PPR of encoder for torque control at low speed

We have an application where we use a motor with encoder to provide torque at low speeds (+- 5 Hz). The drive is Sinamics G120 in closed loop control. The drive has an encoder with 1024 ppr. Because the type of application the velocity of the motor is not that constant (although it should be).
Could there be an improvement in raising the line count of the encoder, or is 1024 enough for good torque regulation at speeds of +- 5 Hz?
Encoder is mounted directly on the motor shaft.

You need to consult the manufacturer's manual on this.
Your statement about inconsistent velocity is confusing...is this a torque control, or a velocity control? It almost sounds as if you are attempting torque regulation with a motor below 5HZ! If that is the case, you need a gearbox. You are operating the motor way below its operating range, IMHO.

Plc_User said:

We have an application where we use a motor with encoder to provide torque at low speeds (+- 5 Hz). The drive is Sinamics G120 in closed loop control. The drive has an encoder with 1024 ppr. Because the type of application the velocity of the motor is not that constant (although it should be).
Could there be an improvement in raising the line count of the encoder, or is 1024 enough for good torque regulation at speeds of +- 5 Hz?
Encoder is mounted directly on the motor shaft.

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If the drive is running in torque control mode, then the velocity will be erratic if the torque is uneven. You'll need to describe the application better to offer much more than that.

The application is an accumulator for textile (synthetic grass). The thing that buffers the textile coming out of a latex oven before it is rolled up. It consists of a frame with rolls going up if the roller is stopped and the oven is running, and going down if the roller runs faster then the oven. The textile comes in and goes out at the bottom of the accumulator.
A motor is used to push the frame up to put tension on the textile (otherwise the frame would never go up, and it has go up accumulate the textile). If difference in speed between the oven and the roller is small, the frame goes up or down very slowly (sometimes only 3-5 Hz). Using another gearbox would limit the maximum fill and empty speed too much. Best performance was achieved so far with closed loop speed control but limiting the maximum torque. Using the pure torque control gave worse results. The drive is Siemens Sinamics G120. I know < 5Hz is very critcal for sensorless vector control, but in this case it is closed loop vector control (1024 ppr).

I would limit the maximum tension not the torque

Torque you measured is the sum of a lot of things that has nothing to do with the tension. Sure there is a tension component but there is also a friction, load and acceleration component. It is no surprise to me that the pure torque control did not work.

This system doesn't appear to be very dynamic and the velocity control doesn't need to be precise. I would have used velocity or tension limit control. That is the motor ramps up till either the max velocity or tension which ever occurs first. This is usually done using a low select.

I would think that torque control is the correct mode on this type of accumulator. I've seen two or three like this in my day and all were in torque control.

With an encoder and the drive operating in vector mode, you should be able to get smooth torque application right down to stall. I suspect that whoever set up the drive parameters did not understand how to get the system to stabilize properly.

As to Peter's comments about the tension being hidden in a bunch of other torque related elements, certainly he is correct but, with a heavy web as the OP describes, I would expect the tension component to far dominate the other nuisance components and reasonably good tension control should be acheivable.

Usually at about this point, I make some comment along the lines of "you need a good drive to get this job done" but, in this case, the Siemens drive is excellent and should not be the problem.

DickDV said:

As to Peter's comments about the tension being hidden in a bunch of other torque related elements, certainly he is correct but, with a heavy web as the OP describes, I would expect the tension component to far dominate the other nuisance components and reasonably good tension control should be acheivable.

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If the web is heavy the amount of torque required just to hold it up will change as a function of the elevation due to the increased weight even if the tension remains the same. A lot depends on the weight of the material compared to the weight of the mechanics that are lifting the web.

We have used torque or velocity limit in the past for grinding and web applications similar to this. The advantage this mode has is that if the load suddenly changes the motor will not spin out of control due to the velocity limit and if the motor stalls it limits the torque so that the nothing gets broken.

I think that the application is not that easy as it may seem. As Peter said there are a lot of influencing factors. Maybe most important is a lot of friction to get the synthetic grass with its thick latexbacking pass across the eight rols in the frame. Also this was the first accumulator that went up to fill, most others fill by going down, which may be a complete different physical behaviour.
As said the drive was Siemens G120, if it had been a S120 we could have done some tracing on the current loop etc. Next month we will commision a simular accumulator with a Control Techniques Unidrive. If the quality of the synthetic grass is the same we can compare the behaviour.