Friday, April 29, 2016

The first person to describe an Inertial Measurement Unit (IMU)

Navigation equipment from the NASA Apollo space program.
The spherical shaped device is the IMU.
While reading an interest book recently ("Inventing Accuracy - A Historical Sociology of Nuclear Missile Guidance" [1]) I came across an interesting bit of historical trivia:

Irish man Joseph John Murphy [2] writing to Nature magazine in 1873 in reply to a letter from Charles Darwin on the topic of animal navigation wrote:

"if a ball is freely suspended from the roof of a railway carriage it will receive a shock sufficient to move it, when the carriage is set in motion: and the magnitude and direction of the shock … will depend on the magnitude and direction of the force with which the carriage begins to move … [and so] … every change in … the motion of the carriage … will give a shock of corresponding magnitude and direction to the ball. Now, it is conceivably quite possible, though such delicacy of mechanism is not to be hoped for, that a machine should be constructed … for registering the magnitude and direction of all these shocks, with the time at which each occurred … from these data the position of the carriage … might be calculated at any moment." [3]
... and thus became the first person to describe a device that took over half a century to realize: the Inertial Measurement Unit (IMU). The fist IMUs where rather large, heavy mechanical devices like the one pictured above from the Apollo space programme. Nowadays they can be implemented in silicon using a technique called MEMS [4] and there's one in every smartphone!


Update: I've been unable to find any photograph Joseph John Murphy. If anyone knows if one exists please let me know.