Design and Characterization of Automated Color

Sensors System
Maher Assaad, Israel Yohannes, Amine Bermak, Dominique Ginhac, Fabrice
Meriaudeau

To cite this version:

Maher Assaad, Israel Yohannes, Amine Bermak, Dominique Ginhac, Fabrice Meriaudeau. Design and
Characterization of Automated Color Sensors System. International Journal on Smart Sensing and
Intelligent Systems, 2014, 7 (1), pp.1-12. <hal-00947786>

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 DESIGN AND CHARACTERIZATION OF AUTOMATED
COLOR SENSOR SYSTEM

Maher Assaad1,*, Israel Yohannes1, Amine Bermak2, Dominique Ginhac3 and, Fabrice Meriaudeau3
1
Department of Electronics and Communication Engineering, American University of Ras
Alkhaimah, Ras Alkhaimah, United Arab Emirates.
2
Department of Electronic and Computer Engineering , Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR.
3
Laboratoire Electronique, Informatique et Image, University of Burgundy, Dijon, France.
Emails: maher.assaad@aurak.ae, izzzrael@gmail.com, eebermak@ee.ust.hk,
dominique.ginhac@u-bourgogne.fr, fabrice.meriaudeau@u-bourgogne.fr

Submitted: Accepted: Published:

Abstract: The paper presents a color sensor system that can process light reflected from a
surface and produce a digital output representing the color of the surface. The end-user
interface circuit requires only a 3-bit pseudo flash analog-to-digital converter (ADC) in place
of the conventional/typical design comprising ADC, digital signal processor and memory. For
scalability and compactness, the ADC was designed such that only two comparators were
required regardless of the number of color/wavelength to be identified. The complete system
design has been implemented in hardware (bread board) and fully characterized. The ADC
achieved less than 0.1 LSB for both INL and DNL. The experimental results also demonstrate
that the color sensor system is working as intended at 20 kHz while maintaining greater than
2.5 ENOB by the ADC. This work proved the design concept and the system will be realized
with integrated circuit technology in future to improve its operating frequency.

Index Terms: Color sensor, light sensor, analog to digital converter (ADC), flash ADC

1
 I. INTRODUCTION

Color sensor systems are increasingly being used in automated applications to detect
automation errors and monitor quality at the speed of production line. They are used in
assembly lines to identify and classify products by color. The objectives of their usage include
to check the quality of products [1-3], to facilitate sorting and packaging [4-6], to assess the
equality of products in storage [7, 8], and to monitor waste products [9]. Consequently, there
is abundant of color sensors and the choice is often application-driven [22, 23]. Low cost and
simple color sensors are preferred over sophisticated solutions for less demanding
applications where the top priority is cost and power consumption. This paper presents such a
sensor that is based on an embedded analog-to-digital converter (ADC) as the end-user
interface circuit. The proposed system is capable of differentiating up to eight different colors
and thus, it is specifically targeted for automated applications where few colored products
need to be detected.

The principle of detection embedded in our color sensor system is based on reflectance
property of a colored surface, as illustrated in Figure 1. When a white light is shined on a
surface, the surface will reflect a specific spectrum while absorbing all the other spectra. For
example, a white light focused onto a red surface is reflected as red. The reflected red light
impinges on the light transducer producing a corresponding output voltage that is typical of
the reflected red color. By interpreting this output voltage using an interface circuit, the color
can be determined.

sensor
LED display (one

source
active)
photodiode

Interface 8
circuit

Surface to be analysed Color sensor system

Figure 1. Surface color detection principle based on reflectance.

The success of a color sensor by reflection depends on the detector, the illuminant (light
source) and the target object. Black and white objects reflect respectively 0 and 100% of the
incident light falling on them while other colors reflect selected parts in the visible spectra.
Rough, glossy or dirty surfaces make the detection rather complicated since their reflectance
property is different, and the source of light affects the color saturation of the object being
detected [10]. For our system, the most appropriate source is a nearly pure white light (i.e.
D65 daylight).
The detector of the color sensor system also plays a crucial role for efficient color recognition.
A photodiode is used in the proposed color sensor system as the principal detector element.
Photodiodes generate electron and hole carriers upon exposure from visible light inducing
photocurrent. The maximum photo-current can be given by (1.1) [11, 12]:

Pin
Iph = (1- R)*(1- ea*d )*q* (1.1)
h* v

where R is the surface reflectivity of the photodiode; d is depth from the silicon surface; α is
the absorption coefficient which is dependent on the material property (e.g. silicon or GaAs);
q is electron charge; Pin is the power of incident light; h is Planck’s constant ; and v is the
frequency of incident light. The following sections present the system design, the operating
principle and the characterization of our color sensor system.

II. COLOR SENSOR SYSTEM DESIGN

The proposed color sensor system incorporates a detector and a dedicated interface circuit.
The interface circuit offers a lower cost and power consumption solution, compared with the
conventional design. As depicted in Figure 2(a), conventional color sensor systems comprise
(i) a light transducer, (ii) a pre-amplifying block to amplify the sensor signal and/or convert
the signal from current to voltage domain, (iii) an ADC, (iv) a digital signal processor (DSP)
for color recognition which may include a memory, digital comparator arrays, arithmetic units
and so on to produce the final color output. In our design, we minimize the component count
by storing the color table (mapping between color and photodiode output voltage) with
variable resistors acting as analog memory, and generate digital color code using an
embedded ADC (refer to Figure 2(b)) eliminating the need for further processing components.
 I
Light I I-to-V Light I-to-V
Transducer Converter Transducer Converter

color li is stored
as a digital word ADC
[b0b1…..b7]
Proposed interface detected color
[B0B1…..B7]
circuit with embedded
ADC
Memory DSP

(a) (b)

Figure 2. Block diagram of (a) conventional color detection scheme (b) color detection
using the proposed color sensor system.

a. Transducer

The complete block diagram of the proposed color sensor system is shown in Figure 3. It
comprises of a monolithic device OPT301 [13], an opto-electronic integrated circuit
containing a light sensing element (photodiode) and a transimpedance amplifier on a
monolithic device. The responsivity of the photodiode is dependent on the wavelength and the
radiant power of the incident light falling on it. The photovoltage versus wavelength response
normalized with the incident power is shown in Fig 3(a). The gain response is adjustable by
using the feedback resistor RF such that the output photovoltage is the product of the
photocurrent (Iphd) and RF (Vphd = Iphd * RF).

b. Interface Circuit

The interface circuit is based on a pseudo flash ADC (shown in Fig 3(b)) that has been
customized to meet the requirements of the targeted application. Indeed, the selection of a
specific ADC architecture is always driven by design requirements. For high-speed, low-
resolution applications, the flash architecture is a promising approach [14]; for applications
such as sensor networks that do not require high speed and can tolerate data latency but have
ultra-low power requirements, successive approximation (SAR) ADCs are usually used [15,
16]. Often designers combine features of one or more architectures to achieve the desired
performance. The ADC designed in this work offers a balance between power consumption
and speed by combining features of a flash and a digital ramp ADC. The interface circuit
needs two comparators only, regardless of the number of colors, Ncolor, to be identified. This
minimizes the power as well as the on-chip area, especially when Ncolor is large.
The proposed design uses a set of tunable resistors, which form part of the ADC to store the
color table. This is easily achieved as the photodiode displays a monotonous response Vphd to
different wavelength/color. If the full scale of the ADC is 2 V, we could thus map different
voltage intervals for different colors as shown in Table 1. These intervals can be easily
adjusted based on calibration result from the photodiode. The proposed design uses eight
indicator blinking LEDs as a simple yet clear means to indicate the final output for user
through a 3-to-8 decoder where each LED represents one color/voltage interval.

CF TCLK active bit

RF
Up-Down duty cycle digital output
Photodiode Vref D/U Counter B[0:7]
0 controller
(Vphd) to S/H
1
V7 2AMUX1
. S[0:2] En
.
Comp1 Decoder
OPT301 Timer
7
Vphd b0
b1
st

S[0:2]
ere
Photo-Voltage (V/µW)

b2
V2
int

S/H DFF
of

V1
ion

7
.
reg

D/U
. Comp2
AMUX2
2
Vo 1 Pseudo flash
0 ADC Complete interface
circuit
Wavelength (nm)

(a) (b)

Figure 3. Proposed color sensor system (a) OPT301 photodiode and its gain adjustable photo-
voltage response. (b) Interface circuit of the color sensor.

Table 1: Reference voltage intervals and their corresponding color values

Color Designation Voltage intervals
Violet B0 0.00 – 0.25
Indigo B1 0.25 – 0.50
Blue B2 0.50 – 0.75
Green B3 0.75 – 1.00
Yellow-Green B4 1.00 – 1.25
Yellow B5 1.25 – 1.50
Orange B6 1.50 – 1.75
Red B7 1.75 – 2.00
The timer block serves the function of adjusting the duty cycle of the LED blinking. In the
case of fast color change, the timer holds the output for longer time so that the output is
visually distinguished by a user. Otherwise for constant outputs, the timer limits the duty
cycle so the LEDs does not draw current continuously there by lowering the power
dissipation. The design also incorporates a sampler after the photodetector to ensure dynamic
(time varying) color detection with minimum error.

III. OPERATING PRINCIPLE OF THE INTERFACE CIRCUIT

The operating principle of the proposed design is depicted in the conceptual diagram Figure 4.
The two analog multiplexers switch through the reference voltages creating a closed interval
dynamic voltage reference. The comparators compare the photodiode response Vphd with the
moving reference (signals AMUX1 and AMUX2) until the correct interval is found (V3 and
V4 in Fig 4(b)). Then the comparators will fire simultaneously as can be seen in Figure 4(b).
At this point, the counter state is registered and outputs the corresponding digital code. This
code is kept in the display until a new color is detected.

b0b1b2 = xxx b0b1b2 = 011

Vref TCLK S0 TCLK
Up-down D Vref S0
D
S1 DFF Up-down S1 DFF
D/U Counter x D/U
S2 Counter S2 0

V4 V4
Vphd D x Vphd D
DFF 1
AM
DFF
V3 UX
V3 1
AM
V2 1 D x
UX
2 1
DFF
V2 D 1
X1 DFF
A MU
1 1
V1 V1
1 1
2
UX
AM
V0 S0S1S2 V0 S0S1S2

(a) (b)
Figure 4. Conceptual diagram illustrating operation of the embedded ADC. (a) The
AMUX’s switch through the ladder voltage until both comparators fire, during this time
the output is not updated from previous detection. (b) Both comparators fire indicating
conversion process is completed.
The multiplexers are controlled by an up-down counter, which in turn is controlled by the
comparison result (D/U). Compared with using one-sided regular counter, the proposed search
scheme provides a faster conversion speed with a small hardware overhead.
As mentioned, the key advantage of the proposed system is the minimization of components
count in the interface circuit which is the number of comparators being fixed to two,
regardless of the number of detectable colors, as opposed to the 2^N-1 comparators in a
conventional flash ADC, albeit with some trade-off in conversion speed. For the present 3-bit
implementation, the average sampling speed of the interface circuit τs, without the timer
effect, is estimated to be 4 clock cycles; in the worst case condition where the AMUXs have
to switch through all reference levels to reach the desired interval, τs is 8 clock cycles; in the
best case where the desired interval is found on its first search, τs is one clock cycle. The
average sampling rate can then be estimated as half the maximum rate. One clock cycle (τclk)
can be measured based on the propagation delay of the constituent ICs, in this case 1/τclk = 2.8
MHz. This gives 1/τs, a sampling rate of 700 kHz for the interface circuit,

τclk ≥ δpd = δCOUNTER + δMUX + δCOMP + δAND + δDFF (1.2)

τclk ≤ τS ≤ 8*τclk (1.3)

(τS)avg. = 0.5*(τclk)MAX = 4τclk (1.4)

where, δpd is the total propagation delay and δcounter, δMUX, δAND, δDFF are the propagation
delay of counter, AMUX, comparator, AND gate and D flip flop (DFF) respectively. As
mentioned previously, the stored voltages using the variable resistors represent color voltages,
which are calibrated to the sensor system before measurement begins.

IV. EXPERIMENT RESULTS

The proposed color sensor system has been implemented using discrete components for a
quick proof of design concept. Validation was carried out by characterizing the embedded
ADC then the complete color sensor system. Experimental setup for a quick proof of concept
implementation on a project board is shown in Figure 5.
 Figure 5. Experimental setup to test the proposed color sensor

a. Interface Circuit Characterization

The principal element of the interface circuit, the pseudo Flash ADC, was realized for a full-
scale voltage of 2 V. The typical static and dynamic performance of the ADC is plotted in
Figure 6. The plots show that the static characteristic of the ADC is good, achieving 0.1 LSB
for both INL and DNL. In addition, regarding the dynamic characteristics, it achieved more
than 2.5 ENOB at 20 kHz input frequency (input color change) on a project board.

0.1
DNL
0.08
0.06 INL
0.04
error, LSB

0.02
0
‐0.02
‐0.04
‐0.06
‐0.08
‐0.1
1 2 3 4 5 6 7 8
(a) Output Code

2.5
ENOB

1.5
5 10 15 20 30 40 50
(b) Input Frequency in kHz

Figure 6. Embedded ADC characteristics in proposed interface circuit. (a)

Differential and integral nonlinearity errors. (b) Effective number of bits against
input frequency (in kHz). The interface circuit is running at 1/τs = 700 kHz.
b. Color Sensor System Characterization

Experiment results from the complete color sensor system, i.e. the digital outputs for different
detected colors, as captured from the TDS oscilloscope are shown in Figure 7. The TDS
oscilloscope has only 4 channels, hence 4 out of the 8 (output code: B0 to B7) decoded bits
are shown at a time. Each bit is connected to one LED, i.e. B0 represents violet, B1 indigo,
B2 blue, B3 green, B4 yellow-green, B5 yellow, B6 orange, and B7 red color respectively. As
illustrated, the duty cycle of the output can be varied by using the timer in order to reduce the
power drawn by the LEDs and also ensure minimum time for output to be discerned by user.

V
B0 (Blinking Violet) V B0

V B1 V B1

V B2 V B2 (Blinking Blue)

V B3 V B3
time time

(a) B0 = 1, color violet detected. (b) B2 = 1, color blue detected.

V B4 V B4

V B5 (Blinking Yellow) V B5
V B6 V B6

V B7 B7 (Blinking Red)
V
time time

(c) B5 = 1, color yellow detected. (d) B7 = 1, color red detected.

Figure 7. Experimental result from the proposed sensor system. The output bits and
their corresponding color is shown. (Only four bits are shown at a time from a 4-
channel oscilloscope).
 V. DISCUSSION

In our previous work [17, 18], we have proposed color sensor based on similar color detection
mechanism. This work includes significant improvements over the previous such as (i) it uses
two-sided counter (up/down) instead on one sided which contributed to improved efficiency
and speed of conversion (ii) it enables dynamic (online) detection by incorporating a sampler
after the light transducer unit (iii) it implements output control by regulating duty cycle using
timer, which attributes to saving power. ADC characterization result of the interface circuit is
also included. The core design concept about the proposed color sensor system is low
component-count and scalability. While the design has been implemented for 3-bit (8
different colors), it can easily be extended to detect more than eight different colors. Apart
from being low component count and low power consumption, the advantage of the proposed
system is its ability to generate a digital output/code representing directly the detected color.
This ensures a straightforward computation and eliminates the need of further data processing.
This is desirable for easy and fast integration with subsequent units in the production line. The
proposed system can be applied for assembly lines where products need to be sorted by color
or colored mark. Experimental results show that the system is working as intended. For a
successful detection, careful manual calibration of the voltage intervals is required before use.
Future work includes integrating a dedicated CMOS photodiodes that suitable for color
sensing as in [19-21] along with the proposed interface circuit on a single chip and
introducing auto calibration mechanism.

VI. CONCLUSION

A new, automated color sensor system based on an embedded 3-bit pseudo flash ADC has
been realized and successful experimental results of the sensor system and the interface circuit
is presented. Admittedly, the operating frequency (successful color detection rate) of the
complete system is rather low, i.e. goes up to 20 kHz for > 2.5 effective number of bits, which
is mainly attributed to its current implementation as discrete components; however it can be
significantly improved by using integrated circuit technology (i.e. much lower gate
propagation delay). The proposed color sensor system design is low-cost, has low component
count and simple yet scalable for higher resolution. We envisage it to be applied in automated
color detection applications such as color sorting and matching applications.
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