Page 123 - Electric Drives and Electromechanical Systems

116 Electric Drives and Electromechanical Systems

aware of errors that can be introduced by the process or system. While in general these
are small, they still can have a significant impact on a machining centres or robots
performance. One of the major errors is caused by temperature rises either caused by the
machining process itself or from the motors driving the actuators as discussed by Mayr
et al. (2012). This research notes that a temperature increase in a ballscrew of 10 C can

lead to positional errors of up to 100 mm. While thermal effects can have significant
impact of the performance of a system, other errors can be introduced though a range of
other factors including (Schwenke et al., 2008):
Kinematic errors: resulting from incorrect setting up of the components for
machining and distortion of the machining centre’s structure.
Mechanical loads: the changes in the mechanical load on the structure as the ma-
chine tool moves can result in changes in the squareness of the machine tool. In
certain robot applications the impact of gravitational forces (particularly robotics
arms) needs to be considers.
Dynamic forces and vibration: As the structure moves, forces are transmitted to
the structure can lead to vibration that is difficult to compensate. In addition,
vibrations can be generated by the motor drives and its power train, in particular
motor torque ripple and problems with gear boxes.
Motion control software: As with any digital control system, the possibility of
errors due to sampling and rounding is always present. These can be minimised by
having a clear understanding of the control algorithms and how to avoid program-
ming errors and related problems.

4.2 Rotating velocity transducers

While the velocity can be determined from position measurement, a number of trans-
ducers can be used to provide a dedicated output which is proportional to the velocity.

4.2.1 Brushed d.c. tachogenerators
A brushed d.c. tachogenerator can be considered to be a precision d.c. generator, consisting
of a permanent-magnet stator, with a wound armature. The output voltage, E g ,isrelated to
1
the tachogenerator speed, N (rev min ), by the voltage constant, K g (V rev 1 min),
E g ¼ K g N (4.9)

In a tachogenerator with a conventional iron-copper armature, a ripple voltage will be
superimposed on the d.c. output because of the relatively low number of commutator
segments; the frequency and the magnitude of this ripple voltage will be dependent on
the number of poles, armature segments, and brushes. A ripple-voltage component with
a peak-to-peak value of five to six per cent of the output voltage is typical for brushed
tachogenerators. The ripple voltage can be reduced, by the use of a moving-coil
configuration which has a high number of coils per pole, this minimises the ripple
voltage to around two to three percent. The armature consists of a cylindrical, hollow