# How to sense displacement without using any camera?

I am working on a hand-held prototype model in which I need to fix motor RPM below a certain displacement. For example, if overall displacement is within 5 mm then the motor should be running at a fixed RPM.

After getting this done (I mean if displacement is > 5 mm) I wish to work on sensing the linear acceleration of the hand-held prototype. I will use this data to control my motor's RPM again. Typical hand-held displacement range would be around 5 cm to 30 cm in 2 sec.

Someone suggested to use IMUs to sense acceleration and rotation but I am not sure how to design my prototype to utilize an IMO. How can I select an IMU and determine an appropriate sensitivity level? References to relevant literature would be very helpful.

• I'm a little confused how your motor and your hand-held displacement device are related/connected? Are you hoping to measure both acceleration and displacement using an IMU? If so then you may struggle to find absolute displacement due to integration errors.
– welf
Feb 17 '15 at 8:46
• Displacement FROM where? - a fixed point or ??? How do you define the fixed point. Maybe it is "antishake" | How does the motor RPM relate to displacement and why ? |} You need to tell us what the basic "problem" is rather than how you think it should be addressed. eg do you REALLY want to control motor RPM or is that driving something else that you have not told us about? || "Tell us what you WANT and we;'ll give you what you need." :-) Feb 17 '15 at 10:12
• An image or sketch of the device you are trying to build would be helpful. Feb 17 '15 at 12:52

I can say using accelerometers to detect displacement won't work. These devices have so much of inherent noise that discerning very slow acceleration from remaining immobile is about impossible. You won't be able to detect acceleration of order of 0.1 mm/s2 and it takes just a several seconds at such acceleration to exceed your 5mm.

You'd have a hard time with a camera as well, especially if the device is hand-held. True the camera looking along X axis would work with Y,Z plane displacement in confined environments (relatively nearby objects in focus) but it would get easily confused by rotation. You'd need at least two cameras looking in different (and not just opposite) directions and environment that is conductive to image detection (you won't get much displacement measurement from the image of the sky). Also, rotation will be a problem.

What will work with ~5mm distance ranges is frequency-based distance metering - preferably ultrasound, though infrared/visible light would be possible too (though obviously more difficult due to frequencies being much, much higher). Unfortunately plain "reflective" ultrasound distance sensors won't help much about rotation/tilt of the hand, distance from walls changing rapidly despite the device not moving. What you need are three ultrasound emitters (speakers), each of a different frequency, placed around the location where the measurement is to take place, and a sampler (microphone) in the "handheld". By filtering given frequency you get signal from one emitter, and then by analyzing the change of phase of the wave in time intervals equal to the wave period, you have displacement of the device relative to the emitter. By triangulating the three distances you have total displacement.