Issue
153 April 2003
Muscle
for High-Torque Robotics
by
Lonne Mays
Is
your favorite robot getting to be a bit too atrophied
for your taste? Does it lack vim and vigor? If so,
don’t worry. What you need is high torque and speed.
With the MC33887D integrated H-Bridge and a little
direction from Lonne, you can bulk up your robot
in no time.
Start
•
Software vs. Hardware • PW-V
Converter
• Analog Switch •
Sources and PDF
Analog
servomotor control ICs—such as the venerable MC33030—have
facilitated my motion control tasks for many years.
Integrated circuits of this genre contain the op-amps
to process the information (i.e., voltage) from analog
position sensors, the comparators to process position-command
voltages (i.e., reference voltages), and the mixed-signal
circuitry to close the control loop while taking care
of drive functions like window detection, drive direction,
braking, and stall detection. The drive capability of
these ICs, however, is generally limited to tiny low-voltage
motors (e.g., less than 12 V at less than 500-mA steady-state).
TORQUE
VS. SPEED
When
you’re limited to such wimpy motors, the issue of obtaining
high torque involves a trade-off between the actuator
speed and the required torque or force. This is because
a high-ratio gear train is required to achieve a torque
multiplication factor. As a consequence, the speed of
the mechanical actuator is reduced (divided) by the
same factor. In essence, it becomes a choice between
torque and speed. For many applications, a high-ratio
gear train is ill suited, because the mechanical delay
imposed between commanding a new position and that new
position being achieved can become greater than what
the system requirements will tolerate.
TORQUE
+ SPEED
To
have both high torque and speed requires you to employ
a more potent motor. Such motors don’t have to be large,
but they do require a heftier V × A product delivered
to their terminals. Of course, the output of the servo
IC can be boosted via a handful of discrete PNP and
NPN bipolar junction power transistors, but the biasing
of this external array of parts is a nontrivial task.
Care
must be taken to provide bias tracking stability over
temperature, given the PN junction’s notorious negative
temperature coefficient. Addition-ally, there is the
undesirable inherent inefficiency of the multiple VCE(sat)
voltage drops in series with the load, as well as the
power wasted in the emitter ballast resistors. Creating
an external H-Bridge with discrete MOSFETS is also problematic,
because this approach requires different drive circuits
for the high-side and low-side FETS and the use of charge
pumps.
Using
a monolithic power H-Bridge ASIC like the MC33887DH
is by far an easier means for boosting the output of
a servo controller (whether analog or digital). Because
this IC incorporates not only the low RDS(on) H-Bridge
but also the gate drive, charge pump, and input logic
circuitry, it’s a simple yet robust solution for boosting
the output of a servo controller IC. The MC33887DH allows
you to select motors requiring up to 6 A steady state
at up to 30 V. This
means motors up to 180 W may be utilized, thus providing
both high torque and speed via low-ratio gear trains.
