13
Data Device Corporation
www.ddc-web.com
SDC-14580
H-05/04-0
When the resolution is changed, the VEL scaling is also
changed. Since the VEL output is from an integrator with a
capacitor feedback, the VEL voltage cannot change instanta-
neously. Therefore, when changing resolution while moving,
there will be a transient with a magnitude proportional to the
velocity and a duration determined by the converter bandwidth.
SPECIAL APPLICATIONS
CONTROL TRANSFORMER (CT) MODE
The SDC-14580 can also be used as a Control Transformer
(CT). The CT mode is used when the AC error (e) is needed to
drive an external control loop by the difference angle of the ana-
log input and the digital input. It is also used for presetting the
converter to a specific angle to reduce the step response time.
Referring to the equation below, the output is an AC voltage (e)
which varies as the sine of the difference between the analog
input angle and the digital angle.
e = sin(
θ - φ) cos ωt
This is analogous to the function of the rotary control transformer
except in this case the rotary shaft input is replaced by a digital
signal.
FIGURE 22 illustrates the CT block diagram. The CT mode is
enabled by placing S (pin 30) to a logic 0 and by using the digi-
tal output lines (now bidirectional) as digital inputs.
When the SDC-14580 functions as a CT the digital inputs are
double buffered, EM is redefined as LM (Latch MSBs), EL is
redefined as LL (Latch LSBs) and INH becomes LA (Latch All).
Control transformers are frequently used as error signal genera-
tors in closed servo loops. The CT mode can be applied in servo
systems, as shown in FIGURE 23. In this application, changes in
position are commanded by the microcomputer through signals
fed to the CT. The CT then drives the motors through dc power
amplifiers.
USING THE SDC-14580 AS AN S/D WITH VEL TO
STABILIZE POSITION LOOP
FIGURE 24 illustrates a typical use of a SDC-14580 connected
as an S/D using the VEL output to stabilize the position loop.
CONNECTING THE SDC-14580 TO A PC BOARD
The SDC-14580 can be attached to a PC board using hand sol-
der or wave soldering techniques. Limit exposure to 300°C
(572°F) max, for 10 seconds maximum.
Since the SDC-14580 Series converters contain a CMOS
device, standard CMOS handling procedures should be fol-
lowed.
VELOCITY RESPONSE
A filter on the VEL output will, for a step input in velocity, elimi-
nate the velocity overshoot (normally critically damped) and filter
carrier frequency ripple. The VEL filter is shown in FIGURE 17.
INTERFACING - DYNAMIC PERFORMANCE
A Type II servo loop (Kv = ∞) and very high acceleration con-
stants give the SDC-14580 superior dynamic performance.
If the power supply voltages are not the +5 Vdc nominal value,
the specified input rates will increase or decrease in proportion
to the fractional change in voltage.
TRANSFER FUNCTIONS
The dynamic performance of the converter can be determined
from its transfer function block diagram (FIGURE 18) and open
and closed loop Bode plots (FIGURES 19 and 20). Values for the
transfer function block can be obtained from TABLE 7.
RESPONSE PARAMETERS
As long as the converter maximum tracking rate is not exceed-
ed, there will be no velocity lag in the converter output although
momentary acceleration errors remain. If a step input occurs, as
when the power is initially applied, the response will be critically
damped. FIGURE 21 shows the response to a step input. After
initial slewing at the maximum tracking rate of the converter,
there is one overshoot (which is inherent in a Type II servo). The
overshoot settling to a final value is a function of the small signal
settling time.
ACCURACY AND RESOLUTION
TABLE 8 lists the total accuracy including quantization for the
various resolution and accuracy grades.
FASTER SETTLING TIME USING BIT TO REDUCE
RESOLUTION
Since the SDC-14580 has higher precision in the higher resolu-
tion mode and faster settling in the lower resolution modes, the
BIT output can be used to program the SDC-14580 for lower res-
olution, allowing the converter to settle faster for step inputs.
High precision, faster settling can therefore be obtained simulta-
neously and automatically in one unit.
TABLE 8. ACCURACY / RESOLUTION
ACCURACY
(MINUTES)
ACCURACY / RESOLUTION
10 BIT
12 BIT
14 BIT
16 BIT
±1 + 1 LSB
±2 + 1 LSB
±6 + 1 LSB
±4 + 1 LSB
22.1
23.1
25.1
27.1
6.3
7.3
9.3
11.3
2.3
3.3
5.3
7.3
1.3
2.3
4.3
6.3