Lecture 7 Flashcards
(20 cards)
What are stepper motors?
Based on electromagnetic principle and used for converting electrical energy into mechanical energy
How is current controlled through motor coils via PWM?
Drive FETs can control the direction of the current through the coil
By having a current being sent through the coil we can sense the voltage across the Rsense resistor
The sensed voltage can control the PWM and control the amount of current flowing through the coil
What is an indexer?
The indexer controles the percentage of current through each of the stepper motor coild and can therefore create ‘Microstepping’
What does microstepping mean for a design?
Smoother motion
Quieter operation
Less torque
Higher resolution
What are the three different rotary incremental encoders?
Optical
Magnetic
Capacitive
What do all three rotary incremental encoders have in common?
They inform that a step has taken place
They inform about the direction of travel
They inform when you have reached a full rotation (index or ‘Z’ signal)
How does a trasmissive multiple light source encoder works?
Three optical transmitters and receivers, carefully positioned, to produce the quadrature output signal A. B and a reference singnal Z
Note that A and B are shifted by 90 degress relative to each other
How does a trasmissive single light source encoder works?
A single light souce with a precision optical mask to produce the same signals A, B and Z
What is X1 encoding?
A pulse is produced on one edge
What is X2 encoding?
Two edges are produced withing the degree range
How can the resolution of encoders be changed?
Can be changed depending on how you decode the A and B signals in either hardware or software
What is PPR?
Rotary encoders are quoted in PPR (Pulses per Revolution) and they refer to X1 encoding
However, the resolution can be improved by either a factor of 2 or 4 depending on the implementation of the hardware or software
What happens when A leads B?
Defines one direction e.g. clockwise
What happends when B leads A?
Defines the opposite direction e.g. counter clockwise
What does the Z signal show?
Indicates one complete revolution
What is the disadvantage of using rotary increment encoders when used in a linear drive system?
If there is any blacklash in the drive system then it is not detected by rotary encoders and therefore errors occur
What are the advantages of using rotary increment encoders when used in a linear drive system?
Instant response to linear motion
Very high resolution up to 1nm
How do linear incremental encoders work?
They work in the same way as rotary encoders, in regards to the positioning of the optics and signal outputs A, B, and Z
However, the precision graticules (series of lines) are printed on a linear surface and can be designed to be any length requires
The graticule bar is fixed to a static structure
The optical detectors are fitted within the moving structre
Reflective optical linear incremental encoders using ‘grating’ mask
What are absolute encoders?
Work in a similat way to incremental encoders, but rather than having a single track graticule they have multiple tracks with encoded data that defines the absolute position
There are many encoding formats
They look just like incremental encoders
They are expensive, but essential in precision positioning systems
They come in both rotary and linear forms and can be optical, inductive, magnetic, or capacitive
What is Gray code?
A binary sequence where only 1 bit changes at a time
It is important because there is not confusion during position transition
