I will be relying primarily on a accelerometer, heart rate monitor and a GPS for theft detection and response. Since the majority of thefts happen at night when the animals are resting, the accelerometer will allow any major disturbances to be easily detected. Especially when multiple animals in a herd suddenly start moving simultaneously, with increased heart rates, it likely indicates some external disturbance. When major movement is detected for a certain amount of time, the GPS will be activated to pin the location and forward it to the base station. This will allow the power consumption to reduced since GPS will only be activated when the animal is moving.
For testing I selected a MPU-9250 intertial measurement unit and a Ublox Neo-7M GPS module.
Temperature and heartbeat sensors can also be added to enhance the tags capabilities. They can be a very good indication of the health of the animal. Heartbeat especially can be a good indication of the animal's stress levels. These two parameters will also clearly show if a tag is removed. Another simple method to detect this is to add a thin wire connection that will be broken when a tag is removed.
I have ordered a PT100 RTD precision temperature probe with amplifier module for initial testing. It will probably only arrive later this month. I have yet to order a heart beat sensor, but am looking at this one for testing. Of course the challenge will be to see if the see if sensors work as expected on animals.
Ear tag or collar?
To enable the implementation of heartbeat and body temperature sensors, a ear tag unit will be the optimal solution. It allows easy access to the animals ear canal for a temperature probe. From a ear tag the arteries just under the skin of the ear is easily accessible. This could allow the use of optical heart beat sensing to be used. Since ear tags are already in use on most farms, fitting animals with tags should be relatively simple.
The ear tag does have some disadvantages however. Firstly it limits the size and weight of the tag severely. An ear tag should be as small and light as possible. Secondly the constant movement of the animal's ear would cause the accelerometer to readings to be very "noisy" possibly limiting it's usefulness A collar will allow a more stable platform for the accelerometer that would allow much "cleaner readings" but makes implementing the temperature a and heartbeat sensors much more difficult.
I will test both approaches first, before looking at more complex/expensive solutions like using both a collar and an ear tag.
Any advice/suggestions are welcome!