Failsafe Object Detection

 In applications where we want to set an alarm when an object appears, we normally program a switch to close when a target is closer than some setpoint.   This problem becomes difficult when we don’t know in advance what the target will look like, or what its position will be, with respect to the sensor.

If sound strikes the object of interest at an oblique angle, the reflection can go away from the sensor, rather than return to be detected.  This can make the application unreliable.  However, there IS a way to improve the situation.

Example:  Detecting a truck backing up to a loading dock.

The objective is to either open the door when a truck backs up, or sound an alarm, so personnel can receive the truck.  What if the truck is small or odd shaped and there is no good target for sound to bounce off from?

The solution?  Don’t try to detect the truck, instead, detect the ground!  Since the ground is a stable target, program the sensor to switch the alarm off when it sees the ground, and turn the alarm on when it sees anything else.


If a truck pulls up that has no good surfaces for bouncing sound from, the sensor will lose the target and not detect anything.   To make this technique failsafe, we have to manage the situation where a truck DOES produce a useful reflection.   To do this, simply set the minimum range close to the ground.   If the distance to the ground is 150″, you could set the sensor to close when an object is detected closer than 130″.   Any object greater than 20″ tall would open the door or trigger the alarm.

We still have to manage variations due to temperature swings (1% for every 10 degrees F). By setting the switch to window mode, and enabling it for a range of 130” to 170”, we should eliminate the impact of seasonal temperature variations.

You may still see false triggers due to animals or people in the bay area.  SenixVIEW software will allow you to experiment with sensor gain and power, and apply filtering such that a vehicle or other interrupting target will have to be present for a specified period of time before the alarm is set.  This should permit you to minimize, if not fully eliminate, false triggering.