LM628N-8 National Semiconductor, LM628N-8 Datasheet - Page 10

• Manufacturer Part Number: LM628N-8
• Description: IC,Motor Controller,MOS,DIP,28PIN
• Manufacturer: National Semiconductor

Specifications of LM628N-8

• Operating Current: 110mA
• Operating Temperature Classification: Industrial
• Package Type: MDIP
• Operating Supply Voltage (min): 4.5V
• Operating Supply Voltage (max): 5.5V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Theory of Operation
for example), the desired position will continue to be in-
creased, resulting in a very large position error. If this con-
dition goes undetected, and the impeding force on the motor
is subsequently released, the motor could reach a very high
velocity in order to catch up to the desired position (which is
still advancing as specified). This condition is easily de-
tected; see commands LPEI and LPES.
All trajectory parameters are 32-bit values. Position is a
signed quantity. Acceleration and velocity are specified as
16-bit, positive-only integers having 16-bit fractions. The
integer portion of velocity specifies how many counts per
sampling interval the motor will traverse. The fractional por-
tion designates an additional fractional count per sampling
interval. Although the position resolution of the LM628 is
limited to integer counts, the fractional counts provide in-
creased average velocity resolution. Acceleration is treated
in the same manner. Each sampling interval the commanded
acceleration value is added to the current desired velocity to
generate a new desired velocity (unless the command ve-
locity has been reached).
One determines the trajectory parameters for a desired
move as follows. If, for example, one has a 500-line shaft
encoder, desires that the motor accelerate at one revolution
per second per second until it is moving at 600 rpm, and then
decelerate to a stop at a position exactly 100 revolutions
from the start, one would calculate the trajectory parameters
as follows:
let
let
then R = 500
and P = 2000
let
let
let
then V = R
and V = 2000
let
then A = 2000
and A = 2.33E−4 counts/sample/sample
The above position, velocity, and acceleration values must
be converted to binary codes to be loaded into the LM628.
The values shown for velocity and acceleration must be
multiplied by 65,536 (as shown) to adjust for the required
integer/fraction format of the input data. Note that after scal-
ing the velocity and acceleration values, literal fractional data
cannot be loaded; the data must be rounded and converted
to binary. The factor of four increase in system resolution is
due to the method used to decode the quadrature encoder
signals, see Figure 9.
P = target position (units = encoder counts)
R = encoder lines
P = 2000
P (coding) = 00030D40 (hex code written to LM628)
V = velocity (units = counts/sample)
T = sample time (seconds) = 341 µs (with 6 MHz clock)
C = conversion factor = 1 minute/60 seconds
V = 6.82 counts/sample
V (scaled) = 6.82
V (rounded) = 446,956 (value to load)
V (coding) = 0006D1EC (hex code written to LM628)
A = acceleration (units = counts/sample/sample)
A = R
A (scaled) = 2.33E−4
A (rounded) = 15 (value to load)
A (coding) = 0000000F (hex code written to LM628)
*
*
T
T
*
*
*
*
*
*
*
4 = 2000
100 revs = 200,000 counts (value to load)
341E−6
desired number of revolutions
341E−6
T
C
*
*
desired acceleration (rev/sec/sec)
desired rpm
*
*
65,536 = 446,955.52
*
4 (system resolution)
*
*
341E-6
1/60
65,536 = 15.24
*
600 rpm
*
(Continued)
1 rev/sec/sec
10
PID COMPENSATION FILTER
The LM628 uses a digital Proportional Integral Derivative
(PID) filter to compensate the control loop. The motor is held
at the desired position by applying a restoring force to the
motor that is proportional to the position error, plus the
integral of the error, plus the derivative of the error. The
following discrete-time equation illustrates the control per-
formed by the LM628:
where u(n) is the motor control signal output at sample time
The first term, the proportional term, provides a restoring
force porportional to the position error, just as does a spring
obeying Hooke’s law. The second term, the integration term,
provides a restoring force that grows with time, and thus
ensures that the static position error is zero. If there is a
constant torque loading, the motor will still be able to achieve
zero position error.
The third term, the derivative term, provides a force propor-
tional to the rate of change of position error. It acts just like
viscous damping in a damped spring and mass system (like
a shock absorber in an automobile). The sampling interval
associated with the derivative term is user-selectable; this
capability enables the LM628 to control a wider range of
inertial loads (system mechanical time constants) by provid-
ing a better approximation of the continuous derivative. In
general, longer sampling intervals are useful for low-velocity
operations.
In operation, the filter algorithm receives a 16-bit error signal
from the loop summing-junction. The error signal is saturated
at 16 bits to ensure predictable behavior. In addition to being
multiplied by filter coefficient kp, the error signal is added to
an accumulation of previous errors (to form the integral
signal) and, at a rate determined by the chosen derivative
sampling interval, the previous error is subtracted from it (to
form the derivative signal). All filter multiplications are 16-bit
operations; only the bottom 16 bits of the product are used.
The integral signal is maintained to 24 bits, but only the top
16 bits are used. This scaling technique results in a more
usable (less sensitive) range of coefficient ki values. The 16
bits are right-shifted eight positions and multiplied by filter
coefficient ki to form the term which contributes to the motor
control output. The absolute magnitude of this product is
compared to coefficient il, and the lesser, appropriately
signed magnitude then contributes to the motor control sig-
nal.
The derivative signal is multiplied by coefficient kd each
derivative sampling interval. This product contributes to the
motor control output every sample interval, independent of
the user-chosen derivative sampling interval.
The kp, limited ki, and kd product terms are summed to form
a 16-bit quantity. Depending on the output mode (wordsize),
either the top 8 or top 12 bits become the motor control
output signal.
n, e(n) is the position error at sample time n, n'
indicates sampling at the derivative sampling rate,
and kp, ki, and kd are the discrete-time filter param-
eters loaded by the users.
(1)