Download Choosing servomotor brakes

Choosing servomotor brakes
JOHN MENDOLIA, API Deltran Inc.
Power-off brakes are used in servo applications to hold the driven load or to provide dynamic braking. Here
are some important factors to consider before making a selection.
n most servo motion control
applications, a servomotor
accelerates and decelerates
the driven system or load.
Servomotor brakes are used
mainly
in
vertical-axis
applications to statically hold
the load in the absence of power.
Typically, the brake is springapplied, power-off type, with a
static torque 50% higher than
required to hold the load.
On occasion, such brakes
are used for dynamic braking,
either as an assist to the motor,
usually taking over just prior to
stopping, or in emergency stop
situations.
Motion control
designers must consider the
worst-case conditions when
selecting the brake. If the need
to dynamically decelerate is
overlooked, for example, the
consequences may be premature
wear, degraded performance,
and
possibly
catastrophic
failure.
I
Brake operation
In most servomotor
applications, either static or
dynamic, it is desirable for the
brake to hold or decelerate the
load in the absence of electrical
power for reasons of safety and
energy conservation. The most
Figure 1 ― Spring-set, poweroff brake components.
common power-off brake type
uses compression springs to
push the brake armature axially
so it contacts and subsequently
stops the rotor and connected
load, Figure 1.
To disengage the brake, voltage
is applied to the coil housing,
thereby generating a magnetic
field. This field attracts the
armature, moving it axially to
the coil housing, thereby
releasing the rotor and the load.
A manual release can be added
to free the rotor if necessary
during a power outage.
To engage the brake,
voltage is removed, allowing the
coil current and magnetic field
to
decay.
When
the
electromagnetic pulling force
(produced by the magnetic field)
between coil and armature
reduces to slightly below the
spring force, the armature
separates from the coil and
engages the rotor.
At this
instant, the brake holds, or in a
dynamic application, begins to
decelerate the load.
Static applications
A typical power-off
brake exhibits the electrical
characteristics shown in Figure
2 during a static disengage and
engage cycle. As voltage is
applied to and removed from the
brake coil, the current rises and
decays respectively.
It is important to
consider the time delay from the
instant power is applied, t1, until
the rotor clamping force is
removed, t2,. Only then can the
servomotor accelerate the load
without restriction.
For this
particular brake model, the time
delay (armature release time) is
0.035 sec.
Equally important is the
time delay from the instant when
voltage is removed, t3, until the
rotor clamping force is applied,
t4. Ina typical power-off brake,
a silicon diode is connected
parallel to the brake coil for arc
suppression, resulting in a time
delay (armature engage time) of
0.028 sec, Figure 3. Obviously,
where the brake is used in a
vertical axis, with little or no
System drag, the servomotor
must hold the load during this
time delay, or until the brake
engages.
If you need a shorter armature
engage time, a Zener diode can
be added to the arc-suppression
circuit, Figure 3, which reduces
the time from 0.028 to 0.008
sec.
mentioned earlier, the stopping
process begins with removing
voltage from the brake. As with
static applications, the rate of
current decay and the stopping
time depends on the type of arc
suppression. Adding a Zener
diode reduces both the engage
time and the stop time. The
time to stop equals the engage
time, t4 - t3, plus the deceleration
time,
t5 - t4, or t5 - t3.
The rate of deceleration
equals the ratio of total torque to
total inertia, where total torque
equals brake dynamic torque
plus system drag, and total
inertia equals brake and motor
rotor inertia plus system inertia.
Accordingly, the rate of
deceleration (rad/sec2) is:
dω/dt = T/J
Application information
needed
Dynamic applications
Servomotor brakes is
dynamic
applications
can
achieve deceleration, Figure 4,
or emergency stops.
As
To select a brake for desired
performance and life, first gather
the
following
application
information:

with a static torque rating
50% greater than the
required holding torque,
thereby
providing
an
adequate safety factor.
High vibration along the
shaft axis can reduce spring
force and the amount of
brake torque.
For this
condition, ask the brake
manufacturer to calculate the
amount of torque reduction.


Braking mode required:
apply when power is applied
or when power is removed.
Type of available power: ac
or dc, constant voltage or
constant current, magnitude,
and tolerance range.
 Type of brake desired:
modular, external add-on, or
integral to the motor. Space
available
and
desired
mounting method.
 Ambient
temperature
extremes, allowable coil
temperature rise, and the
duty cycle.
 Brake operation: hold
only, stop every cycle, or
stop only in emergencies.
 Maximum speed and
direction of rotation.
 System drag or friction
torque and inertia.
 Allowable deceleration
time
or
number
of
revolutions after a stop is
initiated.

Cycle-to-cycle stopping
tolerance and variation
over life.
Brake selection,
dynamic
Brake selection, static
Selecting a brake to
accomplish static holding is
straightforward:
simply
choose a brake with a
torque rating reasonably in
excess of the worst-case
holding torque required.
Make sure that the brake
fits into the available space
and has the desired
mounting configuration.
Also consider the effect
of ambient temperature
extremes. For example, the
amount of current (amperes)
that is available to disengage
the brake decreases as coil
temperature
increases.
Therefore, you may want to
consult the manufacturer
about this effect.
When
selecting a static brake, plan
for occasional dynamic
stops. Engineers find that
operators sometimes use a
static brake dynamically,
which often leads to brake
failure.
A reasonable
approach is to select a brake
Here, the brake can be used
to achieve one of two
objectives: stopping within a
specified time, and stopping
within a specified amount of
travel.
Example 1 ─ Stop within
specified time. Start the
selection
process
by
calculating
the
torque
required to stop the system
inertia within half the
available time. The reason
being that at this point you
don't know the brake rotor
inertia or the brake response
time.
Therefore, the
estimated torque is:
T = 0.1047 Js (dω/dt)
This formula uses different units
than the earlier formula dω/dt =
T.
For convenience, ω is
expressed in rpm rather than
rad/sec. All of the following
formulae use rpm as well.
At this point evaluate the
system drag. If possible, it
should assist deceleration or
stopping.
If so, reduce the
required
brake
torque
accordingly:
Tb = T + Td
If counteractive overrunning
torque
exists
during the
deceleration cycle, increase the
required brake torque:
Tb = T + Td
From the suppliers catalog
choose a brake model that has
25% more than the above
calculated torque Tb to allow for
the brake rotor inertia. Make
sure that the brake is
recommended for dynamic
applications and the torque
listed is dynamic rather than
static.
After
making
this
preliminary selection, calculate
the deceleration time using the
brake rated torque and the total
system inertia (including brake
rotor).
dt = 0.1047Jdω/T
The deceleration time plus the
brake engage time must be equal
to or less than the desired
stopping time. If the time to
stop slightly exceeds the time
required, consider improving the
arc suppression circuit before
evaluating a larger frame size.
Next,
calculate
the
amount of energy the brake can
absorb as described later under
the section "Energy absorption."
Example 2 ― Stop
within specified travel. Start
by selecting a model that best
fits the available space and has
the desired mounting features.
Then calculate the total travel,
which includes travel during the
armature engage time plus the
travel required to decelerate the
load. Referring to Figure 4, the
total travel is:
Eb per cycle = 4.6 (Jω2) 10-4 ftlb/cycle
If the system's friction drag is
significant compared to the
brake torque, modify the energy
calculation above b y the ratio of
brake torque to total torque:
Eb per cycle = [Tb / (Tb + Td)] X
4.6(Jω2) 10-4 ft-lb/cycle
If the braking action will occur
at a rapid cyclic rate, N:
S = [(t4 - t3) + (t5 - t4)/2] ω/60
If this travel is unacceptable,
recalculate using the armature
engage time with a Zener diode,
or select another model and
repeat the calculations. If the
travel is satisfactory, calculate
the energy absorption as
described in the following
sections.
Energy absorption
So far, you have selected
a brake based on its single cycle
stopping capability.
Next,
verify that the brake can
dissipate the kinetic energy
absorbed per cycle and at the
worst-case frequency in the
application without generation
excessive heat. To do this, first
calculate the energy absorbed by
the brake per cycle:
Eb per minute = [Tb / (Tb+Td)] X
4.6(Jω2) 10-4 (N) ft)-lb/min
Figure 5 ― Allowable brake
energy absorption chart (25%
brake duty cycle) from
manufacturer's catalog.
To ensure that the brakes
are sized properly, compare the
calculated energy absorption per
cycle and energy absorption per
minute to the recommended
values listed in the
manufacturer's catalog. Must
manufacturers publish energy
absorption graphs in their
catalogs, such as Figure 5, to
assist in the process.
Estimating life
Servomotor brakes offer
years of maintenance-free
service if they are properly
applied. If there is any doubt
about the application, request
assistance from the brake
manufacturer.
For brakes used in
dynamic applications, the
manufacturers usually provide
wear life data, either estimated
or gathered from tests. From
this information, you can
estimate the brake life in terms
of the total number of braking
cycles expected.
To estimate the number
of cycles of maintenance-free
life, divide the total allowable
energy absorption (ft-lb) of the
brake (specified by the brake
manufacturer) by the calculated
energy absorbed per cycle in
your application (ft-lb/cycle). ■