Exercise: Electromechanical Oscillation
Note: this is a new exercise, please provide feedback on errors and clarity.
Objective
Design and test a mechanical oscillator pumped via electrical actuation.
Overview
Physical oscillators are at the core of many machines: clocks, timers,
motors, pumps, quartz crystals, washing cycles, musical instruments, and
on and on. Oscillations are a product of a physical process which cycles
through a set of states, with physical dynamics that determine the rate of
the process. Practical oscillators lose energy over time to friction and
need a source of power which delivers energy in phase to maintain the
oscillation energy.
A few examples follow:
- A pendulum clock has a well-defined time constant based on the
gravitational constant, the length of the pendulum, and (weakly) the
magnitude of the motion. Energy is added via an escapement
mechanism which applies force in phase with the pendulum swing.
The force is provided from a descending weight which stores
gravitational potential energy.
- An electric motor has a rotor and stator with varying
magnetic fields which repel each other, precisely timed to apply torque
in phase with the rotation. In a small DC motor, the stator holds
permanent magnets and the rotor has an electromagnet winding with
current controlled by commutation brushes which mechanically
switch the current direction as a function of angle. The motor lacks
the single-frequency resonance of the pendulum; the rotational rate
emerge as a function of the voltage, applied load, friction, and the
intrinsic parameters.
Exercise Details
Creating and controlling a mechanical oscillation involves creating a
device which incorporates a physical dynamic process and adding actuation
to provide energy at the right time. The following exercise is framed in
abstract terms to allow for many possible outcomes. But keep in mind that
this can be very simple: a simple pendulum activated by solenoid or motor
is perfectly fine. And try to build as little as possible: a pendulum
device might comprise simply a solenoid and switch taped to a board and
wired together to a power supply to make a 'kicker', with a pendulum
formed by a weight on the end of a string.
Steps and observations
- Choose a mechanical process which either cycles periodically or can be
actuated to reset to an initial condition. Some ideas: swinging
pendulum, 2D pendulum, ball rolling down track, mass rotating on a
torsion spring, ball bouncing on plate, ball descending through liquid,
water flowing between reservoirs.
- Choose a relevant intermittent actuation which can add energy to the
process. Some ideas: a solenoid kicker, a winch or arm to reset object
position, pumps.
- Choose a relevant state-based condition under which to activate the
actuation. Some ideas: a particular object position, change of center
of mass, liquid level. The key is that it is not based on time
so that the natural time constant of the mechanical process will govern
the oscillation rate.
- Design and construct a simple mechanism to implement the oscillator.
Basic electronics are preferred but microcontrollers are permissible as
long as they are not serving as a time reference.
- Test your mechanism. What is the frequency? Is it stable? Is is
adjustable? How slow or fast can it go? Can it be dynamically
modulated?