Design of animal motion detection unit
Passive vs active sensor
After getting an idea of what users needed in a camera triggering device in this post, this article details the design of the animal motion detection unit being prototyped. The most important feature that the users wanted is that the system is reliable to trigger with consistent detection field of view and distance. Low cost and low latency to trigger were the other characteristics that the survey respondents wanted. Good battery life is also a feature that the participants cared for.
Motion detection methods
The two major criteria for choosing a reliable motion detection are low cost and low power consumption. Low cost here means a Bill of Material (BoM) less than 5$ in quantities of hundreds. For power consumption we are looking at less than 1 mA average current consumption. With these criteria the only two options left are a Passive Infrared sensor and an active laser beam sensor.
Passive Infrared Sensor (PIR)
PIR sensor and its working is well described in this article by Lady Ada. Essentially they detect the motion of warm bodies. It is passive because it does not emit energy to detect motion. The setup of a PIR based system is simple, its just making sure that the sensor faces the targeted zone upright. The detection range and field of view can be customized by choosing the appropriate Fresnel lens. We have shown that PIR sensor with a narrow angle lens can detect motion up to 24 m. The PIR sensor and the lens are quite inexpensive, costing about 2–3$ combined. The power consumption of a PIR sensor can be quite low, even less than 0.05 mA.
The disadvantage of PIR sensors is that they can’t detect cold blooded animals, so this sensor is ruled out if they are the targeted species. Also PIR sensors are known to false trigger when there’s gusts of warm wind.
Active laser beam sensor
An active laser beam sensor system consists of a laser transmitter (Tx) and a laser receiver (Rx). As shown in the image below, one method of motion detection is where an animal intercepts the stream of light pulses emitted from the transmitter. Based on the number of light pulses the receiver misses it can make some guess of the size and speed of the animal. This system can be setup to target a particular species, even cold blooded animals.
The primary difficulty of the beam interception based system is its setup. Getting the alignment of the transmitter and receiver, especially over tens of meters can be quite difficult and time consuming. Also the Tx and Rx units’ batteries have to be managed and replaced separately. To overcome these limitations there is the beam reflection based system where the transmitter and receiver are on the same unit. The receiver can infer presence of animals based on the reflected light that it senses. The range of the beam based reflection system is highly dependent on the optics used for the Tx and Rx.
These active sensor systems are generally more power hungry, although it can be optimized by tweaking the duty cycle of the light pulses and be bought down less than 1 mA. They are more expensive than PIR based systems. The certification of a device emitting laser is an additional task to be taken up.
Microwave doppler motion sensors
Microwave Doppler motion sensors as described in this blog post works well for detection of animals and are low cost. Their drawback is that they are power hungry, needing 30–40 mA.
Motion detection method for Prototype 1
Based on the above description of motion detection sensors, a PIR based system was decided to be used in the first prototype. The main reasons were the previous experience of using them, low cost, low power consumption and ease of prototyping.
The first prototype would have two PIR sensors in the same device, one with a wide angle lens and the other with a narrow angle lens. This will achieve a detection region as shown in the image above. The user will have the option to choose if one or both the sensors are to be used.
At the expense of a device twice the size of a single PIR sensor and a bit more cost, the user has the availability of wide angle and long range sensing. The assumption here is that this is a better trade-off than developing separate long range and wide angle PIR devices.
