Identification of paper media using impedance analysis
A microcontroller-based system for identifying a paper type of a sample of paper from a measurement of its electrical impedance. An interdigital dielectric sensor ( 55 ) is deployed in the paper path of a printer (PTR), and the electrical impedance at the sensor, as affected by a sheet of paper (P) near the sensor, is measured over a plurality of frequencies of a stimulus signal. The stimulus signal may be sinusoidal or a square wave. The impedance characteristic, in magnitude or phase, or both, is compared against a plurality of reference impedance characteristics, each associated with a paper type, to identify the closest match and thus the type of paper of the sample sheet.
1. A system for identifying a paper type of a paper sample, the system comprising:
a memory configured to store reference impedance characteristics of paper types;
stimulus generator circuitry having a stimulus output, the stimulus generator circuitry configured to generate a stimulus at the stimulus output;
a sensor having: a stimulus component coupled to the stimulus output, the stimulus component configured to generate electric fields through the paper sample at fundamental frequencies responsive to the stimulus; and a sense component configured to sense the electric fields and generate analog signals representative thereof;
an analog-to-digital converter having: an analog input coupled to the sense component; and a digital output; and
integrated circuitry coupled to the digital output and the memory, the integrated circuitry configured to: receive digital information from the digital output, the digital information representative of the analog signals; responsive to the digital information, determine a sample impedance characteristic for the paper sample; and compare the sample impedance characteristic with the reference impedance characteristics to identify the paper type of the paper sample.
2. The system of claim 1 , wherein the stimulus is a sinusoidal stimulus.
3. The system of claim 1 , wherein the stimulus is a square wave stimulus.
4. The system of claim 3 , further comprising:
clock circuitry configured to generate a base clock signal; and
an anti-aliasing filter, coupled between the stimulus output and the stimulus component, the anti-aliasing filter configured to filter the stimulus to remove frequencies above a threshold;
the stimulus generator circuitry comprising a digital driver circuit configured to generate the square wave stimulus at a fundamental frequency having a period equal to a first integer number of cycles of the base clock signal;
the analog-to-digital converter being configured to receive the analog signals at a sample frequency having a period equal to a second integer number of cycles of the base clock signal; and
the integrated circuitry being configured to execute a discrete Fourier transform on the digital information, using a window of samples numbering an integer multiple of a separation number, the separation number defined by: the first integer, divided by a greatest common divisor of the first and second integers.
5. The system of claim 1 , wherein the sensor comprises:
an interdigital dielectric sensor.
6. The system of claim 1 , wherein the stimulus generator circuitry, the analog-to-digital converter, and the integrated circuitry are integral in a single integrated circuit.
7. The system of claim 1 , wherein the sample impedance characteristic comprises:
a sample magnitude versus frequency characteristic; and
a sample phase versus frequency characteristic.
8. The system of claim 7 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
detecting a polarity of the first derivative of the sample phase versus frequency characteristic over frequency; and
selecting one or more of the reference impedance characteristics exhibiting a phase versus frequency characteristic with a same polarity of the first derivative over frequency.
9. The system of claim 1 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
comparing a value of the sample impedance characteristic at a first frequency with values of at least first and second reference impedance characteristics at the first frequency;
determining which of the at least first and second reference impedance characteristics most closely matches the sample impedance characteristic at the first frequency; and
repeating the comparing and determining at a second frequency.
10. The system of claim 1 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
applying the sample impedance characteristic to an adaptive network.
11. A method of identifying a paper type of a paper sample, the method comprising:
placing a paper sample near a sensor;
applying a stimulus signal at fundamental frequencies to the sensor;
sampling a response signal from the sensor, in response to the stimulus signal;
processing the sampled response signal for the fundamental frequencies, to determine a sample impedance characteristic for the paper sample; and
comparing the sample impedance characteristic with the reference impedance characteristics to identify the paper type of the paper sample.
12. The method of claim 11 , wherein the stimulus signal is a sinusoidal stimulus signal.
13. The method of claim 11 , wherein the stimulus signal is a square wave stimulus signal.
14. The method of claim 13 , further comprising:
generating a base clock signal; and
filtering the stimulus signal to remove frequencies above a threshold;
wherein applying the stimulus signal comprises generating the square wave stimulus signal at a fundamental frequency having a period equal to a first integer number of cycles of the base clock signal;
wherein sampling the response signal comprises sampling the response signal at a sample frequency having a period equal to a second integer number of cycles of the base clock signal; and
wherein processing the sampled response signal comprises executing a discrete Fourier transform on the sampled response signal, using a window of samples numbering an integer multiple of a separation number, the separation number defined by: the first integer, divided by a greatest common divisor of the first and second integers.
15. The method of claim 11 , wherein the sensor comprises:
an interdigital dielectric sensor.
16. The method of claim 11 , wherein the sample impedance characteristic comprises:
a sample magnitude versus frequency characteristic; and
a sample phase versus frequency characteristic.
17. The method of claim 16 , wherein comparing the sample impedance characteristic comprises:
detecting a polarity of the first derivative of the sample phase versus frequency characteristic over frequency; and
selecting one or more of the reference impedance characteristics exhibiting a phase versus frequency characteristic with a same polarity of the first derivative over frequency.
18. The method of claim 11 , wherein comparing the sample impedance characteristic comprises:
comparing a value of the sample impedance characteristic at a first frequency with values of at least first and second reference impedance characteristics at the first frequency;
determining which of the at least first and second reference impedance characteristics most closely matches the sample impedance characteristic at the first frequency; and
repeating the comparing and determining at a second frequency.