Author Topic: storing an analog voltage  (Read 2363 times)

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Offline BEAMerTopic starter

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storing an analog voltage
« on: November 21, 2008, 09:57:02 PM »
hi,

i am building a pressure sensor array and reading the pressure profile in a computer. i have problem of scanning the array so i thought of storing the analog voltage generated by each sensor during a run. later this can be used as an input to the ADC and the data can be transmitted to the computer.

can such a circuit be designed. i though of using a capacitor to charge upto the analog value(like the ones used in sample and hold circuits). is this right? or are there any better circuits to store analog voltage?

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Offline szhang

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Re: storing an analog voltage
« Reply #1 on: November 21, 2008, 11:49:24 PM »
Usually capacitors are used, though they take time to charge, and their voltage will fall if they get disconnected for too long...

What problems are you having scanning the array with the ADC?
« Last Edit: November 21, 2008, 11:51:55 PM by szhang »

Offline BEAMerTopic starter

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Re: storing an analog voltage
« Reply #2 on: November 22, 2008, 02:09:45 AM »
This is the post that i had posted earlier....

"

This is not very much related to robotics but i have a doubt on how to read data form a single pressure sensor that  has been arranged in the form of a linear array. the sensor is a resistive ink sensor whose resistance decreases linearly with pressure. for an application, the sensors are arranged in a linear array (19 sensors arranged in a single line) and the finger is moved over the array. my aim is to build a circuit that can read this finger movement and display the results in a computer. from the data in the computer, i can see how much pressure i have applied during the whole course of movement of my finger.

This is my purpose and i dont know the circuit and the algorithm of how to scan the sensor data using a microcontroller. i am planing to use serial communication to send the data to the computer where a graph is drawn with the data. moreover the ADC part is only comfusing me. should i use a single ADC or many? are there  any circuits like this or should i design one? please guide me.

Thanks in advance!

BEAMer



Offline BEAMerTopic starter

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Re: storing an analog voltage
« Reply #3 on: November 22, 2008, 02:15:25 AM »
i think you understood the problem.

The response time of the sensors is very low (< 5 microseconds), if iam scanning the sensor one by one, and using a single ADC to convert to digital, all the 19 sensors will take time in the order of milli seconds. But the response time of the sensor is very low, and if the person is moving his hand, and if by any chance i am scanning some other sensor,  by the time i come and scan this sensor, i will be registering a lower value than that was applied...

This will affect all the readings and this error cannot be corrected because it is random.... so what to do? you have any better ideas?

Thankyou

 

Offline izua

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Re: storing an analog voltage
« Reply #4 on: November 22, 2008, 03:41:52 AM »
You can't scan sensors one by one, because you can't predict the speed the user's finger will move at.
You need to store all their values at a given moment in time, we'll call this unit frame. You will need to store several frames, if you wish to analyse the motion over the sensors. Higher sample rates will require more storage space for the same amount of time, but will yield more time-accurate results.
« Last Edit: November 22, 2008, 03:42:41 AM by izua »
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Offline BEAMerTopic starter

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Re: storing an analog voltage
« Reply #5 on: November 22, 2008, 04:07:15 AM »
oh.......!
so how do i store it...  the sensors produce a analog voltage depending on the pressure. so should i store the analog value or the digital? if i have to store the digital then should i use 19 ADC (this would become very costly)


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Offline szhang

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Re: storing an analog voltage
« Reply #6 on: November 22, 2008, 01:23:51 PM »
A decent ADC should be able to take thousands of samples per second.  The chips I am using are rated to 1msps @ 10bits and 500ksps at 12bits.  Even at 500ksps you can scann the whole array at ~25KHz.

Usually for embedded stuff a 1kHz control loop is more than fast enough.  If you must, some microcontrollers can do simultaneous ADC readings (in my case, 4 channels @ 10bits), that would mean you can scann the array alot faster.

If you want to slow down the response time, you can attach a capacitor to the output of each sensor.

Offline Admin

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Re: storing an analog voltage
« Reply #7 on: November 28, 2008, 12:26:10 AM »

Offline BEAMerTopic starter

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Re: storing an analog voltage
« Reply #8 on: November 28, 2008, 12:24:29 PM »
sorry, the thing is, i didnt get a reply for a long time for the other post. thats why i had to start this new post. i will make sure this will not happen again.

BEAMer

 


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