Servo indexing conveyor question

[AcuBill]

Hi

I have an application where we are updating an indexing conveyor. I am using a timing style belt with 1/2" pitch teeth and it indexes 8" at a time.

We will be using a Balodr servo motor and reducer existing is 10:1. The drive pulley should be around 3" in diameter.

How do I determine the number of teeth required on the head pulley and the ratio for the reducer? Somebody metioned that if they are not in relation to the servo resolution it would accumulate positon errors.

I am not very familiar with servos and not sure how to procced.

Thanks

[Al_The_Man]

With timing belts the reduction is the ratio of the pulley teeth as it is in gear reduction.
With the majority of CNC controllers, you can enter a value that equals the encoder counts per Inch/mm of travel, so it normally does not have to be a round number.
Al.

[One of Many]

If you do not have access to the controller program or parameters that runs the servo duration, then you might just hook the motor up and see how many revolutions it is currently programmed for.

What is unclear is whether the ratio needed is between the motor and gearbox or gearbox and conveyor. May or may not matter, but the following should still apply in a similar vein.

Although if there were something you could tap into the encoder to get counts, it would be more accurate if the encoders counts per revolution were known.

You will also need to find the revs at the input of the gear reduction to the conveyor belts linear motion of 8"

Once you have that info, then the ratios will be a product of the motor revs to the input of the gear reduction revs.

Example: If you found 200.4 revolutions of the servo motor "on time", and 8" of conveyor belt movement required 100.2 revs of the gear box input shaft. That would be a 2:1 timing belt pulley ratio. The gearbox input shaft pulley will have twice as many teeth as the servo pulley.

Chances are you will find it a standard ratio. The pulleys that came off the original assembled unit may even give you the answer to purchase your pulleys accordingly.

If you are starting over with a new motor and controller as well, then the ratio is going to be important in obtaining enough torque capacity to move the works and adjust the parameter that meets the encoder counts as required to get 8" of belt travel without over loading the servo drive. Since power is work done over time, you might also need to have ramping on the accelleration and deaccelleration to start and stop the converyor belt accurately within the load limits of the driving system. Programming manual would have this information.

DC

[AcuBill]

Hi

Thanks for your help so far.

The existing belt conveyor will be similar but not the same as the new one so I can't "re-use" everything. The head pulley number of teeth and reducer ratio is where I am having the problem. When I say timing belt I am refering to the actual belt on the conveyor. (So that there will be no slippage.) The reducer will direct drive to the headshaft.

The motor I am using is a Baldor #BSM80N-250AA and a Baldor #FPH4A15TR-RN23 drive.

Thanks

[Al_The_Man]

When you say drive pulley, do you mean Driven pulley? i.e. the one driving the belt, if you already have the servo motor and gearbox reducer at 10:1, then the output shaft of the reducer is obviously motor rpm/10.
If the driven pulley is a timing belt type and mates with the 1/2" pitch of the belt, the amount of linear travel per 1 rev of the pulley can be obtained by the number of teeth on the driven pulley, or the diametric pitch.
If the pulses/rev of the encoder is known, then from the above, the amount of pulses/" of travel can be entered into the controller parameters.
Or a picture of what you have would help.
Al.

[One of Many]

Originally Posted by AcuBill Hi Thanks for your help so far. The existing belt conveyor will be similar but not the same as the new one so I can't "re-use" everything. The head pulley number of teeth and reducer ratio is where I am having the problem. When I say timing belt I am refering to the actual belt on the conveyor. (So that there will be no slippage.) The reducer will direct drive to the headshaft. The motor I am using is a Baldor #BSM80N-250AA and a Baldor #FPH4A15TR-RN23 drive. Thanks

So, I take it the head pulley is driving the conveyor belt, which is mounted on the head shaft and driven by the reducer?

The motor is direct coupled to the reducer or is there another pulley ratio there? Is the encoder mounted on the Servo Motor or somewhere else?

Is the original controller sending an analog signal to this new servo drive and monitoring the encoder without feedback to the drive itself?

If not and this is a stand alone drive, you just trigger it to run. The Flex Drive with its own internal looped program that is monitoring the encoder counts from an encoder mounted to the motor will track very well. Who ever is going to set up the program for the servo drive can program that count for whatever head pulley you choose. 8" travel is what you want on the surface of the belt, not the pitch diameter or based on the number of teeth you have. The teeth are just there to keep it from slipping. There is nothing to accumulate error with it all coupled mechanically and tracked electronically. What ever the encoder pulses per inch of belt travel end up at, are locked in that as well. If the headshaft pulley were running several revolutions to move the belt 8", then you could see some accumulation, but I think it is self canceling where encoder tracking can compensate for some minor loss or gain for reliable long run distances.

You can try to look at it as degrees of rotation of the head shaft, which will relate to arc length based on the circumference of the belt riding on head pulley. Certainly this will translate from rotary to linear motion. But, technically, as a stand alone controller, the pulley tooth count wouldn't be critical if you still need to program the drive to monitor xxxxx counts from the encoder feedback equal to 8" of belt travel. I suppose that depends on your encoder resolution, but that is also at 10:1. I'd suspect that will be well within the start/stop tolerance of the belt. The pitch of the belt teeth may not even be that accurate over the entire length of the conveyor belt.

If the head pulley were 3" diameter and the belt running on it we another 1/8" thick, you could reference that at 3.25" diameter or roughly 10.210" circumference for one full revolution of the head pulley to belt travel. We only need a portion of one rev to move 8". Now if the encoder were on the motor and xxx lines/rev internally counted by the controller x1 or by x2 or x4. One could guestimate that 7.87 turns of the motor or 7.87 x the encoder ppr, would move the headshaft and belt .787 of of one full rev of 10.21" (through the reducer), which should be equal to 8" of belt travel. Actuals will vary so some fine tuning of the true count over 8" will be required, but reasonably repeatable if you can get the start/stop and ramping under control.

DC