Device and method capable of converting a position signal into a frequency signal using a photoelectric position sensitive detector

A device and method capable of converting a position signal into a frequency signal using a photoelectric position sensitive detector is provided. The device capable of converting a position signal into a frequency signal comprises: a PSD, two current mirrors, a variable optimum constant current source, and a converter. The method capable of converting a position signal into a frequency signal comprises: providing a PSD, providing two current mirrors, providing a variable optimum constant current source, and providing a converter to convert a position signal into a frequency via a relationship.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device and method capable of converting 
a position signal into a frequency signal, and more particularly relates 
to a device and method capable of converting a position signal into a 
frequency signal using a photoelectric position sensitive detector (PSD). 
2. Description of the Related Art 
In the prior art of a photoelectric system capable of displaying a position 
signal, two terminals of a PSD output two photocurrents when a light spot 
is detected by the PSD. FIG. 1 shows a device and method of calculating a 
light spot position according to the prior art. As shown in FIG. 1, two 
photocurrents (I1, I2) are respectively converted into two voltage signals 
(Vx1, Vx2) by two operational amplifiers (OP AMP) A1 and A2. An adder 
outputs a (Vx1+Vx2) voltage signal; a subtracter outputs a (Vx2-Vx1) 
voltage signal; and a divider outputs a (Vx2-Vx1)/(Vx1+Vx2) i.e. 
(I2-I1)R1/(I1+I2)R1! voltage signal. The (Vx2-Vx1)/(Vx1+Vx2) voltage 
signal will be converted into a signal via an analog-to-digital (A/D) 
converter and a single chip microprocessor. Then a signal corresponding to 
the light spot falling on PSD will be shown on an LCD panel. 
However, this kind of prior art has the following disadvantages. First, it 
is too complicated due to usage of a lot of circuits, for example, a 
divider, or an A/D converter . . . etc. Second, a DC drift signal due to 
DC amplifier circuit will influence the displaying result. 
SUMMARY OF THE INVENTION 
Accordingly, there is a need to improve the prior art. 
The main object of the present invention is to provide a device and method 
capable of converting a position signal into a frequency signal using a 
photoelectric position sensitive detector. 
One object of the present invention is to simplify the prior art circuit. 
Another object of the present invention is to provide a device and method 
unaffected by a DC drift signal resulting from DC amplifier circuit. 
A novel device and method capable of converting a position signal into a 
frequency signal using a photoelectric position sensitive detector is 
provided according to the present invention. FIG. 2 is a simple diagram 
presenting the present invention. Now referring to FIG. 2, a device 
capable of converting a position signal into a frequency signal, 
comprising: a PSD 102, having a first output terminal X1, a second output 
terminal X2, and a voltage terminal Vt wherein said first output terminal 
X1 of said PSD 102 outputs a first value I1 of a first photocurrent and 
said second output terminal X2 of said PSD 102 outputs a second value I2 
of a second photocurrent when a light spot is detected by said PSD 102; a 
first current mirror 125, coupled with said first output terminal X1 of 
said PSD 102, mirroring said first photocurrent to generate a first mirror 
current I1m having a current value substantially equal to said first value 
I1; a second current mirror 124, coupled with said voltage terminal Vt of 
said PSD 102, providing a working current 10 equal to a sum of said first 
photocurrent and said second photocurrent to said PSD 102 and generating a 
second mirror current 10m having a value substantially equal to said 10; a 
variable optimum constant-current source 110, coupled to said second 
output terminal X2 of said PSD 102, generating a constant current having a 
third value Ib, wherein Ib is larger than both the maximum values of said 
I1 and said I2; and a converter 127, coupled to said second output 
terminal X2 of said PSD 102, converting said first mirror current, said 
second photocurrent, and said constant current into a frequency signal. 
FIG. 2 also shows a method of the present invention. A method capable of 
converting a position signal into a frequency signal comprises: providing 
a PSD having a first output terminal, a second output terminal, and a 
voltage terminal wherein said first output terminal of said PSD outputs a 
first value I1 of a first photocurrent and said second output terminal of 
said PSD outputs a second value I2 of a second photocurrent when a light 
spot is detected by said PSD; providing a first current mirror coupled to 
said first output terminal of said PSD, mirroring said first photocurrent 
to generate a first mirror current 11m having a value substantially equal 
to said I1; providing a second current coupled to said voltage terminal of 
said PSD and a power supply, providing a working current I0 equal to a sum 
of said first photocurrent and said second photocurrent to said PSD and 
generating a second mirror current I0m having a value substantially equal 
to said I0; providing a variable optimum constant-current source coupled 
to said second output terminal of said PSD, generating a constant current 
having a third value Ib, wherein Ib is larger than both the maximum values 
of said I1 and said I2; and providing a converter coupled to said second 
output terminal of said PSD, having a resistance value and a capacitance 
value, converting a synthesis current Ic=Ib+I2-I1m into a frequency signal 
via a relationship X=f=kIc/VC, wherein X is the position signal of said 
light spot; f is the frequency signal of said light spot; k is a constant 
depending on aspect of the voltage divider in the resistors across both a 
negative input terminal of an operational amplifier and a positive input 
terminal of another operational amplifier of said converter; C is said 
capacitance value of said converter; and V is the voltage level of said 
power supply. 
The device of present invention is simpler than that of prior art because 
the position calculation method is changed from a voltage signal 
(Vx2-Vx1)/(Vx1+Vx2) to a current signal Ic=Ib+I2-I1m. The relationship 
between a synthesis current and a frequency of a frequency signal (or a 
position) is X(position)=f=kIc/VC. In other words, a different position 
signal corresponds to a different frequency signal, as shown in FIG. 3. If 
a light spot is located on the middle point of PSD, a frequency 
corresponding to a condition I1=I2 is referred as a middle frequency. If a 
light spot is located near X2 terminal, a frequency corresponding to a 
condition I1&lt;I2 is higher than the middle frequency. If a light spot is 
located near X1 terminal, a frequency corresponding to a condition I1&gt;I2 
is lower than the middle frequency. 
Now referring to FIG. 2, the variable optimum constantcurrent source can be 
used to compensate for a dark current difference between the first 
terminal X1 and the second terminal X2 of the PSD. The negative current 
mirror coupled with the PSD provides a working current I0 and a second 
mirror current I0m. The second mirror current I0m can be used to identify 
whether the light spot is located on the PSD. 
Compared with the prior art, the present invention does not require a 
divider and an A/D converter, so the circuit of the present invention is 
simpler than that of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now referring to FIG. 4, two photocurrents (I1, I2) are output from two 
terminals (X1 and X2)of a PSD 102 when a light spot is detected by PSD 
102. Working current I0 is provided by PMOSFET (p-channel metal oxide 
semiconductor field effect transistor) 109 coupled with voltage terminal 
of PSD 102. A negative current mirror 124 comprising of PMOSFET 107 and 
PMOSFET 108 is used for mirroring the working current I0. Comparator 106 
is used to provide an identification of whether light spot is located on 
PSD 102. If no light spot is located on PSD 102, a condition I1=I2 still 
occurs. This condition is the same with that of a light spot located at 
the middle point of PSD. However comparator 106 can identify the condition 
to be invalid. 
A positive current mirror 125 comprises NMOSFET 103, NMOSFET 104, and 
NMOSFET 105. Photocurrent I1 output from terminal X1 of PSD 102 is passed 
through NMOSFET 103; and NMOSFET 104 and NMOSFET 105 are used to mirror 
the photocurrent I1 to generate a first mirror current I1m having a value 
substantially equal to I1. NMOSFET 105 is connected to terminal X2 of PSD 
102 and to a variable optimum constant-current source 110 wherein the 
value of Ib should be larger than both maximum of I1m and 12. Then a 
positive synthesis current Ic=Ib+I2-I1m will flow into capacitor 112, the 
positive input terminal of a first OP AMP 113 and the negative input 
terminal of a second OP AMP 114. 
When capacitor 112 is charged by the synthesis current Ic over a 
predetermined voltage level, OP AMP 113 outputs a high voltage level, and 
OP AMP 114 outputs a low voltage level to drive output terminal Q of RS 
flip-flop 120 to a high voltage level and to turn on switch transistor 111 
to discharge capacitor 112. 
When capacitor 112 is discharged below a predetermined voltage level, OP 
AMP 113 outputs a low voltage level and OP AMP 114 outputs a high voltage 
level to turn off switch transistor 111. Capacitor 112 is charged by the 
synthesis current Ic again. The above operations of charging and 
discharging are repeated over and over. As shown in FIG. 4, a buffer 121 
is coupled to the output terminal Q of RS flip-flop 120; a single chip 
microprocessor 122 is coupled to buffer 121 and calculates a position 
based on the formula Ic=Ib+I2-I1m; a LCD panel 123 is coupled to the 
single chip microprocessor 122 to display a position signal corresponding 
to said light spot falling on PSD 102. 
If a light spot is located at a different position on PSD 102, variable 
photocurrents are output from terminal X1 and terminal X2 to form a 
variable photocurrent difference and output a different frequency, as 
shown in FIG. 3. One terminal of resistor R117 and one of resistor R118 
are coupled to the negative input terminal of OP AMP 113. The other 
terminal of resistor R118 and one terminal of resistor R119 are coupled to 
the positive input terminal of OP AMP 114. The other terminal of resistor 
119 is coupled to the ground terminal. The other terminal of resistor R117 
is coupled to the power supply. The relationship between the frequency and 
synthesis current is X(position)=f=kIc/VC, wherein X is the position of 
the light spot, f is the output frequency, k is a constant which depends 
on aspect of the voltage divider in the resistors across both the negative 
input terminal of OP AMP 113 and the positive input terminal of the other 
OP AMP 114; V is the voltage level of power supply; and C is the 
capacitance value of capacitor 112. Furthermore, when the values of the 
resistors 117, 118, 119 are all the same, k is equal to 3. 
From the relationship X(position)=f=3Ic/VC, when I1=I2, i.e. a light spot 
is located at the middle point of PSD 102, synthesis current Ic will be 
equal to Ib. So the variable optimum constant-current source Ib can be 
used to define the middle working frequency. On the other hand, if an 
unbalanced dark current occurs on PSD 102, variable optimum 
constant-current source 110 may be used to compensate for the unbalanced 
dark current. If the dark current of terminal X1 is greater than that of 
terminal X2, the setting value of variable optimum constant-current source 
110 should be a little larger than original setting value of variable 
optimum constant-current source 110 to compensate an unbalanced dark 
current. If the dark current of terminal X1 is smaller than that of 
terminal X2, the setting value of variable optimum constant-current source 
110 should be a little lower than original value of variable optimum 
constant-current source 110 to compensate an unbalanced dark current. 
As shown in FIG. 4, a method capable of converting a position signal into a 
frequency signal using a photoelectric position sensitive detector 
according to the present invention comprises: 
(1)providing a PSD 102 having a first output terminal X1, a second output 
terminal X2, and a voltage terminal Vt wherein the first output terminal 
X1 of the PSD 102 outputs a first value I1 of a first photocurrent and the 
second output terminal X2 of the PSD 102 outputs a second value I2 of a 
second photocurrent when a light spot is detected by the PSD 102; 
(2)providing a first current mirror 125 coupled to the first output 
terminal X1 of the PSD 102, mirroring the first photocurrent to generate a 
first mirror current I1m having a current value substantially equal to I1, 
the first current mirror comprising: 
a first NMOSFET 103, having a drain coupled to the first output terminal X1 
of the PSD 102, a source coupled to a ground terminal, and a gate coupled 
to a third NMOSFET 104; 
a second NMOSFET 105, having a drain coupled to the second output terminal 
X2 of the PSD 102, a gate coupled to the first output terminal X1 of the 
PSD 102, and a source coupled to the gate of the first NMOSFET 103; and 
a third NMOSFET 104, having a gate coupled to the gate of the first NMOSFET 
103, a source coupled to the ground terminal, and a drain coupled to the 
gate of the first NMOSFET 103; 
(3)providing a second current mirror coupled to the voltage terminal Vt of 
the PSD 102 and a power supply, providing a working current I0 equal to a 
sum of the first photocurrent and the second photocurrent to the PSD, the 
second current mirror comprising: 
a first PMOSFET 109, having a drain coupled to the voltage terminal Vt of 
the PSD 102 and providing the working current I0 to the PSD, a source 
coupled to a power supply, and a gate coupled to a second PMOSFET 108; 
a second PMOSFET 108, having a source coupled to the power supply, a gate 
coupled to the gate of the first PMOSFET 109, and a drain coupled to the 
gate of the first PMOSFET 109; 
and a third PMOSEET 107, having a gate coupled to the voltage terminal Vt 
of the PSD 102, a source coupled to the drain of the second PMOSFET 108, 
and a drain coupled to a comparator 106 that is utilized to provide an 
identification whether the light spot is located on the PSD 102; 
(4)providing a variable optimum constant-current source 110 coupled to the 
second output terminal X2 of the PSD 102, generating a constant current 
having a third value Ib, wherein Ib is larger than both the maximum values 
of the I1m and the I2; and 
(5)providing a converter 127 coupled to the second output terminal X2 of 
the PSD 102, having a capacitance value, converting a synthesis current Ic 
into a frequency signal via a relationship X=f=kIc/VC, wherein X is the 
position signal of the light spot falling on the PSD 102; f is the 
frequency signal of the light spot; k is a constant which depends on 
aspect of the voltage divider in the resistors R117, R118, R119 across 
both the negative input terminal of OP AMP 113 and the positive input 
terminal of the other OP AMP 114; C is the capacitance value of the 
capacitor 112 of the converter; and V is the voltage level of the power 
supply. 
Having described the invention in connection with preferred embodiments, 
modifications will now doubtlessly be apparent to those skilled in this 
technology. The foregoing description of the preferred embodiments of the 
invention has been provided for the purposes of illustration and 
description. It is not intended to be exhaustive or to limit the invention 
to the precise embodiment disclosed herein. The disclosed embodiment has 
been chosen and described to best explain the principles of the invention 
and its practical application, thereby enabling others skilled in this 
technology to understand the invention, to practice various other 
embodiments thereof and to make various modifications suited to the 
particular use contemplated of the present invention. As such, it is 
intended that the scope of this invention shall not be limited to the 
disclosed, but rather shall be defined by the following claims and their 
equivalents.