Machined from a chunk of a virtually indestructible platinum-iridium alloy, the international prototype kilogram (IPK) was built to last for an eternity. And yet, being the last remaining, physical artifact in a club of fundamental SI units, the current definition of the kilogram has worn out. Most certainly the watt-balance will take its place, and redefine the kilogram with a true, physical phenomenon. [Grady] just built his own watt-balance from scratch, and he provides you with a decent portion of scientific background on the matter.
A watt-balance doesn’t look or operate much different from a regular balance, only instead of weighing an unknown mass against a calibrated counterweight, a watt-balance employs a calibrated electromagnetic counterforce. That force is produced by an electromagnetic actuator. [Grady] scratch-built his watt-balance from wood, however, even though he carefully turned and wound his own coils for the electromagnets, he cannot be quite sure about the exact amount of force his actuators exercise per ampere of consumed current. Before a watt-balance weighs redefined kilograms, the contraption needs to be calibrated.
Thanks to algebra and physics, this can be done by measuring the voltage the electromagnetic actuator itself generates when its permanent magnetic core is moved at a given velocity by an external force. Therefore, the watt-balance features a second, electromagnetic actuator with optical position feedback (practically a servo), to slightly rock the balance back and forth. [Grady] realized the optical feedback by installing a shadow sensor, made from a line laser and a photodiode, to the arm of the balance. An Arduino Uno polls the sensors, samples the induced voltage and sends the measurement data to [Grady’s] host software. After being calibrated in this velocity mode, the balance is operated in its weighing mode, where the gravitational force of the mass in question and the electromagnetic force exercised by the actuator are brought to an equilibrium by simply adjusting the current through the electromagnet’s coil.