Linearity relationship test

AN2791

8/18

3

Linearity relationship test

The measurements of the values of the mean current to compare with the "ideal" linear

relationship of the Root Part Number 1 current control channels have been carried out on

the "hold-phase" of the current waveform. The Figure 5 describes a comparison between

the results obtained on two different loads:

●

stand-alone coils (i.e. blue stars, crosses and balls);

●

coils on the valves (i.e. red stars, crosses and balls);

The main difference between the two considered different load conditions is that for the coils

stand alone you have an equivalent R-L circuit with a fixed inductance.

On the other hand, when as load you consider a coil on a valve, from the point of view of the

equivalent R-L circuit there is an inductance changing with the opening/closing dynamics of

the valve.

As the results of our analysis show in the Figure 5, 6 and 7 the current control loop of the Q3

and Q4 channels has been conceived in order to drive variable inductance loads, in fact, the

spread between maximum and minimum values of the current for a fixed duty-cycle value of

the PWM input signal is minimum in the case of a variable inductance load.

Figure 4.

Current waveform produced on the load by the test coil energizing

strategy