33991 datasheet(21/36 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33991

TIMING DESCRIPTIONS AND DIAGRAMS

DEVICE FUNCTIONAL DESCRIPTION

The motor is stepped by providing index commands at

intervals. The time between steps defines the motor velocity,

and the changing time defines the motor acceleration.

The state machine uses a table defining the allowed time

steps, including the maximum velocity. A useful side effect of

the table is, it also allows the direct determination of the

position the velocity should reduce to allow the motor to stop

at the desired position.

The motor equations of motion are generated as follows:

The units of position are steps, and velocity and

acceleration are in steps/second, and steps/second²

From an initial position of 0, with an initial velocity u, the

motor position, s at a time t is

For unit steps, the time between steps is:

This defines the time increment between steps when the

motor is initially travelling at a velocity µ. In the ROM, this time

is quantized to multiples of the system clock by rounding

upwards, ensuring acceleration never exceeds the allowed

value. The actual velocity and acceleration is calculated from

the time step actually used.

Using

and

and solving for v in terms of u, s and t gives:

The correct value of t to use in this equation is the

quantized value obtained above.

From these equations, a set of recursive equations can be

generated to give the allowed time step between motor

indexes when the motor is accelerating from a stop to its

maximum velocity.

Starting from a position p of 0, and a velocity v of 0, these

equations define the time interval between steps at each

position. To drive the motor at maximum performance, index

commands are given to the motor at these intervals. A table

is generated giving the time step

∆t at an index position n.

where

indicates

rounding up.

300

-0.866

222

0.5

128

315

-0.707

181

0.707

181

330

-0.5

128

0.866

222

345

-0.259

0.966

247

Notes * Denotes Normalized Values.

Table 16. Coil Step Value

1 at

−

⇒

−

= 2

⎥

⎤

⎢

⎡

−

∆

−

⎡⎤

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