A Simple Model for Handwriting

Abstract

An understanding of the human motor system, though interesting as a facet of intelligence, would also have application for computer controlled manipulation. To enhance this understanding, handwriting is appropriate for study because of its commonality, its susceptibility to measurement, and the possibility of its simulation with a manipulator. A theory of handwriting has been developed and implemented on a 6-joint manipulator, the MIT VIC ARM The theory views handwriting as a constrained modulation of an underlying oscillation pattern. This pattern, chosen from a repertoire of carrier waves such as sawteeth or garland chains, is established by programming of horizontal and vertical actuators. The two actuators assume different roles: the vertical actuator drives the movement with rhythmic down-up movements. The horizontal actuator, acting within time constraints imposed by the vertical rhythm, produces letter shapes. The choice of underlying oscillation pattern restricts the shapes the horizontal actuator can produce, and is the primary factor influencing writing style. A simple set of constraints on actuator force patterns, derived in part from an analysis of some handwriting measurements, imposes a certain structure on the handwriting programs: 1. a basic time quantum in which force is constant 2. synchronous joint activation 3. force magnitude restriced to a few levels The basic time quantum for the VICARM, representing a compromise among such factors as speed of response and jerkiness, was chosen as 24 msec. A cycle of the carrier is a down-up movement comprised of several time quanta. The fastest cycle consists of one quantum acceleration-deceleration bursts by the vertical actuator, resulting in a four quantum down-up cycle of 96 msec. When coupled with a horizontal pattern that rounds the bottoms of the oscillation, a smoothed sawtooth results (la below). Horizontal (H) and vertical (V)