Inertial DampersUsing inertial dampers to reduce resonanceStepper motors are affected by resonance. The resonance can become so severe that the stepper will stall and lose position; stalling can also damage the stepper.Resonance is caused by parts vibrating in concert with each other. It can be lessened by altering a variety of factors including micro-stepping, acceleration rates, speed, looser or tighter mounting hardware, thicker or thinner stepper mounts, longer or shorter mounting bolts, and a myriad of other variables. Resonance sounds like a grinding or rattling noise; this is usually followed by a high pitched whine as the stepper stalls while still receiving step signals. ..............
A damper attached to the stepper's shaft can reduce resonance. It is essentially a wheel that turns with the stepper; however, it is not simply a flywheel. The damper reduces resonance by remaining slightly out of sync with the stepper. The out of sync vibrations help to counter the stepper's harmonics, which otherwise would crescendo and cause the stepper to stall. ..............
A
simple damper made from a 2-1/2 inch caster works well; it performs
better than one made from 1-3/4 inch caster.The caster has a 5/16 inch bore; an axle is made by drilling a 1/4 inch hole in the head end of a 5/16 inch bolt. This hole accepts the stepper's shaft. A hole is drilled and tapped for a set screw in the side of the head; the set screw is brass colored in the right image. ..............
The hole in the bolt for the stepper shaft can be drilled by placing the bolt in a power drill's chuck, and clamping the drill bit in a vise. Start with a small bit and incrementally work up in bit size. It is easier to center the hole when the smaller bit is used to drill the pilot. A drill press makes this easier, but it can be drilled with a hand held drill and set of Vise Grips holding the bit. ..............
Washers sandwich the caster and the spring that is on the threaded end
of
the bolt. A lock-nut is used to adjust the spring's tension against the caster. The caster does not easily freewheel, but it will turn on the axle bolt. The caster slips so it cannot be used as a knob to turn the stepper. This unit is easy to make and all of the parts are off the shelf from Lowes. The difference in performance is impressive. The cheap 18 turn per inch leadscrews with the Xylotex 425s can approximately double their speeds when the damper is part of the system. The rapids for the Xylotex improved dramatically from an un-damped 25 ipm to a damped 60 ipm. Background and Experiments A variety of different shop built dampers were tried since commercially made dampers are expensive and difficult to find. An option was made of one of the knobs shown on Hold Down page.  Recesses were drilled in the knob's back, and ball bearings were sealed in the holes with a plastic cover. A 1/4 inch aluminum hub from a stray XL pulley was pressed into the knob. There seemed to be no difference whether the bearings were larger or smaller. BB shot was also used in varying quantity and the results were all similar. This unit also worked relatively well, but did not seem to help in a wide range of speeds. It seemed that something more massive was needed. ..............
A thrift store roller-blade wheel was tried; it worked well.
Two notches were cut in the bore to hold a stud that would prevent the wheel from freewheeling, left image. 
The axle is made of 1/4 inch ID and 5/16 inch OD copper pipe. The bearings do not turn completely; they are simply used as flexible bushings to hold the axle in the wheel. A hole is drilled through the copper pipe, and a cut nail is pressed through it. The pipe to nail fit is tight; otherwise the hole in the copper will quickly stretch and the nail will rattle in the hole and allow too much play. The nail fits in the notches that were cut into the plastic wheel. The fit should be relatively tight, a little play is okay, but too much allows the wheel to coast too far, which will cause the stepper to stall on deceleration. 
The damper is held onto the stepper's shaft with a set screw that is tapped into a 3/8 inch nut. A matching hole is drilled in the copper pipe so the set screw tightens against the stepper's shaft rather than against the copper pipe. The unit can be used to turn the stepper when the axis is disabled. However, turning the damper to force-turn a powered stepper will tear the roller-blade wheel's plastic. This unit worked well, but the wheel is not easily found. The caster version shown at the top of the page worked just as well and was made of home center components. |