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4.1 Off-the-self Amplifier vs. IC solution
PMD’s products are used across a wide variety of motion control
applications. As of yet PMD only offers the “smarts” behind
the control and the customer is responsible for selecting the components
necessary for driving their motor with a PMD control solution. In the
simplest terms this device will act as a signal amplifier. The current
needed to create even a minimal torque in the motor is far beyond what
can be produced by the PMD device. The motor signal from the PMD devices
needs to be amplified and in many cases further signal conditioning
will be done by the amplifier/driver.
The details of connecting these devices to the PMD chip and to the motor
will be addressed in the next chapter. For now the various packaging
options available to the designer will be discussed.
Some devices that perform the task described above come in a prepackaged
enclosure as shown in Figure 4.1. The device shown contains screw terminals
for making connections to the PMD device, the motor, and to the external
power supply. Most of the time one such device will be required for
each motor/axis being controlled. The smallest enclosures start as small
as 5”x 3” x 1” and get much larger depending on current/voltage
requirements and functionality.
At the opposite end of the spectrum is the amplifier/driver Integrated
Circuit (IC). An IC package will provide the same basic functionality
as the “box-type” amplifier. The IC solution provides many
benefits that the designer may find valuable. The two biggest advantages
to the IC solution are cost and size. In almost all cases the IC solution
will be far less expensive in terms of parts cost and for obvious reasons
the IC solution will occupy much less space. Many times a single IC
can handle multiple motors (in low current/torque application).
The two biggest reasons why a system designer may shy away from an IC
solution pertain to the increase in complexity. Use of an IC implies
that a circuit board will be created on which the IC will reside along
will several other devices. The IC solution requires time and at least
a minimal amount of experience in regards to designing and fabricating
a circuit board. Time may not be a resourceavailable to the system designer.
Another disadvantage is that the cost of both a circuit board design
(labor) and the actual fabrication of the board may offset any initial
cost advantage. In applications that involve high motor currents, the
IC solution may require a main IC to handle the
switch timing logic and additional devices, MOSFETs or IGBTs, will be
required to do the actual switching. This allows the designer to use
the same IC driver and select a MOSFET (or IGBT for very high currents)
to exactly meet his current requirements. PMD offers a brushless DC
motor control IC with a built-in current loop. The MC73110 is not detailed
in this document but should be considered an option for high-end brushless
DC motor control.
4.2 What is Current Control?
Generating motion with an electric motor requires driving a current
to the motor that will produce torque. When the torque produced by the
motor overcomes frictional and inertia loads, angular displacement of
the motor shaft will occur and the load attached to the shaft will be
in motion. A servo control mechanism is introduced into this system
for the purpose of controlling the torque. The amplifier the designer
chooses to use with the PMD product will determine how directly the
PMD product can control the motor torque. This will ultimately affect
the behavior of the whole control system.
Figure 4.2 depicts a standard DC brushed motor model. The most common
use of the PMD device is for positional control of a load. In the vast
majority of mechanical systems the position of the load is deterministically
related to angular displacement, theta (?), of the motor shaft. When
velocity control of the load is desired then the derivative of theta,
angular velocity, can be controlled. A mechanical equation can be generated
that relates the motor torque (T) to theta.
J is the Moment of Inertia of the load and B is a viscous
damping constant. Modeling the motor/load as a rigid body and then summing
the Newtonian forces acting on the body create the terms used in the
torque equation. The torque produced by the motor (T) is assumed
to be linearly related to the current through the motor windings (i)
by the motor torque constant (Kt). Solving these equations
for torque leads to:
Assuming that J and B remain constant leads to the conclusion that controlling
the current through the motor will control the angular displacement
 .
For this reason current control becomes very important.
Figure 4.2 also relays some other important information
about the electrical characteristics of the motor. The motor model has
an electrical resistance (R) and an electrical inductance (L). These
parameters will be used later for creating a voltage equation for describing
the motor behavior.
Depending on the type of amplifier/driver selected either
a voltage source or a current source will be applied to the terminals
of the motor. The applications of the two sources to the motor terminals
are seen in Figure 4.3. The PID filter from the PMD device is shown
outputting a voltage. This voltage becomes the input to an amplifier/driver
device. The input terminals of the amplifier are high impedance and
therefore draw a minimal amount of current from the PMD device. What
the amplifier does with this voltage input depends on the type of amplifier
selected.
In the case of voltage control, the amplifier creates
a voltage source (V’) at the motor terminals that is linearly
related to the input voltage by the amplifier constant (Kv). In order
to model the torque generated in the motor an equation is needed that
relates the terminal voltage (V’) to the current through the motor.
Based on the upper diagram in Figure 4.3 a voltage equation can be generated.
This equation introduces another constant that has not
been discussed yet. Because a motor can also behave as a generator,
the rotation of the motor will produce what is commonly referred to
as “back EMF”. This is modeled as a voltage that opposes
the voltage at the terminals which is linearly related to the angular
velocity of the motor by the constant Kb.
In the case of current control, the amplifier creates a current source
(I) at the motor terminals. This time it is the current source that
is linearly related to the input voltage by the amplifier constant (Ki).
The existence of a current source (as opposed to a voltage source) at
the motor terminal greatly simplifies determining the current through
the motor. Because there is only one path for the current, the current
through the motor will be equal to the current being supplied by the
current source. So the torque produced by the motor can be expressed
as the value of the current source multiplied by the torque constant
(Kt).
Using an amplifier for current control is also referred
to as “Torque Mode” because the motor torque is linearly
proportional to the input to the amplifier (which is also the output
of the PID filter). The relationship between current and voltage is
not linear because of the existence of inductance and back EMF. A specific
voltage value at the motor terminal does not guarantee a specific current
and therefore does not guarantee a specific torque. This is unlike a
current control system where the motor torque is always proportional
to amplifier input signal. |
