1. A method of controlling the level of gain applied to a photodetector, comprising the steps of:
providing means for establishing a selected level of gain;
providing a mechanism for limiting the level of gain applied to the photodetector irrespective of the selected level of gain; and
compensating for differences between the selected level of gain and the level of gain applied to the photodetector, which differences being attributable to the mechanism.
2. The method of claim 1 wherein the compensating step includes constructing a lookup table reflecting differences between the selected level of gain and the level of gain applied to the photodetector.
3. The method of claim 2 wherein the compensating step includes adjusting the applied level of gain in accord with the look-up table.
4. The method of claim 1 including the step of protecting the mechanism from excessive current that would damage the mechanism.
5. An automatic gain control circuit for a photodetector, comprising:
a bias control for the photodetector;
an operational amplifier receiving an output from the photodetector and a reference level selection, the operational amplifier providing an output signal to the bias control representing an amount of bias to be applied to the photodetector;
a limit means for limiting the output from the reference level selection received by the operational amplifier to a predetermined maximum level; and
means for establishing the reference level selection and for compensating for differences between the established reference level selection and the reference level selection received by the operational amplifier.
6. The circuit of claim 5 wherein the limit means includes a Zener diode.
7. The circuit for controlling the gain of a photodetector including:
a bias control for driving the photodetector;
a gain adjusting circuit connected to the bias control for providing to the bias control an output signal representing a comparison between an output of the photodetector and a selected level of gain, both the selected level of gain and the output being received as voltage inputs to the gain adjusting circuit;
a limit device for limiting the voltage level input corresponding to the selected level of gain, the limit device also reducing the voltage level input corresponding to the selected level of gain; and
compensating means for compensating for the voltage level reductions attributable to the limiting device.
8. The circuit of claim 7 wherein the compensating means includes a stored look-up table and a controller for adjusting the selected level of gain in accord with the look-up table.
9. The circuit of claim 7 wherein the limit device is a Zener diode.
10. The circuit of claim 9 including a current protection device for limiting the level of current applied to the Zener diode.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A woodworking machine comprising:
a cutter adapted to cut a workpiece; and
a contact detection system adapted to detect contact between a person and the cutter, the contact detection system including:
an excitation system adapted to supply an electrical signal having a first amplitude and period, the electrical signal being coupled to the cutter to induce a corresponding electrical signal of a second amplitude on the cutter, and
a sensing system adapted to sense the second electrical signal induced onto the cutter, where the sensing system is adapted to detect contact between a person and the cutter based on a change in the sensed second electrical signal over a detection period, where the detection period is between 5 and 150 microseconds and at least twice the period of the first electrical signal.
2. The machine of claim 1, wherein the excitation system is adapted to adjust the first amplitude based on the properties of the second electrical signal sensed by the sensing system.
3. The machine of claim 2, wherein the excitation system is adapted to adjust the first amplitude to attempt to maintain the amplitude of the second electrical signal sensed by the sensing system at a predetermined level.
4. The machine of claim 2, wherein the excitation system is adapted to adjust the first amplitude at an adjustment rate of less than 10% per millisecond.
5. The machine of claim 2, where the sensing system is adapted to detect contact between a person and the cutter when the amplitude of the second electrical signal sensed by the sensing system drops below the predetermined level by a specified amount.
6. For use in connection with a woodworking machine having a motor-driven cutting tool, a method of detecting contact between a person and the cutting tool, the method comprising:
imparting an electrical signal onto the cutting tool, where the electrical signal has an alternating voltage amplitude defining plural voltage peaks;
sensing the electrical signal imparted onto the cutting tool; and
detecting contact between a person and the cutting tool by detecting that a predetermined plural number of the voltage peaks of the sensed electrical signal have amplitudes outside a specified range of amplitudes.
7. A woodworking machine having a cutting tool adapted to cut workpieces, the machine comprising:
means for driving the cutting tool;
means for detecting contact between a person and the cutting tool;
means for distinguishing contact between a person and the cutting tool from contact between green wood and the cutting tool; and
means for stopping movement of the cutting tool upon detection of contact between the person and the cutting tool.
8. A woodworking machine comprising:
a support structure;
a motor supported by the support structure;
a blade supported by the support structure and moveable by the motor;
a reaction system adapted to perform an action to mitigate injury if a person contacts the blade when the blade is moving; and
a contact detection system to detect contact between the blade and a person, where the contact detection system is configured to generate a signal, to apply the signal to the blade, and to detect a change in the signal on the blade due to an electrical load applied to the blade, and where the contact detection system is further adapted to adjust the signal in the presence of a changing electrical load applied to the blade.
9. The machine of claim 8, where the contact detection system is configured to detect a change in the signal on the blade that exceeds a threshold, and where adjusting the signal in the presence of a changing electrical load applied to the blade corresponds to adjusting the threshold.
10. The machine of claim 8, where the contact detection system is configured to detect a change in the signal on the blade that exceeds a threshold, and where the threshold adjusts when the signal is adjusted in the presence of a changing electrical load applied to the blade.