Success Stories

Quadrant Feeder

Control System Solution

Double Acting Quadrant Feeder.

A 4 Axis servo capable of producing 400Hp per axis was needed but could not be justified.

When faced with a 60,000lb assembly 76ft wide running at 15Rpm, power delivery and control is paramount.

Requiring almost 800Hp to operate and with the width involved a single drive point was not practical.

Therefore the decision was taken to simultaneously drive each end of the assembly.

The machines primary function is to separate stems prior to processing in the sawmill.

There are 2 main parts,

The Lift shown in Red

The Kicker Shown in Yellow

The lift must reach top position at a synchronized point with the Kicker, and the Kicker is synchronized with a transfer chain running into the mill.

In total 2 pairs of motors each required to be synchronized.

The lift pair needs to be synchronized with the kicker pair, and the kicker pair needs to synchronize with the transfer chain.

A Total of 4 control loops.

One idea was to set the motor pairs as master & slave forcing the slave to be identical to the master.

On most VFD’s master / Slave is speed based and therefore would not allow us the required control over synchronizing one pair with the next. Also as both motors would be tied together on speed if one motor was lagging it would not be able to catch up and therefore stress the assembly.

This is where Servos would be the ideal solution, having the option for tight position control. Each motor could be given an identical position profile and the servo would ensure each motor followed that profile by adjusting the relative speed accordingly.

The problem being we had a total of 4 motors each rated at 40Hp driving 100:1 reducers.

Although large horse power servos are now available the financial implications of a 4 axis 40Hp servo controller was not viable.

Having worked closely with the Delta RMC motion controller we turned our interest on turning standard induction motors with VFD’s into cost effective servos.

To ensure we did not twist the assembly the motor encoders were removed and installed directly on the assembly. The encoders were setup to provide a 0.043 degree resolution Each VFD was configured with zero acceleration and deceleration ramps, And in Open Loop Frequency Mode. This ensured that the VFD performs directly as the RMC is requesting.

Utilizing the onboard programming ability of the RMC no on-site PLC was required.

In theory we have relational positional control. The position of each area would be dependent on another.

The Required position of the kicker would be dependent on the position of the transfer chain.

The Required position of the lift would be dependent on the Kicker’s position.

The RMC would take the encoder feedback from each end of the assembly and adjust each VFD’s command speed to ensure each encoder was at precisely the right position. Should 1 motor be lagging the RMC would increase the speed to bring it back on course, likewise if one was leading the associated motor would be slowed down.

Operating in pure relational position control worked well but had one flaw; the transfer provided 2 signals a running, and a Stem Request indicating the correct time to deposit a stem. When the transfer stopped it could take up to 6 seconds for the assembly to reposition itself for a restart, When the transfer was running continuously both assemblies’ ran at 15 rpm but 180 degrees out of phase due to the transfer point.

Therefore if the transfer was paused for a second due to a hang-up in the mill the system would become out of sync and could leave up to 2 empty spaces on the transfer chain. An empty space means reduced efficiency.

The internal program was modified to allow both assemblies to operate independently, with an overdrive speed setting.

Specific conditional timing points were added to the program.

The new program forced both assemblies to accelerate to a maximum speed as soon as it was detected that the transfer had stopped.

If the transfer restarted with the kicker in position and the lift running, then the kicker would start out of sequence with the lift. The lift would then park and wait for the kicker, to rejoin on the fly. Ensuring the correct transfer point was maintained between the lift & the kicker.

At the same time the system ensured that both end of the assembly were at the target position at the required time.

The balancing act:

No VFD likes to operate with a constantly varying set point; As the RMC adjusted the set point every 2ms, The VFD was constantly accelerating or decelerating.

Acceleration / Deceleration ramps were set to zero in the VFD to avoid the possibility of a Double Loop Conflict. Where 2 PID loops are running simultaneously in series and therefore contradicting each other.

Without acceleration & deceleration ramps in the VFD, a drive trip was a major concern as this could severely stress the assembly or possibly blow the remaining drive. This was handled by braking the remaining drive as fast as possible then releasing the drive control to ensure that one motor was not trying to hold the weight of the assembly.

In a attempt to reduce the risk a drive trip stall prevention was set to 75% in the VFD, This resulted in the drive overriding the RMC’s requested speed should the motor demand be in excess of 75% FLA. The stall prevention although protecting the drive increased the risk of following errors where the position of the assembly is more than 2 degrees off course, Resulting in a controlled shutdown.

Tuning:

Tuning PID loops is always challenging but having 2 loops connected by a 76Ft assembly weighing in at 60,000lbs makes life a little more interesting. Not only are Loops interconnected but speed, load, and position are also dynamic making finding the happy medium difficult.

Tuning was completed with a maximum error of less than 0.7 degrees error end to end, while performing a high speed transition on the fly, Average error margin was less than 0.2 degrees end to end.

Dynamic Tensioning System

GLC Controls was presented with the challenge of creating a control scheme that would allow existing planers run more efficiently, more quickly, and with less downtime. Traditionally, planers use air bags or just the roll pressure themselves on the feed rolls of the planer. A problem arises when a planer which is designed to run at 1000 ft/min is increased to 1600 ft/min. The feed rolls cannot come to rest on the lumber beneath them before the board exits the roll; therefore, the roll becomes less effective at higher feed speeds. Because of the ineffectiveness of feed rolls at higher speed, planer jam-ups are more common and therefore create more downtime.

GLC Controls has designed an effective solution to address this challenge. Working in conjunction with Wolftek Industries to control the Planer Feed Rolls, and with Setworks to improve planer performance and reduce downtime, GLC Controls’ “Planer Tensioning System” uses hydraulic cylinders to control the movement of the feedrolls as illustrated below.

Controlling the hydraulic cylinders with pressure feedback required three main components a Delta RMC, Allen Bradley Control Logix, and a Touch Screen Computer. The Delta RMC industry-proven closed-loop controller that controls the position and the pressure of the roll is placed on the lumber. The RMC has a very fast scan time of 2 ms and the ability to run and store programs locally on the controller to ensure high-speed, accurate control of the planer roll. The RMC connects to Temposonics to give a position feedback and pressure transducers on the rod and cap end of the hydraulic cylinder to give accurate pressure feedback.

The Allen Bradley Control Logix PLC interfaces the Dynamic Tensioning System to the existing planer system. The PLC monitors IO from the field and controls which mode the planer roll is to operate. The PLC will monitor system performance and capture any alarms that may occur in the control system and send alarm information to the HMI (Touch Screen Computer).

The Touch Screen Computer (HMI) serves as a user interface to the control system. The HMI displays all diagnostic information for ease of troubleshooting. Active alarms and parameter changes are all logged in a historical report.

The HMI also includes:

A product recipe selection for running a variety of different combinations of dimensional lumber.
Automated roll position and pressure calibration control to ensure the system runs at optimum performance.
Dynamic roll timing of feedrolls based on line speed to ensure system performs accurately at different line speeds.

The HMI also has a virtual control console to control all aspects the Dynamic Tensioning System.

Basic Control System Overview

The Dynamic Tensioning System has been a huge success for GLC Controls and our customers who have installed our system. Many of our customers have increased productivity by 10 percent or more, with less downtime and fewer planer jam-ups, while running faster line speeds.