VSD Leader-Follower Applications

VSD Leader-Follower Applications

In the past machine ‘synchronisation’ were done by using one large electric motor with a long central line shaft and many belts and cams to mechanically fix different axes together. This design had many flaws with belts that could slip, line changes being very difficult, etc. There are now better options with Leader-Follower (Master-Slave) drive topologies where an individual motor is used for each axis.

A Leader-Follower (Master-Slave) application is where two or more drives are electronically synchronised together with one of the drives configured as the Leader (Master) and subsequent drive/s that follow the Leader are referred to as the Follower/s (Slave/s).

Load sharing, synchronisation, and redundancy are common reasons that Leader-Follower arrangements are used (e.g., winder/unwinders, printing presses, gantry cranes, etc.). Since the shafts are only electronically coupled, machine operation can be changed with a few parameter adjustments (much simpler than changing mechanical line shafts). This allows for much more flexibility to accommodate quick changes (for example for changes in product sizes). There are many different implementations of Leader-Follower arrangements, but here are some of the more common applications:

• Speed/Velocity Following: This scenario is to allow multiple motors to run at the same speed (or perhaps at a scaled/proportional speed of each other). The Leader operates in speed control mode and then outputs a speed reference command to one or more Follower drives. If the different drive axes are geared differently, then the Follower speed command might be a scaled version of the Leader’s speed (speed offset is also possible). Important to note in speed following mode is that the drives are always trying to regulate the instantaneous speed command, so if there was a Follower error that happened in the past, the Follower drive only adjusts to the latest speed setpoint and does not compensate for any accumulated error.
• Position Following: This scenario is to allow a Leader axis to provide a position reference to a Follower axis (also referred to as Angular Synchronisation since the rotary position is angular). For position following the drives need to be set up for closed loop control with some sort of feedback on each axis (the Follower axis must follow the position reference), this is to allow for the Follower axis to correct any sort of error as quickly as possible. These scenarios are more difficult than speed following applications.
• Load Sharing (Torque Sharing): This scenario might be desired as a way to distribute the torque and work through different drivetrains and motors or for redundancy in critical applications (e.g., solar panel tracking or wind turbine control). Motors would thus share the load, resulting in less heating and wear on the mechanical components. For load sharing applications it works best to assign slightly different torque values for the Leader and Follower, so that they do not fight each other. An advantage with a torque sharing topology is that motor sizes can be different.

The Leader can provide a signal to a Follower in a few different ways, including Analog Output, Encoder Feedback (much tighter closed-loop regulation) or more complex bus systems (complex PLC synchronisation using bus technologies such as RS485/MODBUS RTU, EtherCAT, Profinet/ProfiBus, CANopen, etc.).

Using VSD Analog Output for Speed Synchronisation

The speed reference from a Leader drive can be a simple analog output based on running frequency. The analog signal is then fed into the Follower drive’s Analog Input (e.g., for conveyor and flight simulator applications). This open-loop topology is the most cost-effective option, however, the response between the Leader and Follower motors might lag somewhat and can increase as more axes are added. Since there is no feedback from the Follower for corrections, this is typically only used for systems that can tolerate following error.

For an example of how to connect and set up a Leader and Follower VSD, please see the following EMHEATER example: Leader/Follower Example.