Exploration of a RISC Distance Sensor

Most of our RISC sensors have focused on optical beam sensors which simply detect the presence of matter between source and detector, or within some range of a reflective sensor. Combinations of these sensors have proved enormously versatile, and can solve most of the sensing tasks we have encountered. Nevertheless, there are some problems that require more information, or information of another type.

Some of the more difficult sensing applications would benefit greatly from an inexpensive distance sensor. An important example is the bin-of-parts problem. This problem is very difficult to solve using intensity information only, whereas it could be solved using coherent tabular methods if only a handful of depth measurements were available from the object to be recognized. This is true even if these measurements were mixed with measurements from other objects. There are commercially available distance sensors, which come in two main types (1) structured light/camera and (2) laser interferometry. Both are very bulky and expensive, and are not currently cost-effective for many bin-of-parts problems.

We are exploring a low-cost distance sensor based on a modulated light beam. The sensor uses off-the-shelf components and computes distance via phase shift of a modulation signal. Because the phase comparison is done using the modulation signal rather than the light itself, the light source need not be coherent, so LEDs or inexpensive laser diodes can be used as the source. Because the modulation is at radio frequency, the phase comparison can be performed electronically rather than using non-linear optics. The hardware elements all have about the same size as our current beam sensors, and comparable cost.

Eric Paulos (paulos@robotics.eecs.berkeley.edu) / 20 Apr 1994