Download After you`ve designed the world`s best servo…

After you've designed the world's best servo…
Don't be tripped up by practical details!
The servo configuration of Figure 1 may well embody the latest in motion control theory.
However, translating a superb theoretical design into a reliable servo requires attention to
the mundane practical details that often eludes a gifted designer's interest.
Poorly designed ground connections, for example, can lead to inadequate protection
against common mode errors—or crosstalk in multi-axis motion control systems. Power
supply selection that's left to last moment decision may inhibit servomotor acceleration.
Or the unusual duty cycles of a PCB drilling system—if not anticipated—cause MTBFdegrading heat dissipation. And don't wait for a power supply explosion to focus
attention on regenerative braking deficiencies.
This article may help differentiate between a superbly performing servo, and persistent
problems that create customer friction or costly field fixes.
CONTROLLER-SERVOAMPLIFIER CONNECTIONS
The servoamplifier and its source of motion commands are frequently located at widely
separated sites. Such geographical separation may create substantial differences in the
ground potential at controller and servoamplifier locations. In the absence of appropriate
protection, excessive common mode voltage (CMV) can interference with the analog
signal fed from controller to servoamplifier. Motion commands are then distorted by this
common mode voltage.
Most servoamplifiers are "immunized" against common mode errors owing to their
differential input circuitry. Control signals are then applied between the amplifier's
ground. This differential input, combined with the amplifier's high common mode
rejection (CMR), vastly reduces the effect of amplifier-controller potential differences.
(See Editorial Box for discussion of common mode errors).
Just because the servoamplifier comes with differential input and high CMR, don't be
lulled into a false sense of security. The basic servo of Figure 1 does not take into
account the fact that industrial plants with heavy machinery suffer ground potential
differences of tens of volts, not merely a few volts. Most servoamplifiers are rated for a
±10 volt common mode voltage range. Therefore evaluate the likelihood of higher
common mode voltages, and select an amplifier with appropriate common mode voltage
(CMV), rating.
POWER SUPPLY CONNECTIONS
Pulse width modulation (PWM) servoamplifiers draw power from their DC supply in the
form of fast-rise bi-directional current pulses at the amplifier's switching frequency. For
modern servoamplifiers, this switching frequency lies between 15 kHz and 100 kHz.
At the MOSFET's fast current rise rates,
even short lengths of wire between power
supply and servoamplifier can have
sufficient inductance to create amplifier
operation problems. As a rule of thumb,
whenever wire length exceeds 16 inches,
Figure 3, connect a decoupling capacitor
directly across the amplifier's power
supply terminals to reduce the wiring's
influence. Capacitor value should be at
least 500 µF for applications where wire
length exceeds 16".
BEWARE LdI/dT
A 1-inch length of 20 gauge wire has an inductance of
roughly 25 nanohenries. Modern MOSFETs switch
from zero to 40A in 20 nanoseconds or so.
Consequently, a considerable Ldi/dt voltage transient
can deveop across enen a short conductor length
L
= 25 x 10-9 H
di/dt
= 2A/ns = 2 x 109 A/s
V
= Ldi/dt
= 2.5 x 10-9 x 2 x 109
= 50 volts. And that's for a one-inch
length of wire!
A further step to minimize wiring length, and reduce power supply source-impedance, is
to connect the power supply output terminals directly—or as closely as possible—to the
capacitor terminals. (Rather than the rectifier terminals). The power supply case should
also be grounded to the capacitor's -VDC terminal, with minimal wiring length between
case and actual earth ground.
MULTI-AXIS SYSTEMS
To minimize mutual interference between the different axes of a multi-axis system, run
separate power cables to each amplifier, as shown in Figure 4 (top). Daisy-chaining the
amplifiers to a common DC bus, Figure 4 (bottom), exposes downstream amplifiers to
the accumulated power line voltage perturbations created by amplifiers' upstream in the
chain. Servoamplifiers draw appreciable accelerating current—20A is typical, 50 amps is
quite common Since this current takes the form of fast rising pulses, we are not dealing
with straightforward DC voltage drops—which the amplifiers' differential input circuit
and common mode rejection capability would handle. Instead, the impedance created by
wiring resistance and inductance creates mutual interference between amplifiers (ie,
crosstalk), and impairs system performance.
The mechanical structure of many automation systems provides a convenient heat sink
for the servoamplifiers. In such instances, the servoamplifiers are mounted directly onto
to the equipment's frame. Most servoamplifiers have signal and power grounds tied
together, and connected in common to the amplifier's metal base. Consequently, bolting
the amplifier to the machine frame grounds the amplifier at that point. If no separate
power supply connection is made to each amplifier's -VDC terminal, the machine frame
itself becomes the ground return path for all amplifiers, Figure 5.
Machine frames—which are not usually designed to provide an electrical return path for
substantial motor currents—can add substantial ohmic resistance—hence I x R voltage
drops—to the current return path. Especially where paint or other coating adds to the
resistance—or creates an intermittent open-circuit. This will vastly exacerbate crosstalk
difficulties. A longer-term and less obvious problem associated with high motor
currents—especially when several motors share the same DC power supply and machine
structure—is galvanic action, which can seriously degrade mechanical joints.
The answer, of course, is not to depend on the machine frame for the amplifiers' current
return path. Copley Controls recommends running a separate -VDC return wire to at
least the part of the machine frame where the amplifiers are located. Alternatively, run
individual cable pairs to each amplifier, as shown in Figure 4 (top). It may even help to
leave the DC power supply un-grounded. Instead, ground the DC bus at the amplifier's
negative power supply terminals, with individual conductor pairs energizing each
amplifier.
AC POWERING
A modern trend towards direct AC operation changes the way servoamplifiers and their
associated servomotors are powered. And grounded. The purpose of AC operation is to
cut cost and bulk by eliminating the isolated DC power supply. Or at least, dispense with
the bulky isolation transformer. For a 3 hp servomotor, the isolation transformer alone
can weigh in the region of 40lb - 60 lb.
In such arrangements, high voltage DC power is provided by a simple rectifier-capacitor
supply built in to the amplifier, Figure 6 (top), or —for multi-axis systems—provided
externally, (bottom). The high voltage DC power circuits of both amplifier and
servomotor are floating—that is, electrically isolated from ground. (Grounding the VDCbus would short-circuit half of the power supply rectifier, since one leg of the AC supply
is already grounded). The signal source (computer, controller…), is normally grounded,
as are the amplifier's internal signal circuits. This introduces the need for internal optoisolation devices to convey control signals to the floating HVDC power stages.
POWER SUPPLY DROOP
Pulse width modulation amplifiers function as true DC transformers. This attribute
enables a servoamplifier to power motors with a wide range of operating voltages. (The
amplifier very efficiently "steps down" the DC supply voltage to whatever voltage the
motor requires). For this reason, PWM servoamplifiers can operate from simple
unregulated rectifier-capacitor power supplies, thereby minimizing system cost.
(Regulated DC power supplies typically lack the reservoir capacitance for motion control
use).
In selecting the unregulated power supply, Figure 7, be aware that its output voltage is
likely to decline significantly with load current. Make sure that this output "droop" likes
within the range of motor requirements. The power supply's isolation transformer is the
major source of the internal droop-creating impedance. This internal impedance Z
develops an IPeakZ voltage drop that reduces the maximum output voltage. (Where IPeak is
the capacitor charging current, which is drawn through the transformer impedance).
Rectifier voltage drop adds to the output decline.
The important selection issue is to ensure that the power supply's output voltage is
sufficient to sustain full motor speed at maximum load. The power supply must maintain
adequate voltage "headroom" when delivering the high peak currents required for motor
acceleration and reversal. (Typically, a servoamplifier's peak current is twice its
maximum continuous output).
In deciding upon power supply specifications, remember that the servoamplifier's own
output transistors function as resistors. Voltage drop in the output MOSFETs reduces
motor drive voltage by roughly 5% - 10%. Accordingly, select a DC power supply with
10% extra output voltage headroom.
POWER SUPPLY RIPPLE
Ripple refers to the power supply's instantaneous output voltage variations, Figure 8,
which occur at twice the power supply frequency. While output droop may be measured
on an ordinary DC-reading voltmeter, ripple viewing requires an oscilloscope. Ripple
creates problems that are less readily diagnosed.
A simple unregulated rectifier-capacitor power supply depends upon energy stored in its
reservoir capacitor to maintain load voltage between charging periods. The twice-percycle charging intervals last for roughly 12% of each AC half-cycle—about 1 millisecond
at 60Hz operation. Between charging periods, the capacitor is "on its own" for the
remaining 7 milliseconds. (Equivalent to roughly 175 complete cycles of a
servoamplifier's 25 kHz switching frequency). Since the capacitor is isolated from the
AC source between charging periods, the capacitor itself, to all intents and purposes,
becomes the actual DC supply.
On light loads, Figure 8, the reservoir capacitor holds output DC voltage close to the AC
voltage peak. For a 120V AC supply, the no-load output voltage approaches 120V x √2 =
170 volts. Ripple is negligible at zero current, but increases as current rises. Between the
twice-per-cycle charging intervals, the degree of capacitor discharge (that is, ripple),
depends both upon both loads current and capacitance value. (In actuality, a bleeder
resistor connected across the capacitor renders "no load" a moot condition).
The diagnosis of ripple's impact on servo operation is less than obvious. The power
supply's output voltage, as measured on a DC-reading voltmeter, may well exceed the
amplifier's requirement at maximum motor current. Ripple, however, reduces the
instantaneous power supply voltage, and may drive VDC momentarily below the motor's
minimum rating.
Fast response servomotors can accelerate from zero to full speed within the 8
milliseconds of a 60Hz half-cycle. Excessive ripple reduces the DC supply voltage
available for acceleration, drastically curtailing the servo's responsiveness. However,
none of this can be diagnosed with an ordinary DC reading voltmeter.
Modern servoamplifiers are provided with an internal current sensing circuit that
furnishes an analog output voltage proportional to load current. This is the diagnostic
key! At the threshold where ripple reduces VDC below the minimum necessary for motor
operation, the current monitor's analog output waveform becomes a replica of the power
supply ripple waveform. This can be viewed on the oscilloscope to confirm the ripple's
degrading effect.
The simplest cure for power supply ripple is to change the power supply transformer tap,
and raise the average DC output voltage. Ripple amplitude won't be significantly altered,
but voltage reductions (ripple), caused by capacitor discharge will remain above
minimum motor requirements. An alternative—especially when tap changing is
insufficient—is to increase the capacitance of the power supply's filter capacitor. A
further possibility is to run the system from a tree-phase DC power supply, which
produces lower ripple than its single phase counterpart.
BIGGER CAPACITOR?
Increasing power supply filter capacitance is not without its penalties. More capacitance
means that the power supply must squeeze more energy into the (larger) capacitor within
the same 1 millisecond charging period. This is why rectifier-capacitor DC power
supplies can draw peak currents amounting to ten times their average AC load current.
The RMS heating effect of high peak currents can be far more serious—and MTBF
degrading—than a meter-reading of the AC line current would suggest.
Adding reservoir capacitance to reduce ripple can therefore raise peak charging current
(and heat dissipation), to the level at which transformer life is degraded. Addition of
capacitance may also be counterproductive in a different way. High peak current drawn
from the AC power line can increase the power supply's internal IPeak x Z voltage drop to
the point of diminishing returns. Ripple will decline, but so will the nominal DC output
voltage.
A final safety precaution. Don't forget to connect a bleeder resistor across the reservoir
capacitor. The resistor won't draw much current from the DC supply, but it will dissipate
the capacitor's charge—and eliminate nasty surprises—when the power is off.
WATCH THE DUTY CYCLE
Servomotors are likely to expose the DC power supply to unusual extremes of load
variation. More so, at least, than better-behaved loads like computers and
instrumentation. Depending on the servo's duty cycle, the power supply can require an
output rating nearer the amplifier's peak current, rather than its average current.
A printed circuit board drilling system, for instance, undertakes many rapid step-andrepeat motions, each involving high current accelerating and braking. Operation at
normal running current occurs only when the drilling head is withdrawing, to allow a new
PCB to be introduced. In other words, the motor spends the bulk of its time either
accelerating or braking. Accelerating current is typically twice full speed running
current. Consequently, the RMS current delivered by the power supply in such step-and
repeat applications is appreciably higher than it would be for an amplifier driving—for
example —a constant-velocity conveyor system.
Coil winders, centrifuges, and similar high inertia loads are slow to attain full speed,
hence draw high accelerating current for long periods. Here again, selection of the power
supply—and the amplifier too—must be guided by an unusual duty cycle—one that
amounts to a sustained overload.
REGENERATIVE BRAKING
Battery powered systems—golf carts, fork-lift trucks, automatic guided vehicles
(AGVs)—turn the kinetic energy of the vehicle's motion to positive use. Regenerative
braking converts the drive motor into a generator which then recharges the vehicle's
battery. The net effect is to extend equipment operating life between charges.
For servos powered by rectifier-capacitor DC supplies, regenerative energy can be a
hazard rather than a benefit. The E = 1/2CV2 energy storage capacity of a 10,000µF
capacitor is measured in watt seconds, whereas a battery's capacity is often rated in watt
hours. For AGVs, it can even be kilowatt-hours. Consequently, a DC power supply's
filter capacitor is easily "overcharged" by regenerative energy transferred from the load.
Excessive kinetic energy—see box: EXAMPLE OF REGENERATIVE BRAKING ENERGY—will
raise capacitor voltage (hence DC bus voltage), to levels far beyond the capacitor's rated
value.
Excessive bus voltage will destroy the capacitor or the servoamplifier. Replacing a $40
capacitor in the field can become a $1000 undertaking. There's an even more perilous
consequence: excessive kinetic energy can overcharge the capacitor and activate the
servoamplifier's over-voltage shutdown circuit. Motor and load are no longer under
servoamplifier control, but are free to coast until friction absorbs the kinetic energy. Or
more likely, until the freely coasting mechanism slams into mechanical stops, or, worse,
causes injury, or damages costly machine tools and workpieces.
Figure 9 shows three different techniques for disposing of the load's kinetic energy.
The brute force way to accommodate regenerative energy is to absorb it in a larger power
supply reservoir capacitor. (Figure 9, top). Increasing reservoir capacitance may be a
cumbersome solution for high inertia loads, coil winders, rolling mills, and centrifuges,
which store large amounts of kinetic energy. The sheer bulk of storage capacitance—not
to mention cost—provides the incentive to seek alternative techniques for dealing with
regenerative braking energy.
An "electronic zener" or "dumper" in parallel with the DC supply offers a compact and
economical energy dissipation method. (Figure 9, center). Regenerative energy charges
the capacitor voltage to the zener's trigger level, causing the zener to conduct and connect
a high wattage dissipating resistor across the DC bus. Commercial dumpers can be
paralleled for a virtually unlimited range of regenerative loads.
A third trick-of-the-trade completely obviates the need for electronic zener or added
capacitance. Here's how! Drive the mechanical load with a low voltage servomotor.
Power the motor from a low voltage DC supply. And control the motor with a high
voltage servoamplifier. Also, make sure the reservoir capacitor and rectifier diodes have
the same high voltage rating as the servoamplifier.
For example, operate a 50V servomotor from the normal 55V - 60V DC supply, but use a
servoamplifier and capacitor rated for, say 180V. The bottom illustration of Figure 9
shows the arrangement.
The capacitor's energy storage capacity (E = 1/2CV2), increases with the square of the
voltage to which it is charged. Trebling its voltage rating, therefore, enables the capacitor
to absorb a ninefold increase in regenerative energy. Neither capacitor nor amplifier is
damaged by the high voltage, and the amplifier's over-voltage protection circuit is set to a
yet-higher threshold.
Protection available by raising amplifier voltage threefold is comparable to matching the
protection from a ninefold increase in reservoir capacitance. A high voltage capacitor
and amplifier add only modestly to system costs. In contrast, a ninefold increase in
storage capacitance raises cost significantly, and may also be unacceptably bulky.
ARMATURE VOLTAGE VELOCITY FEEDBACK
For speed control application that can accept 3% or so speed variation—conveyors,
automatic guided vehicles, radar antennas—a cost-cutting technique is to dispense with
tacho feedback, and depend upon the armature's back-emf instead.
Armature back-emf is a true measure of shaft speed. However, a motor's back-emf can't
be measured directly. This is because the voltage at the armature terminals includes
armature voltage drop (Iarmature x Rarmature), as well as back-emf. To obtain a true value for
back-emf, the armature voltage drop must first be subtracted from the motor's terminal
voltage.
An analog subtraction circuit that removes armature voltage drop is available in many
modern servoamplifiers. Figure 10 shows the evolution of the subtraction circuit, and its
embodiment in a typical servoamplifier. The resulting post-subtraction output voltage is
then a signal closely proportional to true motor back-emf which can be used for velocity
and control feedback.
END
Run separate cable to each servoamplifier
Don't power amplifiers from common DC bus
Figure 4—Avoid crosstalk between servoamplifiers in multi-axis systems by running
separate power cables to each amplifier (top). Don't "daisy chain" the amplifiers from
common DC bus (bottom), unless the bus is very short and has low impedance. Power
line perturbations created by upstream amplifiers (#1 and #2 for example) impair
operation of amplifiers later in the chain.
Figure 5—Servoamplifier's cover and metallic frame are bonded to -VDC terminal.
Consequently, bolting amplifier to machine's metal frame grounds the amplifier. Using
the machine as return path for load current introduces possibility of high resistance joints
and large voltage drops. Downstream amplifiers are then exposed to interference from
amplifiers electrically-closer to the power supply. Copley recommends running
individual -VDC return cables to each amplifier, as shown in Figure 4 (top). In some
circumstances, to minimize current flowing through the machine frame, the power
supply's negative output terminal is left ungrounded. Instead, grounding of the DC
supply occurs at the amplifier's -VDC terminal—separate power cables are run from the
DC supply to individual amplifiers.
Fig. 2 - Ground Loops