Drive circuit for photoelectric sensor

Photoelectric sensors are used in many applications ranging from detecting the presence or absence of an object to high speed counting or edge detection applications. Photoelectric sensors use a light source or emitter, e.g. an LED, which is generally pulsed at high currents to achieve long scan distance and conserve power. An object is sensed by the reflectance or interruption of the serial stream of light pulses caused by the object. In order to ensure reliable detection of objects, especially at long distances or in situations where nearby objects provide stray reflections which appear as a background signal to the sensor, sensors are commonly designed so that a fixed number of received pulses in a given time period is required before the output of the sensor turns ON. The time delay associated with the turning ON or OFF of the sensor defines the response time of the sensor. Response time can range from tens of milliseconds to less than one microsecond.

The response time of a sensor is directly related to the sensing distance or range of the sensor, which can be less than an inch to several hundred feet. When long ranges are required, the internal power dissipation limits the speed of response. For example, a sensor with a rated scan distance of 10 feet typically has a 5 millisecond response time, and a sensor with a rated scan distance of 1 inch, typically has a 1 millisecond response time.

Photoelectric sensors of the modulated or pulsed type generally employ an oscillator to drive an LED light source at a low duty cycle and for setting the response time of the detection circuitry. In order to achieve long scan distances and not exceed the power dissipation rating of the LED, high currents are driven at low duty cycles. Currents up to several hundred milliamps can be driven without exceeding the continuous rating of the LED, which is usually 100 milliamps or less.

Additionally, because of the internal power dissipation, as higher pulse currents are driven through the LED to achieve long scan distances, the clock frequency or transmit frequency of the LED is slowed down. The result of slowing the clock frequency results in a longer response time of the photoelectric sensor, especially because multiple pulses are counted in the detector prior to the actual turn ON of the output.

Therefore, sensors are either designed to have long scan distances with slow response times or short scan distances and fast response times. A sensor for a particular application is, therefore, typically chosen or designed depending upon the particular sensing distance and response time required by the user.

To date, however, no photoelectric sensor has been developed which can provide a variable response time that changes with an operator adjustable sensing distance. Accordingly, there is a long felt need in the art for a photoelectric sensor drive circuit which automatically adjusts sensor response time based upon the drive current to the sensor light source required for a desired sensing distance.