07 Position control of feed drives

Question 1

Formula Speed of electric motor

Answer 1

Frequency of current (e.g. 50Hz)/number of pole pairs

• Number of pole pairs is fixed
• Stator frequency needs to be adaptable to be able to set different speeds

Question 2

Three limits of electric motors regarding torque and rpm

Answer 2

Maximum torque
-> Highest possible torque you can use without damaging the motor shaft

Maximum voltage
-> Results from the maximum feeding voltage of the motor, which has to compensate the counter voltage in the stator/armature winding induced by the rotation

Maximum current is also depending on rotational speed, due to the dependence of the counter voltage on the rotational speed

Maximum rotational speed -> Maximum speed that does not damage the bearing or the rotor of the motor.

MUST NOT BE EXCEEDED OR DAMAGE WILL OCCUR

Question 3

Controlling a synchronous motor

Answer 3

• 3 phases required due to AC-Current
• Diodes and transistors left of the condensator build up a rectifier that generates direct voltage -> three pulse width modulations are fed, which head up to the single motor windings
• Pulse width modulation decreases the dynamics of the drive because an adjustment of voltage can only be done during a fixed time period
• Voltage cannot be adjusted continuously from zero state, because of the limited switching speed of the transistors -> Minimal pulse width that cannot be undercut

Question 4

Common feed axis set-up (Elements)

Answer 4

• Control system
• Mechanics
• Measuring system

Question 5

Feed Axis Set-Up: Control System

Answer 5

Interpolates the target values of NC-programs and compares them to measured actual values -> Manipulated variables are determined out of the difference between control variable and measured actual values

Question 6

Common Feed Axis Set-Up: Mechanics

Answer 6

Links the motor with the tool/workpiece

Question 7

Common Feed Axis Set-Up: Measuring System

Answer 7

Determines the actual position, velocity of the different axes and actual current values of the motor and transfers them to the control system -> Closes the control loop of the feed axis

Question 8

Tasks of the feed generation

Answer 8

• Quick and precise positioning without overshoot
• Compensation of Disturbances
• Interaction of multiple axes

–> Quick and precise generation of the required workpiece shape

Question 9

Closed-Loop control technology
–> Difference Control Chain and Loop
–> Controller Deviation
–> Location of control loop

Answer 9

• Difference between a control chain and a control loop is the return of measured actual values
• Controller deviation is determined from the difference between target and actual values -> From the deviation the controller calculates the measured variables, which are transferred to the performance electronics or subordinate control loops
• Control loop is usually integrated in the Numerical Control Unit (NCU)

Question 10

Closed-Loop control technology: Objective of the Control Loop

Answer 10

Keep Output signal at all times closely corresponding to the input signal
o Cannot always be met -> Contour deviations between desired geometry and workpiece

Question 11

Control-Loop Elements: P-System

Answer 11

System with proportional behavior

o Output signal is always proportional to the input signal.
o No frequency-dependent amplitudes and phase differences
o Classic examples are electrical resistors, mechanical springs, lever or gear ratios

Question 12

Control-Loop Elements: I-System

Answer 12

Output value corresponds to the time integral of the input variable

o At a constant input signal the output signal corresponds to a ramp with a constant gradient, corresponding to the height of the input signal
o Classic examples are capacitors, forcing a damper or the filling of a water tank

Question 13

Control-Loop Elements: PT2-System
(Characteristic Elements)

Answer 13

Characteristic elements of a PT2 element are the cut-off frequency, Eigenfrequency, damping factor and amplification

Question 14

Amplification (PT2-System)

Answer 14

Amplification indicates how much the system amplifies the input signal at subsided oscillation

Question 15

Eigenfrequency (PT2-System)

Answer 15

Eigenfrequency corresponds to the frequency at which the system swings after single excitation

Question 16

Cut-Off Frequency (PT2-Sys
stem)

Answer 16

Cut-Off frequency indicates the frequency at which a phase shift of 90° happens between the input and output signals

Question 17

Damping Factor (PT2-System)

Answer 17

Damping factor indicates how much the system is damped and if the system is able to oscillate

• Oscillatory system has complex poles, which is satisfied when the damping factor is below 1 (D<1)

Question 18

Stability of a PT2-System

Answer 18

 Stability in the control engineering sense means that an externally excited system response with decaying amplitude after the excitation ends
 An unstable system responds to an input signal with an oscillating and rising output signal

Question 19

Stability of a PT2-System: Linear Systems

Answer 19

• Stable control circle is stable for any input signal
• Unstable control circle is unstable for each input signal

Question 20

Stability and Poles

Answer 20

Stability depends only on the poles of the system
* All poles have negative real parts: Control loop is stable
* Complex poles: PT2-System oscillates

Question 21

Components of a Control-Loop of a single machine axis

Answer 21

o Controller: Must be designed according to the section
 Influences the section behavior through appropriately manipulated variables

o Section: Must be well known for designing the controller

o Measuring System: Measurement of motor current, slider speed and position of the slider
 Returns speed and position values to the drive electronics to close the loop

Question 22

Accuracy and synchronous control of multi-axis systems:
Velocity Amplification

Answer 22

KV-Factor: Relation between the actual velocity to the position deviation (tracking error) xW in the steady state -> Characterizes the possible dynamics of the feed drive

 Parameter to identify the exactness of the machine when moving graphs in 2D or 3D
 Typical Values: KV = 0,6 … 4,8 m/min*mm
 Comparable function between systems: The bigger KV can become without producing overshoots the quicker the system.

Question 23

Accuracy and synchronous control of multi-axis systems: Circular Shape Test

Answer 23

o Allows a simple to implement, meaningful assessment of geometric and drive technical accuracy
o For machines with linear axes
o Deviations from set shape result from static friction in the mechanical axis system and the limiting KV factor causing a tracking error
 KV-Factor of all axes must be the same to create a circle  Limitation is given by the slowest axis

Question 24

Simulation of closed-loop systems: Capabilities

Answer 24

• Evaluation of non-linear influences
• Safe observation of the experiment
• Determination of the machine behavior without the actual machine
• Evaluation of parameters which might lead to collision
• Evaluation of potential modifications
• High availability
• Transfer of experience between simulations and reality

Question 25

Simulation of closed-loop systems: Limitations

Answer 25

• Requires precise model of the actual system
• Only qualitative results
• Only those effects can be considered which are included in the underlying model