AN2388 datasheet(40/54 Pages) STMICROELECTRONICS

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Software design

AN2388

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the reason why the current gain has been set to have the maximum wide range voltage

during the transient stage, avoiding any saturation.

The two phase currents are sampled through the microcontroller and converted using the

ADC module. The channels of the A/D peripheral are multiplexed and the time of conversion

is 3µs per channel. A complete conversion (two phase currents) needs 6µs.

In this application, channel 0 (port 5.0) and channel 1 (port 5.1) have been configured as

inputs for A/D conversion.

Figure 34.

Phase current detection

As can be seen in the Figure 34 a slight offset appears on the two phase currents acquired

by Hall effect sensors. Many reasons could explain it (offset voltage on the op-amp and

conditioning circuitry) and in order to compensate for it there are some offset values

introduced in the current sensing routine. The offset value fixes the output voltage, coming

out from the current sensor, in order to have a 2.5V mean value.

4.5.2

Rotor mechanical position sensing

In this AC induction drive a 1024 pulse incremental encoder produces the rotor position.

The two sensor output channels (A and B) are wired directly to the incremental interface

mode for the core timer T3 selected by setting bit-field T3M in register T3CON to ‘110’. In

incremental interface mode the two inputs associated with timer T3 (T3IN, T3EUD) are used

to interface to an incremental encoder. T3 is clocked by each transition on one or both of the

external input pins which gives 2-fold or 4-fold resolution to the encoder input

AN2388 Datasheet(PDF) 40 Page - STMicroelectronics