Amplitude vs. Duty Cycle Modulation

The blog Ultrasonics – Ultrasonic Generators – Power Control discussed several ways in which the power delivery of ultrasonic generators can be varied.  One of the ways is by modulating the amplitude while another is through the use of time proportioning or duty cycle modulation.   There are other ways as well, but these are the “biggies.” They provide the basis of most of the others which are variations of these.  This blog discusses the workings of these two options. Upcoming blogs will reveal the merits and some limitations of amplitude as well as time proportioning or duty cycle modulation as applied in several applications.  But first things first.

Because it is easy for me to understand and illustrate, let’s use an electrical heater as an example for these two options.  To make the following illustrations useful for comparison, let’s also assume that all represent the same heater configuration under the same load conditions.

In an amplitude modulated system, the temperature depends on the amount of power supplied to the heater.  Think of it as the voltage supplied. As the power to an electrical heater is increased, the number of BTU’s produced increases according to I²R heating.  Once the capacity of the heater at a particular input power level is reached, the temperature remains stable as long as nothing else changes.

The higher the voltage, the higher the terminal temperature.  It should also be noted that the rate of heating as indicated by the slope of incline of the straight part of the curve is higher when the input power is higher.  This is due to the fact that the heater is producing more heat at the higher power input.

As a basis for our discussion of duty cycle or time proportioning modulation, let’s now look at what happens when maximum power is applied to an electric heater and then turned off before the terminal or stable condition is attained.

 

 

Notice that the  rate of heating is higher due to the higher power being applied.  Also notice that the temperature continues to rise after the heater is turned off.  This is due to the heat inertia provided by the residual heat of the heater

In time proportioning or duty cycle modulation control the rate of energy delivery remains constant (full power) but is intermittently interrupted (turned on and off) to produce the desired average power delivery.  The technique works particularly well in cases where the response lags input as is the case in most applications involving heat.  Consider an electric heater.  When electricity is applied, it takes time (lag) for the heater to produce the first heat.  When electricity is disconnected the residual heat in the heater continues to provide heat raising the temperature above the target temperature then cools slowly as the heat stored in the heater is dissipated.

 

The result is a fluctuation in temperature as illustrated above as the heat source is turned on and off to provide the required average temperature.

Remember, we have used an electric heater here as an example.  It so happens that in the case of an electric heater time proportioning works well with the proper controls.  In other cases amplitude modulation is the better choice.  Imagine a car that moved at 25 miles per hour average but in 100 mph spurts?  We will in an upcoming blog. 

 –  JF  –