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Why is my Trilogy Ripped Ironcore Linear Servo Motor so smooth?

Ironcore linear motors have traditionally suffered from a phenomenon known as cogging. This is seen as a periodically varying resistive magnetic force when the motor is pushed by hand along the magnet track.  Cogging is caused by the motor coil having preferred positions in relation to the magnet track, and resists attempts to move it off of these preferred positions. Cogging limits the smoothness of motion systems using ironcore motors because the force generated by the motor must change with position in order to maintain a constant velocity.  Ironless linear motors do not have these preferred positions and therefore can achieve very smooth motion.

 

Parker-Trilogy has developed Anti-Cog technology that virtually eliminates cogging and enables ironcore motors to be used in applications where only ironless motors would have been considered before.  Since ironcore motors use half the magnets of equivalent ironless motors, this helps lower the price barriers traditionally seen with linear motor solutions.

 

Anti-Cog technology eliminates cogging without skewing the magnets or laminations or otherwise compromising motor efficiency.  Parker Trilogy uses two methods to nullify cogging.  For one, the motors are designed using a fractional winding technique, in which the teeth of the motor stator do not correspond equally with the magnet track magnets.  The result is the horizontal component of net magnetic force from the inner teeth will be lower.  Many of them will cancel each other out however, the cogging forces from the leading and trailing edges of the stator still remain.  This is where the second technique for eliminating the edge effects comes into play.  Parker-Trilogy has developed Anti-Cog blocks (Patent Pending) which create a cogging pattern equal and opposite the cogging pattern of the leading and trailing edges of the lamination assembly.  Each Ripped Ironcore Linear Motor has two Anti-Cog blocks of which produce an opposite and equal cogging pattern, effectively using destructive interference to negate undesirable cogging at the ends.

 

Below are Force vs. Position Plots which demonstrate the Anti-Cog block‚Äôs ability to cancel cogging forces.  Figure 1 shows an R10-1 without cogging blocks incorporated into the motor assembly, whereas, Figure 2 shows an R10-1 with cogging blocks incorporated.  Both plots were created with a linear motor moving speed of 2.5mm/s and about 35N of force is due to mechanical friction, mostly from the bearings.  Notice the the periodic waveform with 30mm pitch in Figure 1, and the absence of it in Figure 2.

 

 

           Figure 1: Plot of Typical Cogging Force vs. Position

Figure 2: Plot of Cogging Force vs. Position with Anti-Cog Technology

Note - Jack Marsh's article "Anti-Cog Technology Application Note" is heavily referenced in this FAQ.  Please contact the Application Dept. for a copy of the article.

JE 11/27/12