Developing battery operated wireless interfaces to manage and read a sensor seems straightforward but to optimize the design can be tricky. The problem is that if we are designing a true wireless sensor system then the remote termination unit (RTU) will provide sensor power from the RTU batteries, and managing the sensor power to ensure a long life from the batteries depends on a number of factors:
1. Power draw and power settling time:
For traditional sensors using a 4-20mA loop or 0-5V output (with 12V power) tests have shown us that battery operated sensor power is generally the most power hungry part of the system. Providing wireless interfaces to older sensors requires battery capacities of 5Ah and upwards to ensure a reasonable battery life.
Note that serial interfaces and most microprocessor digital based sensors are not usually suited for dry battery operation due to their boot (start up) time.
2. Processor and radio power:
Today’s micro controllers and processors are ideal for battery operated sensor management. draw very little power, particularly in sleep mode. The radio interface can draw a significant current depending on the power output and if a low noise amplifier (LNA) is used on the input.
3. Read frequency:
This tends to be application dependent. Targeting 3 to 5 years life span, applications that require readings once or twice a minute are generally acceptable for dry battery operation, but if a higher read frequency is required the duty cycle increase can significantly affect the battery life.
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