Detection systems for power equipment
Improved methods to detect when a human body contacts a predetermined portion of a machine are disclosed. The methods distinguish contact with a person from contact with other materials. The methods are particularly applicable in woodworking equipment such as table saws to distinguish contact between a person and the blade of the saw from contact between the blade and wet or green wood.
1. A method for detecting contact between a user and a cutting tool on a piece of woodworking equipment, the method comprising:
applying an electrical signal to the cutting tool, wherein the cutting tool has an apparent electrical impedance dependant at least in part on whether the cutting tool is in contact with the user, and wherein the electrical signal has an associated property related to the apparent electrical impedance of the cutting tool;
integrating the property to obtain integration results that are dependant at least in part on the apparent electrical impedance of the cutting tool over time;
generating a value based at least in part on the integration results; and
deciding there has been contact between the user and the cutting tool if the value compares to a threshold in a predetermined way.
2. The method of claim 1 , wherein the integrating is done with an analog circuit.
3. The method of claim 1 , wherein the integrating comprises multiple integrations over discrete time intervals.
4. The method of claim 1 , wherein the property is digitized prior to integrating.
5. The method of claim 1 , wherein the property is related to the amplitude of the signal applied to the cutting tool.
6. The method of claim 1 , wherein the property is related to the phase of the signal applied to the cutting tool.
7. The method of claim 1 , wherein the property is related to the current associated with the signal applied to the cutting tool.
8. A method for detecting contact between a user and a cutting tool on a piece of woodworking equipment, the method comprising:
inducing an electrical signal on the cutting tool by generating an electrical current between a drive circuit and the cutting tool, wherein the electrical signal induced on the cutting tool has a phase and an amplitude, wherein the cutting tool has an apparent electrical impedance dependant at least in part on whether the cutting tool is in contact with the user, and wherein the electrical current, phase and amplitude are dependant at least in part on the apparent electrical impedance of the cutting tool;
sampling one or more of the current, phase or amplitude a plurality of times at discrete intervals to generate multiple data points, wherein each of the data points is the result of a discrete integration;
calculating a value based at least in part on more than one of the data points; and
deciding there has been contact between the user and the cutting tool if the value compares to a threshold in a predetermined way.
9. A method for detecting contact between a user and a cutting tool on a piece of woodworking equipment, the method comprising:
inducing an electrical signal on the cutting tool by generating an electrical current between a drive circuit and the cutting tool, wherein the electrical signal induced on the cutting tool has a phase and an amplitude, wherein the cutting tool has an apparent electrical impedance dependant at least in part on whether the cutting tool is in contact with the user, and wherein the electrical current, phase and amplitude are dependant at least in part on the apparent electrical impedance of the cutting tool;
sampling one or more of the current, phase or amplitude a plurality of times at discrete intervals to generate multiple data points;
calculating a value based at least in part on more than one of the data points, wherein the value is based at least in part on a change in at least one of the data points relative to a sum of a plurality of data points; and
deciding there has been contact between the user and the cutting tool if the value compares to a threshold in a predetermined way.