9. Additional Loading

Commonly, CMUTs are loaded with a DC Bias to create electrostatic forces and deflect the membrane before application of an AC signal for ultrasound generation. However, there can often be other loading or force conditions acting on the model, for example

  • Atmospheric pressure
  • Pre-stress of the membrane

In the case of atmospheric pressure, users can employ plod to apply the requisite pressure on the center of the membrane in order to cause initial deflection. This is best implemented before the application of the Bias DC voltage, to effective give two static loading stages: Atmospheric Pressure and DC Bias. Both these stages requires Dynamic Relaxation to be on.

A separate function can be created to generate the Pressure loading as this prevents complex manipulation of the electrical drive function. It is important, however, to get the model timings correct:

  • Ramp up Atmospheric pressure load and allow steady-state to be reached
  • Ramp up DC bias and allow steady state to be reached.
  • Turn off DRLX and drive with AC signal for Impulse response.

Pre-stressing the membrane is a little more complex as the user either has to have knowledge of the forces involved, or run a thermo-mechanical model first to calculate residual stresses from the manufacturing process. Either way, forces can be applied using a plod command, or more commonly, a rigd command. 

Note: Care must be taken when applying pre-stress to ensure simulation input conditions are correct.

/* function for loading ATM - smooth ramp would be preferable to step.
data hist atmprs 3 0.0 0.0 5.e-7 $atm 1 $atm c ***************************** c         PRESSURE LOAD c **************************** plod pdef pldatm atmprs vctr negy 0. -1. 0. sdef pldatm negy $i1 $i3 $j4 $j4   /* Pres applied in 'fake' element (center of Membrane) end