Frequency inverters

Smart positioning of waffles

With the "Integrated Motion Control" functionality, Danfoss offers an alternative to servo control, which, directly integrated in the VLT AutomationDrive series inverter, very easily takes over position, positioning and synchronization tasks with and without an encoder. At the Bühler Group in Leobendorf, Lower Austria, the technicians appreciate these advantages - especially the resulting motor independence.

The machine manufacturer of production plants for wafers, cookies and confectionery has been using Danfoss VLT frequency inverters for many years. Due to the increasing need to synchronize and position, the company has recently gradually converted its machine drives to servo technology. When Danfoss launched the "Integrated Motion Control" (IMC) functionality in 2017, Haas - at the time - wondered whether the new control technology would be a practical alternative to servo controls for their machines. Above all, they expected advantages from the free motor selection, the independence of technology and the simple system optimization through automatic motor adaptation.

Integrated Motion Control offers three types of positioning

In principle, the IMC functionality is universally applicable, since it can position absolutely, relatively and with sensor. For absolute positioning, the position values refer to a defined machine zero point. This is determined when the machine is started by the so-called reference run (homing). If an optional feedback system with absolute position is used, this is not necessary because the zero point is determined by the encoder. A typical application is the "downstream" of the bag baking oven with absolute positioning of linear axes. These have the task of placing paper bags in front of the continuously moving sugar bags so that they can be pushed into each other in the next step. The motors used are PM motors that are operated in open loop.

The advantage of IMC is that the encoder can be omitted, thus saving the costs for the encoder as well as for cabling, evaluation and the time needed for installation and commissioning. The use of the VLT AutomationDrive with IMC itself also brings savings, because the acquisition and installation costs for a corresponding servo system are much higher compared to frequency converter technology.

Relative (exact) positioning

Another application concerns relative positioning. In contrast to absolute positioning, the position specification does not refer to a zero point of the machine, but to the respective current actual position of the drive.
An example of such an application at Buhler is dosing pumps for dough. Here, the distance covered by the motor corresponds to a certain amount of dough that is pumped. After positioning (by the defined travel distance) with the exactly correct amount of dough, the drive is moved in the opposite direction of rotation to create a small vacuum. This causes the dough in the line to be pulled back a little and it will not drip. Both movements are relative positioning. The advantages for the user are the same as in the previous version.

Touchprobe on stop

Another application variant is sensor or touchprobe positioning. Here the drive starts and runs until an external sensor sends a signal to the frequency converter. From the position of this sensor the desired target position is calculated. The sensor positioning is used wherever an exact target position cannot be calculated without external specifications. Often the sensor is also used to correct the target position, e.g. to compensate for the error caused by the slip of an asynchronous motor.

This method is used in the so-called cushion press. Here, an asynchronous motor operated in an open-loop with a gearbox makes several revolutions to complete a complete cycle of the mechanics. Then it stops at a certain position. Only in this position can material be picked up again and a new cycle started. The advantage of touch probe positioning is even more precise positioning than with start/stop on the VLT AutomationDrive frequency converter. In addition, the target position is very easy to adjust because only one parameter needs to be changed. This also makes it extremely easy to adapt the machine to different products.

Touchprobe in Speed Mode

As a variant of the Touchprobe, there is also the Speed Mode as a method of positioning. Here the drive is started in normal speed control and operated continuously. The monitoring of the Touchprobe input is activated via a field bus signal. From this point on, the frequency converter waits for the sensor signal and carries out the positioning. Switching from speed mode to positioning is done bumplessly while the motor is running. This is a function that can only be solved so easily with a frequency converter.

Synchronization becomes very simple 

Synchronization means to achieve angular synchronism of two or more axes. The transmission ratio of the two axes to each other can be continuously adjusted. The setpoint signal for position and speed is either taken from an external encoder, generated by a PLC as a pulse signal or specified via a "virtual master signal". This virtual master signal is a pulse signal from which direction of rotation, speed and position can be derived. The input to the master drive is either done classically via an analog signal or via a setpoint value on the field bus. The master drive then converts the signal into a virtual master signal, which serves as a setpoint for the slave drives and itself.

The Leobendorf-based wafer machine specialists use synchronization with a master signal, for example, in the downstream drives of the bag baking oven. In the subsequent processes, up to eight axes each move in angular synchronism with one another. A pulse signal from the PLC is used as the master signal. Before the drives are started, they are all moved to a reference position (homing). The PM motors used in open loop with gearboxes achieve angle-synchronous synchronous running despite different gearboxes and different motors by exact adjustment of the transmission ratio. This saves the complete encoder system for the eight axes and the servo controllers. In addition, the furnace can be put into operation much faster and the engineering has also been simplified.

Start from defined position

The reference run of the drive is necessary to define a machine zero point when using incremental encoders or in open-loop operation and to bring the drive to this defined starting position. There are different ways to define the reference point. The VLT AutomationDrive FC302 IMC supports the following possibilities:

  • The current position can be defined as zero point. 
  • An external sensor defines the zero position; this sensor is approached at the set speed.
  • An analog input defines the reference position.
  • The zero pulse of an incremental encoder is the reference position.
  • A combination of an external sensor and the zero pulse defines the zero position.
  • The set torque limit defines the reference position (travel to stop, mechanical).
  • "Home-Sync" function

The "Home-Sync" function offers the possibility to define the reference position during the positioning or synchronization process. The acquisition is done by means of a fast digital input, all running positioning or synchronization movements are directly adapted to the changed zero position. It is not necessary to stop the drive.

In addition, the reference position can be adjusted by setting the offset via parameters. This offers the advantage that a different position from the zero position can be selected as the reference position. For example, an end position of the machine could be the zero position, even if the reference position is at a different location. 

Simple troubleshooting 

In addition to simple parameterization, IMC offers high reliability and easy troubleshooting in the event of a fault thanks to proven frequency converter technology: With IMC, recommissioning can usually be carried out by the operator without the need for specially trained personnel. In this way, IMC saves costs for OEMs and users both through simple engineering with fewer components and through reduced downtimes.

What the IMC function brings

  • Motor independence - allows free choice of motor and motor technology (PM, asynchronous)
  • Easy adaptation to any engine by automatic engine adaptation (AMA)
  • Operation with encoder (incremental, absolute) or encoderless possible
  • Fieldbus independence
  • own IMC-Motion-Profile for fieldbus operation with adaptable control word
  • Supports linear and rotating systems
  • Handling of limit switches integrated
  • No extra software and no programming by specialists necessary - all parameter settings are made via the control panel "LCP" or optionally via the configuration software "MCT10
  • Easy exchange in case of service, parameter copy via "LCP" or "MCT102
  • Manual operation via control panel "LCP" possible

 

Author: Michael Hitter is Business Development Manager at Danfoss Drives in Austria

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