Optical position sensor allows "human" touch

March 1, 1995
Robots may soon be able to "feel" much the way humans can with a new optical position sensor developed by engineers at Tchoupitoulas Research (Metarie, LA).

Robots may soon be able to "feel" much the way humans can with a new optical position sensor developed by engineers at Tchoupitoulas Research (Metarie, LA). Numerous sensors are necessary for robotic systems to determine the complete shape of an object. Approximating the tactile capabilities of a human fingertip, for example, can require as many as 100 sensors. The compactness of the Shadow Sensor, as well as its high performance and durability, may make this type of high-density sensor system feasible.

Sensor design

The basic Shadow Sensor design consists of a light-blocking plunger and the "shadow rod" moving in a channel formed by a pair of lightpipes. Light from an infrared light-emitting diode (IR LED) in the source lightpipe travels across the plunger gap and is collected by a second lightpipe. This lightpipe contains a detector, a phototransistor circuit that converts an optical signal to electric current. As the plunger moves between the source and detector, it progressively blocks transmission of the IR LED signal, resulting in the decrease of detected intensity. When the plunger is pulled out of the channel, the intensity increases.

Variations in the design include a rotational sensor, in which different configurations of the light-blocking element allow the device to detect pitch or roll. If the plunger is connected to a compressible element with a known spring constant, the sensor is capable of registering pressure variations. The aperture in the plunger can be triangular, so that the detected signal as a function of displacement is nonlinear. This gives good response for very small displacements.

The design is simple and robust as well as inexpensive to manufacture. The prototype was made with an aluminum shadow rod and housing and acrylic lightpipes, making the IR LED and the phototransistors the most expensive elements. Mechanical motion in the sensor is completely isolated from the optoelectronic elements, so the assembled device can stand up to repeated use without sustaining significant wear.

A current prototype registers a displacement range of greater than 2 cm, with a maximum error between full extension and compression of 0.2%. The prototype measures 1 ×× 4 in., but smaller prototypes have been made. Minimum size of the sensor is driven by the electronics and light source. Incorporating fiberoptics in the design as illumination and detection elements will significantly reduce the size of the final product.


Because it is compact, highly sensitive, and reliable, the device has strong potential as a robotic sensor. A robotic hand using the sensor could "feel" an item it was holding. It could sense if the item was slipping and, in response, tighten its grip. It is difficult for robots to reliably drive screws into parts without periodically stripping some of the screws. Using an array of these sensors, a robot could detect and correct for thread mismatch, increasing complexity and capability of robotic manufacturing. An intriguing variation on the tactile sensing application is the use of these devices as sensors in prosthetic limbs, allowing the wearer a better assessment of the amount of force used for lifting, carrying, and manipulation.

The small size, low cost, and large displacement range of the Shadow Sensor make the design attractive for use in virtual-reality gloves. Such gloves require an array of sensors spread over the user's hand to detect movement and are relatively expensive and difficult to manufacture. A three-layer design consisting of an inner glove, a sensing layer, and an outer protective layer could be easily manufactured and adjusted to various wearers. Such modularity could also be rapidly disassembled for cleaning and repair.

Inventors Philip Chin and Michael Lynch envision other applications for the Shadow Sensor, ranging from motion/shock-detection elements in car alarm systems and angle positioners for helicopter blades to low-cost, durable position sensors in video-game joysticks. In December 1994, they received a patent for their design and are currently working on commercialization.

About the Author

Kristin Lewotsky | Associate Editor (1994-1997)

Kristin Lewotsky was an associate editor for Laser Focus World from December 1994 through November 1997.

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