Simple Signal Injector Aids Control-Loop Analysis - Power Electronics

Simple Signal Injector
Aids Control-Loop Analysis
A signal-injection circuit for control-loop
analysis is flat from dc to 200 kHz, isolated from
chassis ground and easily constructed with a
readily available instrumentation amplifier.
Network analysis is a powerful and wellestablished
method of characterizing and
optimizing a control system. [1] Unfortunately,
making a successful measurement can be
difficult and frustrating without the proper
instrumentation. Having a good network analyzer is not
enough. There must be a means for injecting a test signal
into a closed loop over the frequency range of interest.
A common method of signal injection is to insert a
100-W resistor in the control loop, typically between the
error amp and the plant, which is everything between the
control output and the feedback input. For example, a buck
converter plant consists of the sawtooth generator and
comparator, the power transistor, the catch diode and
the LC filter. The injection point must be between a lowimpedance
output and a high-impedance input. A trans-
High Z side B
B
Low Z side B
V REF
High Z side A
Low Z side A
Control
100 W
Feedback signal
generation
A
100 W
ERR IN
Plant
Fig. 1. In a typical control loop the error amplifier sends an error
signal to the plant. Resistors A and B are possible points for
signal injection.
V OUT
By Brian Keeney, Senior Electrical Engineer, Oxford
Instruments X-Ray Technology Group, Scotts Valley, Calif.
former is then used to generate an ac test signal across the
resistor. The reference signal is then measured at the plant
input and the response is measured at the error-amplifier
output. [2]
Fig. 1 shows a typical control loop and two common
locations for signal injection and measurement. It is very
difficult to design a transformer that will provide a flat
signal both at very low frequencies (< 1 Hz) and at higher
frequencies (200 kHz). An engineer can forego this design
and purchase a transformer, but the commercial versions
still have the same frequency limitations, typically only
operating over one or two decades. Electronic injection
circuits exist but are expensive, around $1000. At least one
open-source design exists according to published literature
[3] , but it suffers from chassis grounding issues.
Another Method
Fig. 2 shows a simple circuit that performs the forementioned
tasks very well. An instrumentation amplifier
(inamp) isolates the sine-wave test signal from the network
analyzer ground. The reference input is driven by the lowimpedance
output of an error amp or buffer. The sine wave
then rides on the reference node, or control point of the
circuit. It is important to realize that the reference input to
an inamp is not a high-impedance node. Therefore, make
sure that this node is truly driven by a low-impedance
source.
The output of the inamp is connected to the highimpedance
input of the plant. A designer does not actually
need the 100-W resistor with this design, but its presence
prevents accidental open-loop conditions if the device under
test (DUT) is powered up before the test fixture. Analog
Device’s AD620 [4] is able to be run on ±18-V rails, allowing
for injection at nearly any conceivable point in most loops.
Lastly, note the 1-MW resistor from the inverting input to
ground. The inamp needs nanoamps of input bias current,
Power Electronics Technology September 2008 20
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which isn’t much, but it is ill-advised to float the
inputs completely. The resistor allows a very small
amount of bias current without changing the return
current paths appreciably.
Design Verification
J1-1
The injection circuit was tested using a Venable
3120 network analyzer. First, the open-circuit
J1-2
performance of the fixture was checked with a test
voltage of 500 mV and the reference input biased at J1-3
1 V (a graph of open-circuit performance is shown
as Fig. A in the online version of this article.) The
region of practical use is from 0 kHz to 200 kHz.
The phase shift is unimportant because it affects
neither the network measurement nor the DUT.
However, gain is important because the injected
signal will start to decrease in the region of gain
roll-off, reducing signal-to-noise and resulting in
an inferior measurement. A higher-performance
inamp would extend the range of the circuit.
It is very easy to believe erroneous results from a network
analyzer. There are innumerable ways to compromise
the measurement, from incorrect test signal amplitude to
forgetting to turn on the injection circuit. It is important to
have an oscilloscope hooked up to both the signal injection
and measurement points. The signals should be relatively
clean — distortion and noise compromise a reading. Once
R2
100 kW
Low Z side
Fig. 2. In the instrumentation amplifier-based signal injection circuit the
output of the error amp drives the “low Z side”, and the “high Z side ” drives
the plant input.
the setup has passed an open-loop test like the one previously
described, it is prudent to test a standard, such as a
simple RC circuit. As a means of verifying signal integrity,
a buffered 10-kW/16-nF low-pass filter was tested in the
fixture with a dc bias of 1 V.
Results of this test (see Fig. B in the online version of
this article) showed that the performance was very good,
matching the expected 3-dB point of 1 kHz. There are
www.powerelectronics.com 21
Power Electronics Technology September 2008
R1
10 kW
R4
1 MW
+9 V
+IN
RG REF
U1 +VS
AD620
RG –VS
–IN –9V
C1
0.68 mF
C2
0.68 mF
Output
High Z side

50
Gain (dB)
–50
1
sigNAl iNjectioN
Plant gain-gain
Plant gain-phase
Frequency (Hz)
–180
20 k
Fig. 3. This closed-loop Bode plot of the high-voltage plant was
obtained by placing the reference node (S1) of the analyzer at
“high Z side A” and the measurement node (S2) at “low Z side B.”
many other logical test circuits, but making a standard
that closely matches the application is best. The results
from a 10-Hz high-pass filter won’t yield much information
about how the test circuit will behave at 100 kHz.
Plant Measurement
After verifying performance open loop and then in a
test circuit, the test fixture was used to take practical data.
The injection circuit was used to make actual network measurements
of a 45-kV X-ray source’s kilovolt-drive circuit.
This particular drive is for a handheld X-ray fluorescence
spectrometer used in RoHS compliance testing. Because
it is battery powered, the spectrometer needs to be stable
over a rather wide input range of 12 V to18 V.
Load conditions also vary widely, from 10 kV to 45 kV
and 0 mA to 50 mA. The initial “guess” compensation worked
well over most line and load conditions, but the supply occasionally
oscillated at high kilovolt and no load. To find
out what was going wrong, the inamp test fixture was used
to probe stability at several line and load combinations,
including the problem region.
Even if the compensation is in place and working, it is
usually very instructive to first look at the plant response.
Run the system closed loop and place the test input at the
input of the plant, but instead of placing the measurement
input at the output of the error amp, place it at the output
of the feedback generation circuit (or the output of the
plant).
A 100-W resistor was inserted between the error amp
and the plant, and the injector was connected as previously
described. The reference input of the network analyzer was
connected to the output of the inamp, and the signal input
of the network analyzer was connected to the buffered
feedback reference (1 V = 10 kV). The DUT was brought
up to 45 kV and 40 mA of load current, and the injected
sine-wave amplitude was adjusted to yield a good signal,
but not so much as to disrupt the operating point (about
180
Phase (degrees)
Frequency (Hz)
Fig. 4. A type-2 integrator was used to stabilize the loop in a
typical Bode plot of the high-voltage supply’s loop gain. Phase
margin in this supply is 60 degrees, which ensures that the
supply doesn’t overshoot, but at the expense of settling time.
100 mV PK-PK ). A sweep from 1 Hz to 20 kHz yielded the
results in Fig. 3.
By knowing that a type-1 integrating error amplifier
provides a constant 270 degrees of phase shift (90 degrees
from the capacitor and 180 degrees from the logic inversion
of the amp), the phase can be shifted to see where the
0 degree crossing point is. The criterion for stability is that
the gain must be less than 0 dB when the phase crosses
0 degrees. Knowing this, the integrator roll-off can be set
for optimal performance.
The measurement described previously was repeated
for several line and load conditions. It was found that the
dip in phase shown in Fig. 4 shifted to lower frequencies
with decreasing load. This effect was so pronounced that
the ample phase margin at 45 kV and 50 mA was reduced
to zero at no load, leading to the oscillation described
previously. H. Dean Venable wrote a landmark paper on
optimizing feedback compensation networks. [1] Through
use of the techniques prescribed by Venable, the phase in
that area was boosted to ensure stability over all line and
load conditions.
Note that there are two practical ways to make this
plant measurement. The loop usually can be stabilized by
swamping the capacitance of the integrator. Even if the
dynamic performance is abominable, it will keep the plant
in the operating region so that the designer can perform the
forementioned test. Once the test is complete, the designer
can calculate the optimal compensation and tune the error
amp to a stable, high-performance state.
A second way to perform this measurement is to run
the plant open loop, using a variable voltage reference to
set the operating point. This dc level is injected into the
reference input of the inamp, and the output of the inamp
drives the plant directly. On pulse-width modulation chips
with internal amps, this node is often available as the COMP
pin. Running the chip in this configuration can be a little
Power Electronics Technology September 2008 22
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50
Gain (dB)
-50
1
Plant gain-gain
Plant gain-phase
Slide bar:
357.2 Frequency (Hz)
0 Gain (dB)
58.12 Phase (degrees)
-2.47 Slope (20 dB/decade)
180
Phase (degrees)
-180
20 k

sigNAl iNjectioN
tricky since ICs such as Linear Technology’s LT3431 [5] pack
a bunch of features into the feedback node, for example
frequency reduction and current limiting. Clamping the
feedback input to the correct voltage will often solve these
problems (e.g., in the case of the LT3431, a voltage of 1 V
keeps the chip operating normally). [5]
Loop Measurement
The Bode plot in Fig. 3 shows a plant gain of about 11 dB
and a 90 degree phase shift at about 1.5 kHz. Using Venable’s
methods, the loop was compensated to have 60 degrees of
phase margin (the phase at the 0-dB point). This results in
a slightly overdamped response to step changes in program,
which minimizes the voltage stress on the components. In
other systems, fast settling time might be more important,
which would indicate a lower phase margin, but resulting
in more overshoot.
Fig. 4 shows the Bode plot of the compensated power
supply at 45 kV and 50 mA. The results of the test match the
DUT connections (banana cables)
Aux amp connections
BNC from N.A.
J4-1
J4-2
J5-1
J5-2
J6-1
J6-2
J8-1
J8-2
J9-1
J9-2
J1-1
J1-2
J1-3
R2
100 kW
Low Z
side
High Z
side
Aux in
Aux in
Aux out
BNC to S1
R11
No load
J10-1
J10-2
J10-3
+
J2-8
-
R12
100 kW
C3
100 pF
Low Z side
10x, 100x Attenuation
R1
10 kW
J2-1
J2-2
R3
1 kW
J3-1
J3-2
R4
1 MW
LT1057
High Z
side
Aux out
+IN
RG REF
RG
–IN
U1
AD620
BNC to S2
+9 V
OUTPUT
J11-1
J11-2
J11-3
LM4D4D, 2.5 V LM4C4C, 2.5 V
+V5
–V5
–9 V
+9 V
D2
NC
NC
C2
0.68 mF
C1
0.68 mF
-9 V
predicted response of the measured plant gain combined
with the integrator, indicating that the test fixture performs
its task well.
Network analysis can be difficult not only because of the
challenges of signal injection, but also due to the practical
difficulties of maneuvering so many test leads and scope
probes. It is very easy to short out a critical point and destroy
equipment or the DUT. The test jig described next
and pictured in the online version of this article overcomes
many of these challenges.
Fig. 5 shows the full schematic of the signal injection
circuit. In the upper left-hand corner, U1 handles the
actual signal injection. Note that there is isolation from
chassis ground at the signal input. R4 provides enough
current for input bias current requirements, but shouldn’t
cause ground loop errors. Jumpers J2 and J3 provide 10x
and 100x attenuation for fine control of the injected signal,
which can be very helpful for analyzers with limited
attenuation control. Two batteries provide low noise and
Fig. 5. Complete schematic of signal-injection pc board. The inamp in the upper left-hand corner does the actual signal injection.
R8
10 kW
D3
Low Z side
R7
10 kW
High Z side
Power Electronics Technology September 2008 24
www.powerelectronics.com
R5
CW
LED +9 V
+9 V
-9 V 5W1
DPDT switch
R6
5 kW
D1
Power on
R10
5 kW
R11
4 kW
REF for Open-loop plant analysis
+ LT1057
U2-A
-
R14
100 kW
C4
100 pF
R16
100 kW
V+
V-
V+
V-
+9 V
J7-2
J7-1
B1
9-V holder
B2
9-V holder
R15
5kW
D5
TL431
R9
5kW
D4
Low battery (< 7.5 V)
Q1
Low Z side

isolated rails (this could be changed to extend the usable
range to ±18 V).
In the lower right-hand corner, U2 (LT1057) provides
buffering for a variable voltage reference (needed for making
open-plant measurements) and an auxiliary buffer (for
providing a low-impedance feedback point). To make the
open-plant measurements, J7 is jumpered and the pot adjusted
until the plant is at the desired operating point. The
plant response is then measured from “high Z side A” to
“low Z side B” (Fig. 1). The TL431 is used for a low-batterydetect
circuit. The various connectors are used to interface
between the DUT and the network analyzer.
The fixture’s mechanical layout is
nearly as important as its electrical
functionality. (A mechanical layout of
the pc board appears as Fig. C in the
online version of this article.) Three BNC
connections at the back of the board go
off to the network analyzer (sine, reference,
measurement). This keeps all the
large cables out of the way and under
control. Two 9-V batteries (mounted
on the underside of the board) provide
power, eliminating another piece of test
equipment from the designer’s bench.
Additionally, the batteries provide some
mass, which is needed to keep the board
from falling off the bench.
Three banana jacks are mounted at the
front of the board. These connect to the
DUT ground and the low-impedance and
high-impedance sides of the resistor. The
resistor should be on the DUT and should
preferably be built in to the pc board.
There is negligible cost or mass penalty
to one extra resistor in the assembly. Test
points on either side of the resistor are
extremely helpful for clipping in both the
banana test leads and the scope probes.
A three-pin SIP header is very useful for
connecting all three signals at once.
When making plant measurements,
the banana leads stay as they are, but the
measurement BNC is unplugged, and the
measurement test node is probed directly
from the network analyzer to the node of
interest on the DUT. Other useful user
features on the board include a poweron
LED and a low-battery LED. Both are
very helpful at preventing measurement
errors. PETech
References
1. Venable, H. Dean. “Optimum Feedback
Amplifier Design for Control Systems,”
Proceedings of Intersociety Energy Con-
version Engineering Conference, August 1986.
2. Venable, H. Dean. “Practical Testing Techniques for
Modern Control Loops,” Venable Industries Technical
Paper #16, www.venable.biz.
3. Galinski, Martin. “Use Op-Amp Injection for Bode
Analysis,” EDN, Sept. 16, 2004, pp. 90, 92.
4. “AD620: Low Drift, Low Power Instrumentation Amp
with Set Gains of 1 to 10000,” Analog Devices data sheet,
www.analog.com.
5. “LT3431: High Voltage, 3A, 500kHz Step-Down Switching
Regulator,” Linear Technology Datasheet, www.linear.
com.
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