Help required for VFD Application.

The braking resistor will safely get the energy out of the VFD so you don't fry it on sudden stops but what you may want to explore for your application is a motor brake.

(cont) You'll still need the encoder but the brake will allow you to perform the instant stop based on the encoder's output position. You may need to lead you stop position by a few degrees to compensate for the brake latency; its a mechanical device so it takes a amount of time to function.

If you have a good quality Closed Loop Vector drive, you can input the encoder directly to the drive and set the drive up to do positioning control, almost as accurately as a Servo Drive. But with only 360PPR, you must not be terribly concerned with accuracy. But either way, I would not use the PLC as the positioning controller directly. Put the encoder signal into the drive, retransmit t back to the PLC if you like, but a good Vector Drive can just take the desired pulse cunt you want for travel and use the encoder to get there, without the latency that would be introduced by the PLC.

If you need EXACT positioning, use a Servo (as long as your load is small enough).

A Dynamic Braking Resistor might be necessary if there is a significant amount of inertia in your load and gearbox, but you have nowhere near enough information posted here to know that. However, there is nothing wrong with having a DBR installed just in case, as long as your budget can afford it. It does not hurt the drive to have it attached.

Since no one has asked yet, what's the gear ratio of your reduction gearbox? If it is high enough then the VFD will be able to slow and stop the output shaft with only a moderate amount of shaft rotational overshoot.

Additionally, what size is the motor and what is the output shaft connected to?

It is also possible to short count the pulses to limit overshoot by stopping the VFD early. It's a quick simple solution but it really depends on what accuracy you really need.

What's the application?

We used to do something similar for crop and cobble shear for rod mills, and also rotary shears for pulp cutters. The drive positions for 1 revolution of the motor. Motor accelerates to rated speed and back to zero in one revolution, or else the blade will contact the material again. (for the crop and cobble actually there is some overshoot allowed since the blade is parked almost 3/4 of a revolution ahead of the cut, it makes the cut by accelerating to line speed and then decelerating once clear of the material, and can over travel before being returned to the park position.) We used the Reliance Automax with resolver feedback directly into the Automax controller. Resolver has 8096 counts per revolution for a x2 resolver. For each scan of the program at a fixed scan rate (say 5 msec) you can create the required displacement and velocity profiles for the entire cycle. Take the derivative of the velocity profile x the WR2 and you have the inertia compensation for feedforward compensation to minimize velocity error. If you use a type 2 (double PI) speed regulator you will have zero steady state position error.

It is a fun exercise in math, and you will need to use the Z transforms for your stability calculations, or at least use a Bode plot and seriously oversample.

Many of the new high speed PLC's can handle the computations, and pulse tachs with marker pulses or a home switch can also be used instead of the absolute positioning of a resolver.

If the motor gear combination is always driving in the same direction,

Determ the number of pulses for one output gear shaft turn.

ie 360 pulses x gear reduction

Start the motor at full speed, and simultaneous start a counter in the PLC.

At 90% from the desired number for one output gear shaft turn, slow down the motor to

+/- 10 % of the full speed.

When the counter is "full", just give a stop command to the drive.

Depending on the inertia of your machine, you have to adjust the maximum, minimum speed and the acceleration / deceleration slope in your VFD.

Good luck

Dear All

Thanks for your comments. I am currently working on it and will get back soon.