33977 datasheet(13/37 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33977

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

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 to define the allowed time

and the maximum velocity. A useful side effect of the table is

that it also allows the direct determination of the position at

which the velocity should reduce to stop the motor at the

desired position.

Motor motion equations follow: [reworded for efficient use

of space]

(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 traveling at a velocity u. 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 the 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.

Note: [chgd for format consistency AND deleted that as PV]

For pn = n, on the nth step, the motor [has deleted as PV]

indexed by n positions and has been accelerating steadily at

the maximum allowed rate. This is critical because it also

indicates the minimum distance the motor must travel while

decelerating to a stop. For example, the stopping distance is

also equal to the current value of n.

The algorithm of pointer movement can be summarized in

two steps:

1. The pointer is at the previously commanded position

and is not moving.

2. A command to move to a pointer position (other than

the current position) has been received. Timed index

pulses are sent to the motor driver at an ever-

increasing rate, according to the time steps in Table 6,

until:

aThe maximum velocity (default or selected) is

reached after which the step time intervals will no

longer decrease.

bThe distance in steps that remain to travel are less

than the current step time index value. The motor

then decelerates by increasing the step times

according to Table 6 until the commanded

position is reached. The state machine controls

the deceleration so that the pointer reaches the

commanded position efficiently.

An example of the velocity table for a particular motor is

provided in Table 6. This motor’s maximum speed is 4800

165

0.259

-0.966

-0.707

180

0.0

-1.0

-0.866

195

-0.259

-0.966

210

-0.5

-0.867

-1.0

225

-0.707

-0.966

240

-0.866

-0.5

-0.866

255

-0.966

-0.259

-0.707

270

-1.0

0.0

-0.500

285

-0.966

0.259

-0.259

300

-0.866

0.5

0.0

315

-0.707

0.707

0.259

330

-0.5

0.866

0.500

345

-0.259

0.966

0.707

* Denotes normalized values

Table 5. Coil Step Value

s = ut + 1/2 at

⇒ t =

- u +

√ u2 + 2a

v2 = u2 + 2as

v = u + at

v = 2/t - u

p0 = 0

v0 = 0

∆t

n =

⎡-v

n -1 + √v

n -1 + 2a

⎤

where

⎡ ⎤ indicates rounding up

vn =

2/∆tn - Vn -1

pn = n

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