Mailing machine having optical sensors with improved immunity to ambient
View Patent ↗Circuitry to control reflective optical sensors is provided that reduces false detections due to ambient light without compromising the performance of the optical sensors to detect dark materials. A reflective optical sensor includes an emitter LED and photo-detector arranged to receive light from the LED that is reflected by an object being detected. A first input of a comparator is coupled to the output of the photo-detector. A second input of the comparator is coupled to the output of the photo-detector through a filtering circuit. The filtering circuit operates to filter the detector's output and adaptively adjust the trigger threshold of the comparator, thereby enabling the photo-detector to be sensitive enough to detect dark mail pieces, i.e., those mail pieces that are minimally reflective, while being immune to repeated false triggers due to excessive ambient light.
1. A mail processing system comprising:
a controller to control operation of the mail processing system;
a transport device to transport mail pieces along a transport path of the mail processing system, the transport device being controlled by the controller;
at least one optical sensor located along the transport device for determining a location of a mail piece along the transport path, the optical sensor including a light emitting diode that is powered based on a pulse width modulated signal, and a photo-detector, the photo-detector causing a voltage level at a photo-detector output node to vary based on an amount of light being received by the photo-detector;
a filtering circuit having a diode coupled to the photo-detector output node and a low pass filter coupled between the diode and an output of the filtering circuit; and
a comparator having a first comparator input, a second comparator input, and a comparator output, the first comparator input being coupled to the photo-detector output node, the second comparator input being coupled to the output of the filtering circuit, the comparator output indicating when an object is detected by changing states based on voltage levels input to the first comparator input and second comparator input, the voltage levels at the first comparator input and second comparator input being dependent on the amount of light being received by the photo-detector,
wherein the filtering circuit adjusts the voltage level at the second comparator input to control a threshold at which the comparator changes states to reduce false detections caused by the photo-detector receiving ambient light.
2. The mail processing system of claim 1 , wherein the low pass filter further comprises:
a filter resistor coupled between the diode and second comparator input; and
a filter capacitor coupled between the second comparator input and ground.
3. The mail processing system of claim 1 , wherein the filtering circuit further comprises:
a threshold resistor coupled between the second comparator input and ground.
4. The mail processing system of claim l, wherein the filtering circuit further comprises:
a current sinking device coupled between the second comparator input and ground.
5. The mail processing system of claim 1 , further comprising:
a pull-up resistor coupled between the photo-detector output node and a supply voltage.
6. The mail processing system of claim 1 , wherein the mail processing system is a mailing machine.
7. The mail processing system of claim 1 , wherein the mail processing system is an inserting machine.
8. A reflective optical sensor circuit comprising:
an emitter and photo-detector, the photo-detector being positioned to detect light output by the emitter that reflects off of an object being detected, the photo-detector causing a voltage level at a photo-detector output node to vary based of an amount of light being received by the photo-detector;
a device to generate a pulse width modulated signal used to cause the emitter to emit light;
a filtering circuit having a diode coupled to the photo-detector output node and a low pass filter coupled between the diode and an output of the filtering circuit; and
a comparator having a first comparator input, a second comparator input, and a comparator output, the first comparator input being coupled to the photo-detector output node, the second comparator input being coupled to the output of the filtering circuit, the comparator output indicating when an object is detected by changing states based on voltage levels input to the first comparator input and second comparator input, the voltage levels at the first comparator input and second comparator input being dependent on the amount of light being received by the photo-detector,
wherein the filtering circuit adjusts the voltage level at the second comparator input to control a threshold at which the comparator changes states to reduce false detections caused by the photo-detector receiving ambient light.
9. The reflective optical sensor circuit of claim 8 , wherein the low pass filter further comprises:
a filter resistor coupled between the diode and second comparator input; and
a filter capacitor coupled between the second comparator input and ground.
10. The reflective optical sensor circuit of claim 8 , wherein the filtering circuit further comprises:
a threshold resistor coupled between the second comparator input and ground.
11. The reflective optical sensor circuit of claim 8 , wherein the filtering circuit further comprises:
a current sinking device coupled between the second comparator input and ground.
12. The reflective optical sensor circuit of claim 8 , further comprising:
a pull-up resistor coupled between the photo-detector output node and a supply voltage.
13. The reflective optical sensor circuit of claim 8 , wherein the device to generate a pulse width modulated signal further comprises:
a programmable logic device.