ST7MC2 datasheet(179/308 Pages) STMICROELECTRONICS

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ST7MC1/ST7MC2

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MOTOR CONTROLLER (Cont’d)

10.6.7.5 Speed Measurement Mode

Motor speed can be measured using two methods

depending on sensor type: period measurement or

pulse counting. Typical sensor handling is de-

scribed here.

Incremental encoders allows accurate speed

measurement by providing a large number of puls-

es per revolution (ppr) with ppr rates up to several

thousands; the higher the ppr rate, the higher the

resolution. The proposed method consists of

counting the number of clock cycles issued by the

Incremental Encoder Interface (Encoder Clock)

during a fixed time window (refer to Figure 100).

The tachogenerator has a much lower ppr rate

than the encoder (typically factor 10). In this con-

text, it is more meaningful to measure the period

between Tacho Captures (i.e. relevant transitions

of the incoming signals). Accuracy is imposed by

the reference clock, i.e. the CPU clock (refer to

Figure 99).

Figure 99. Tachogenerator period acquisition using MTIM timer

Figure 100. Encoder Clock frequency measure using MTIM timer

Comparator

Output

Tacho

Capture

Compare

Value

MTIM Counter

Value

Interrupts

Decreasing Speed

To interrupt generator

(Capture Event)

(Speed Error Event)

Encoder

Clock

Capture

(triggered by software

MTIM Counter

Value

Interrupts

Decreasing Speed

To interrupt generator

(Capture Event)

or Real Time Clock)

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