Method and apparatus for measuring physical parameters

Remote sensing with two wires and daisy chained sensors


Sequential sensing was developed to fill a need to read the temperature of a heating pad using a microprocessor with a single pair of wires connecting remote sensors within the pad. The sensors were placed at strategic locations chosen so the average temperature could be read and any dangerous hot spot temperatures sensed.

Microprocessors are able to easily and accurately read time intervals so a method of converting temperature readings to time was developed. In order to read multiple temperatures with a single pair of wires, the sensors were daisy-chained so that each one receives power from the previous one in the chain. When power is supplied to a sensor, a thermistor begins charging a capacitor. The length of time to charge the capacitor is easily predicted using the thermistor resistance vs. temperature curve and the capacitor value. When the capacitor reaches a fixed voltage, a comparator switches on power to the next sensor. This process continues until the last sensor fires. A simple circuit located at the microprocessor converts the changes in current to pulses that are input to the microprocessor.

The microprocessor starts the reading data process by applying power to the first sensor starting a timer and waiting for the first current pulse. It stores the time of the first pulse and waits for the second pulse etc. The time from the first application of power to each pulse is captured in the processors memory. The micro then removes power from the pair of wires and, using an algorithm, converts the time difference between sequential pulses and the previous one to temperature.

Temperatures are averaged and the maximum sensor temperature determined. If the average temperature is below a set temperature, power is applied to the pad heater providing the maximum temperature is below a safe limit. Under normal conditions, no hot spots, the pad temperature is maintained at the set limit. The microprocessor also insures that the time between pulses is less than a maximum limit to guard against defective sensors or broken wires. If the limit is exceed, heater power is turned off and disabled until power is removed and reapplied.
Sequential Sensing Technology

This invention is used to measure physical parameters such as temperature, humidity, air speed, switch open/closed status, light level and any reading that can be converted to resistance or voltage. A controller consisting of a PC or microprocessor is connected to a pair of wires. The sensors are then connected in series with the one of the wires in a daisy-chain fashion. The second wire serves as a ground connection for each sensor. The controller applies voltage to the first pair of wires connected to the first sensor. The first sensor delays by an amount related to the value of the parameter being measured and then applies power to the second sensor etc. until all sensors are actuated. As each sensor is powered, the amount of current drawn from the controller increases.

The time from power application to current increase is read by the controller, and knowing the characteristics of the sensor, the physical parameter value can be determined. For example, if the input is connected to a thermistor, a device whose resistance changes with temperature, and the thermistor is connected to a capacitor, a comparator and an output voltage switch. The time from application of power and switching power to the next device will be determined by the temperature of the thermistor. The temperature sensing application is suitable for zone control of a house or controlling the temperature of a heating pad or blanket.

Intrusion alarms
HVAC zone controls
Heat pad and heating blankets
Pet bed heaters
Garden soil moisture monitor

US 7,412,347 issued 2008-08-12   [MORE INFO]

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Asking Price: US$30,000

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