07 Position control of feed drives Flashcards
Formula Speed of electric motor
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
Three limits of electric motors regarding torque and rpm
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
Controlling a synchronous motor
- 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
Common feed axis set-up (Elements)
- Control system
- Mechanics
- Measuring system
Feed Axis Set-Up: Control System
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
Common Feed Axis Set-Up: Mechanics
Links the motor with the tool/workpiece
Common Feed Axis Set-Up: Measuring System
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
Tasks of the feed generation
- Quick and precise positioning without overshoot
- Compensation of Disturbances
- Interaction of multiple axes
–> Quick and precise generation of the required workpiece shape
Closed-Loop control technology
–> Difference Control Chain and Loop
–> Controller Deviation
–> Location of control loop
- 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)
Closed-Loop control technology: Objective of the Control Loop
Keep Output signal at all times closely corresponding to the input signal
o Cannot always be met -> Contour deviations between desired geometry and workpiece
Control-Loop Elements: P-System
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
Control-Loop Elements: I-System
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
Control-Loop Elements: PT2-System
(Characteristic Elements)
Characteristic elements of a PT2 element are the cut-off frequency, Eigenfrequency, damping factor and amplification
Amplification (PT2-System)
Amplification indicates how much the system amplifies the input signal at subsided oscillation
Eigenfrequency (PT2-System)
Eigenfrequency corresponds to the frequency at which the system swings after single excitation
Cut-Off Frequency (PT2-Sys
stem)
Cut-Off frequency indicates the frequency at which a phase shift of 90° happens between the input and output signals
Damping Factor (PT2-System)
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)
Stability of a PT2-System
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
Stability of a PT2-System: Linear Systems
- Stable control circle is stable for any input signal
- Unstable control circle is unstable for each input signal
Stability and Poles
Stability depends only on the poles of the system
* All poles have negative real parts: Control loop is stable
* Complex poles: PT2-System oscillates
Components of a Control-Loop of a single machine axis
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
Accuracy and synchronous control of multi-axis systems:
Velocity Amplification
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.
Accuracy and synchronous control of multi-axis systems: Circular Shape Test
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
Simulation of closed-loop systems: Capabilities
- 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
Simulation of closed-loop systems: Limitations
- Requires precise model of the actual system
- Only qualitative results
- Only those effects can be considered which are included in the underlying model
