jittery DC motor cause

One or more reasons, check:

Brushes and spring.

Connections including on battery side.

Armature for deposits or worn out or shorts.

Bearings.

Like Hendrick says...but also check for stiff spots in the drive train...does the motor jerk when it's dissconnected from the drive train? Have you substituted a different motor? Maybe add a flywheel if the load is very uneven?

Give the brishes/commutator a good squirt of some suitable stuff...Have you got supression across the brushes to minimise wear....?

Add a 1ohm (or other suitable low value) resistor in series with the supply and put a 'scope across it, this may well show up what's happening.

Del

Also check the voltage across the motor terminals. Use an oscilloscope if you have one handy.

as in previous post i would check the load and make sure that your mtr is not oversized.

If you are using a drive control check it. IRcomp, gain settings, etc can do strange things if set a bit off. I have also found that many Minarik dc drives seem to be full of cold solder joints.

Hendrik is 100% correct.

Hendrik is correct, unless Switchman is. The others aren't necessarily wrong.

Brushes, springs, commutator eccentricity.

Is the motor running at full rated speed or could it be going slow enough to find the dead spots between the commutators to pause for the brief periods. Gearing down the drive might help. Also between these dead spots the chain gear could backlash. A consent motor drive circuit could help, or a stepper motor. Not sure what ya doing tho.

What is the load being driven by the motor? If it is anything that pulsates like a reciprocating compressor then you might need to increase the HP of the motor.

Also, if the drive or driven sprockets are worn, they can also cause the problem.

Has it always had the hesitation or is it something that has recenly started to ocur?

Some 12VDC motor/controller combinations behave like this. It's the relationship between the pulse rate coming from the controller and the programmed terms inside it. One particular combination that is like this is the ECM Compspeed CA and the Portescap 1219 motor-gearbox. There simply isn't enough rotational inertia in the system in order for it to behave itself properly.

One cheap-and-nasty solution is to add a resistor in parallel across the motor terminals, thereby slugging it. A 75Ω 2.5W-rated resistor works fine on 12VDC with this combination, so consider 300Ω 2.5W-rated on 24VDC, and see what happens.

Another solution is to add a flywheel (usually with the CA/1219 there isn't enough room to do this without voiding the motor manufacturer's warranty).

If you are trying to move slowly it could be one of several things

Stiction - the static friction is bigger than the dynamic friction and so the motor moves until it catches up to the commanded position, then the commanded ouput drops and it sticks

cogging - the dead spots between phases. you dont notice when things are going fast but do when it is going slow

Slot packing - constantly running slow will sometimes allow dust from the brushes to build up in the commutator slots.

this is on top of the problems mentioned by others already. I would disconnect the motor from the controller and run it from a lab power supply if you can. if the problem goes away you know it is the controller. If it persists then disconnect the load and try running the motor at a very low voltage to see if the flaw is the motor. If the motor runs smoothly once it starts, then look at the load and the drive. usually you can figure out if the glitch corresponds to the once around frequency of one of the components. If the encoder has an index pulse you can use that with a recording scope to nail it pretty quickly if you know all the gear ratios in the system. My recollection is that the micro-mo coreless motors have little cogging and little inertia. the low inertia means that the servo needs a fairly fast response, or the load needs to be a reasonable inertia, in order for the servo time constant to be faster than the motor/load mechanical time constant.

If you are going to run slow, then don't use carbon brushes. I think the precious metal ones may be better, but ask your motor vendor