09 Signal Processing, Condition and Process Monitoring Flashcards
Signal Sources
o Internal Data (e.g. motor current)
o External sensors mounted on machine parts (e.g. acceleration/force sensors, strain gages)
goal of process control
optimization of the process guidance
Control Loops - Basics
Work pieces are being measured periodically for the systematic process control and systematical errors by wear and thermal drift of the machine are being compensated.
Machines settings are chosen on the “safe side” due to unpredictable tool and material deviations Does not lead to a full exploit of the machine performance
-> Improvement can be achieved by the use of a machine intern control loop
-> Processing quality and safety can be improved by the collection of disturbing influences and feedback By closing the open loop control with a process controller
Quality Assurance Tasks
o Can be performed directly or indirectly -> Machine and monitoring parameter are evaluated locally and immediately, directly or indirectly by their effects on measureable variables
o Time grid for quality assuring tasks has to be created, depending on the product and process quality and quality assuring apparatus and maintained during the manufacturing process
Reasons for collisions
o Programmatic errors -> Wrong paths; wrong programmed tool
o Control errors -> Error in measurement system; error in workpiece; read error
o User errors -> Wrong movement commands/program used; set-up error; wrong tool
o Machine errors -> Tool change error; Positioning error
o Work piece errors -> Wrong over measure
Collision Detection Strategies
o Without sensors (calculation of geometry)
o With sensors
Collision Area
Reduction of the number of points to be considered for a possible collision -> Reduction of necessary computing power
o One point of the tool (collision point) should have a prescribed movement path (e.g. tool tip)
o Parts of the working space that can be involved in a collision are artificially enlarged in such a way that moving the collision point outside this artificially enlarged surface does not lead to a collision with any point of the moving machine part.
Look Ahead Function (Collision Avoidance)
Checks if next NC line is within boundaries
Application of Sensors in Process- and Condition Monitoring
o It is important, that the sensors do not affect the characteristics of the machine or that this influence is insignificant (e.g. the dynamometers influence on the stiffness)
o Machine internal signals (current of drives, positions etc.) could be used in addition to external sensors
Mounting Options for structure borne sound sensor
o The resonance frequency limits the sensors range of application -> Should only be used in the range, where it has a linear frequency response
o Screw and wax connections are to be preferred for measuring high-frequency vibrations
Fourier Transformation
o (Almost) each periodical function can be described as a sum of sine and cosine terms
o Transforms a signal s(t) from the time domain into the frequency domain S(f)
o In the frequency domain, it is easier to determine and identify (unwanted) oscillations
o The frequency often gives a hint towards its origin
What Filter do you add if you only want to analyze low frequencies
Low Pass filter
Digitalization of Signals
o Either first quantization and then sampling or the other way around
o Data must be converted into a binary system, in order to digitally transmit and store the values after sampling and quantization
o Then, the binary data can be transferred from the physical layer via technologies such as ethernet or radio technologies
Quantization
o Quantization transforms value continuous signals in value discrete ones
o The resolution determines, how exact the original signal values can be described
o A resolution of 3 Bit yields in a range of 2^3 = 8 values
o With 16 Bit there are 2^16 = 65.536 possible values
Quantization - Target
The best possible utilization of the range of values
