Labs Idea #3: Intrusion / Open and Close Door Event Detection
Author: Sensirion Innovations Team - November 2017
Motivation
A surveillance camera will only detect intruders within its field of view. However, breaking into a house through a door or window will generate a pressure pulse, which can be detected with a sensitive pressure sensor. Also the opening and closing of doors generates an air pressure wave, which is reflected from the walls of the building. The frequency of reflection depends on the size of the resonating room. If the door is closed the room volume is smaller. Therefore, the resonance frequency of the reflected air wave is higher. If the door is opened the resonance frequency is lower due to adjacent rooms adding to the total room resonance volume.
A very sensitive air flow or pressure sensor can detect the air wave generated by breaking windows or the opening and closing of doors in a building. Thus, it is possible to detect an intrusion event happening somewhere in the building, even if the sensor is in a different room (as long as the pressure wave can travel freely within the house). It is also possible to distinguish between the opening and the closing of a door, if the air flow or pressure sensor resides within the room.
This valuable information can be used to drive actions for building control or surveillance purposes. A smart home might turn on lightning or a smart thermostat might increase or decrease temperature based on room occupancy and load.
Approach
Sensirion's flow sensor is a semiconductor MEMS version of the traditional hot wire anemometer. The sensing principle is based on thermal conductivity through the gas flowing over the sensor surface (figure 1).
In opposition to diaphragm based differential pressure sensors, Sensirion's CMOSens flow sensor characteristic shows high sensitivity especially at low air flows (figure 2). This allows to reliably measure ultra low flows and differential pressures.
Sensirion's differential pressure sensors (SDP8xx or SDP3x) use the flow through principle explained in figure 1, but are calibrated for differential pressure output in Pascal (1 mbar = 100 Pa = 1 in. water column) instead of flow output. Whenever air flow is generated a pressure difference exists. Therefore, a differential pressure sensor can also detect air flow.
A Sensirion differential pressure sensor (SDP8xx or SDP3x) is capable of detecting pressure differences even below one Pascal. In order to detect the air wave from a breaking window or moving door, a set-up is required, which forces the air pressure wave to enter the differential pressure sensor through one port and leave the sensor through the other port. Therefore, the outgoing port of the differential pressure sensor needs to be isolated from the entry port. Otherwise the air pressure wave would enter the sensor through both ports and cancel out.
To isolate the outgoing port of the differential pressure from the entry port, the outgoing port was connected to a dead space (see figures 3-5).
The event of an opening or closing door is also detected by its generated air turbulence (figure 6, 7).
The breaking of a window by an intruder is detected by the high preak of the generated air pressure wave (figure 8). For this measurement a SDP8xx was used with a tiny dead volume of one milliliter attached to the sensor's outgoing port (similar to the set-up in figure 5).
The measurement results below demonstrate that Sensirion's SDP8xx and SDP3x differential pressure sensors can measure smallest air flows generated by a opening door (figure 9-10). While a large dead volume generates a higher differential pressure signal, even a small dead volume still allows to measure the resonance frequency of the air wave travelling through the room upon an open door event.
Depending on the resonance frequency of the reflected air waves in the room an «open door» event (larger room volume → lower frequency) could be distinguished from a «close door» event (smaller room volume → higher frequency).
A characteristic «open door» and a characteristic «close door» wavelet signal are convoluted with the measured air wave. If the air wave frequency matches the characteristic wavelet frequency a non zero output signal is generated in the time domain, allowing "real time" signal analysis and event classification (figure 11).
Conclusion
- Sensirion's differential pressure sensor can detect the burst of air generated by a breaking window
- Closing and opening of doors is detectable
- Good signal-to-noise even with very small dead volume
Implementation
This paragraph should give in a how to style how to build the solution for the own purposes.
To build an air movement detection sensor a Sensirion differential pressure sensor SDP8xx or SDP3x series (figure 12) can be sourced from distributors listed on the Sensirion website.
Communicate with the digital sensor over I2C according to the explanations in the SDP800 or SDP3x series data sheet or use the appropriate evaluation kit.
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