Westinghouse Modules 1C-,5X- Emerson VE-,KJ-
Honeywell TC-,TK- Fanuc motor A0-
Rosemount transmitter 3051- Yokogawa transmitter EJA-
Do I Run a Hard Servo or a Soft Servo?
High gain servos have come to be known as hard and low gain servos
as soft. The higher the gain (A), the
greater the bandwidth (the frequency when A = 1). If one tries to
raise the bandwidth too much, the
natural frequencies of the machine come into play and eventually
instability occurs causing the machine
to oscillate indefinitely. In addition, as one increases the
bandwidth, the servo tries to force the machine
more and more to follow the command. A step input in the command
will force the machine to jump.
Over time, a machine could be beat to death by an oversized drive
system that is forever jerking it back
and forth in response to commands. The practical approach is to
vary the gain and test the machine's
response to a step in command. That response can be viewed on an
oscilloscope for number of overshoots
or one can rely on gut feel based on how the machine looks when the
step occurs. For heavy industry, 1
IPM/MIL (inches per minute per mil of error) is a typical gain. It
is unusual for a piece of industrial
machinery to get above 5 IPM/MIL.
Phases, Poles and Stepping Angles
Usually stepper motors have two phases, but three- and five-phase
motors also exist.
A bipolar motor with two phases has one winding/phase and a
unipolar motor has one winding, with a center tap per phase.
Sometimes the unipolar stepper motor is referred to as a “fourphase
motor”, even though it only has two phases.
Motors that have two separate windings per phase also exist—these
can be driven in either bipolar or
unipolar mode.
A pole can be defined as one of the regions in a magnetized body
where the magnetic flux density is concentrated.
Both the rotor and the stator of a step motor have poles.
Figure 2 contains a simplified picture of a two-phase stepper motor
having 2 poles (or 1 pole pairs) for each phase on the stator, and
2 poles (one pole pair) on the rotor. In reality several more poles
are added to both the rotor and stator structure in order
toincrease the number of steps per revolution of the motor, or in
other
words to provide a smaller basic (full step) stepping angle. The
permanent magnet stepper motor contains an equal number of rotor
and stator pole pairs. Typically the PM motor has 12 pole pairs.
The stator has 12 pole pairs per phase. The hybrid type stepper
motor has a rotor with teeth. The rotor is split into two parts,
separated by a permanant magnet—making half of the teeth south
poles and half north poles.The number of pole pairs is equal
to the number of teeth on one of the rotor halves. The stator of a
hybrid motor also has teeth to build up a higher number of
equivalent poles (smaller pole pitch, number of equivalent poles =
360/teeth pitch) compared to the main poles, on which the winding
coils are wound. Usually 4 main poles are used for 3.6 hybrids and
8 for 1.8- and 0.9-degree types
It is the relationship between the number of rotor poles and the
equivalent stator poles, and the number the
number of phases that determines the full-step angle of a stepper
motor. Step angle=360 ÷ (NPh × Ph)=360/N
NPh = Number of equivalent poles per
phase = number of rotor poles
Ph = Number of phases
N = Total number of poles for all phases together If the rotor and
stator tooth pitch is unequal, a more-complicated relationship
exists
