Using hardware to save energy in MCU-based sensing applications
In the case of a capacitive-sensing subsystem, it is possible to build a circuit that periodically excites the oscillator, sets a timer and then counts the number of pulses before the timer expires. At the end of the process, if the counter value is below a set minimum, it can raise an interrupt that wakes the MCU.
In this implementation, there is no need for the MCU to wake up periodically, unless there are other management functions to handle. The MCU will only be active when it has work to do, which reduces power consumption to the minimum level possible based on user activity.
A similar problem with sensor predictability arises with inductive sensing. These are electronic proximity sensors, which are able to detect the presence of a conductive target. Some common applications of inductive sensors include metal detectors, traffic lights, car washes and various automated industrial applications. Because there is no need for physical contact, inductive sensors are particularly used in harsh environmental conditions where dirt is prevalent and which may obscure optical or infrared proximity sensors.
High-performance inductive sensors, also known as eddy-current sensors, can do high-resolution measurements of the position and/or change of position of any conductive target. Lower-cost inductive sensors are used as proximity switches giving a simple on-off output indicating whether a conductive target is present or not. It is this latter group that causes more problems for power consumption when it comes to algorithm implementation on an MCU.
Inductive proximity sensors detect magnetic loss due to induced current generated on a conductive surface or target by an external magnetic field. When an AC current is applied to a coil, it generates an AC magnetic field. If a conductive target approaches the sensor it generates currents also known as eddy currents, on the sensed object due to the alternating magnetic field.
One way of producing oscillation on an inductance is using what is called a tank circuit, which consists of an inductive coil and a capacitor – broadly similar to the RC circuit used in capacitive sensing but with the addition of the inductive coil.
The capacitor stores energy in the form of an electric field while the coil stores energy in the form of a magnetic field. When the switch is in its first position, the capacitor is charged up to the DC supply voltage. When the capacitor is fully charged the switch changes to its second position, placing the capacitor in parallel with the inductor coil and starts to discharge through the coil.
The voltage across the capacitor starts falling as the current through the coil begins to rise. This rising current creates a magnetic field around the coil. When the capacitor is fully discharged the energy previously stored in the capacitor is now stored in the inductive coil.
As there is no external voltage in the circuit to maintain a current within the coil, current flows back to the capacitor, which is then charged with the opposite polarity of its original charge. After that the whole cycle is repeated resulting in a periodic energy transfer between the two circuit elements. The polarity of the voltage changes as the energy is passed between the inductor and capacitor producing an AC voltage and current waveform.
Every time energy is transferred between the two circuit elements losses occur which will decay the oscillations. This is due to the resistive circuit components, which will dissipate energy over time. The amplitude of the oscillation decreases at each half cycle of oscillation until the circuit loses all power.
This damping process happens more quickly, however, if a metallic object is near the coil. By detecting this change in decay, MCU algorithm can determine whether an object is within range of the sensor. This is normally performed by reading the change in voltage at regular intervals and comparing it to a reference voltage. If the input voltage falls below this reference, the MCU can trigger a proximity event.
As is the case for capacitive sensing, a software implementation requires that the MCU be awake to generate the excitation signal and process the analog comparator inputs. In the case of a flow metering for water or gas utilities, it can be hard to determine how often the MCU needs to wake up to be sure of registering all turns of the impeller. The supply may not be used for long periods, which will mean the MCU is waking to process no useful input.
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