. Due to the rapid expansion of this bull market it's not worthwhile us sending you the actual bulbs, so we retain them for you. As the market fluctuates, we re-invest your money in other carefully selected funds (such as Porcine Avionics), and operating a Brazilian office helps us to trim the margins. Your money is safe with us, but do remember that stocks can go down as well as up.

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#15
In reply to #12

Re: How Do Laptop Touchpads Work?

10/04/2008 6:27 PM

Hello Kris,

I left a bit out of my last post to you..................

B.........................................ulbs cut off! As in hedge trimming? oowwy, hurts just to think of it.

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#20
In reply to #12

Re: How Do Laptop Touchpads Work?

10/10/2008 8:29 AM

I've seen your adverts!

"We serve as an excellent tool for investors, allowing them to easily and inexpensively bankrupt in their business."

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#13
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Re: How Do Laptop Touchpads Work?

10/04/2008 6:18 PM

Hello Del,

You say "esrate", thats, among other things is what I call them especially when they are working for me and seem to be able to put every 'lickle' idsy bitsy clause in the contract to charge me more than the 'basic' sum for a sale? Do "esrate" agents understand the word 'basic'? I doubt it veerryy much!.......Well, you got to have a go sometimes, right?

By the way, how much is the shipping for the stupendous bubble kit please?

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#9
In reply to #4

Re: How Do Laptop Touchpads Work?

10/03/2008 3:53 PM

"I just googggggled it up"

Sounds like the process by which you'd cough up a hairball...are we absolutely certain it isn't little mice running about inside the fool thing?

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#8

Re: How Do Laptop Touchpads Work?

10/03/2008 10:02 AM

Just in case anyone is interested. (or if your actually looking to build/design one)

www.microchip.com has a ton of information about this in support of their Mtouch technology. There is a link right on the front page.

Good luck with your endeavors!

(God knows I need it with mine!)

bill12780

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#21

Re: How Do Laptop-Touchpad-Touching Fingers Work?

11/04/2008 6:32 AM

Maybe the real question is: What is the finger property that allows the touch pad to respond and work? For pads that respond to breaking a field, how does the finger accomplish that? How could the pad still be used with something other than a living finger? Maybe this is what the OP was getting at?

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