Drives switched for sequential servos

Written by: Tom Shelley | Published:

Many machines tend to do one thing at a time. For example, a pin tester for printed circuit boards has an axis for the belt conveyor that has to be stopped before closing the cover with the test probes. In addition, both motors need to be switched off when undertaking the tests.

However, a German company has come up with a way of switching between servo axes, so that only one servo drive is required to drive up to four axes. This not only saves the cost of up to three servo drives, but also reduces cabinet space and wiring costs.

The idea, conceived by Stober, uses one drive and a switch rather than four separate drives controlled by a PLC. When switching from one axis to another, the MDS 5000 or SDS 5000 drive unit has to save the data from the active axis that has to be stopped, and then restore data to the axis that has to be started. This process, including switching the motor contactors, is accomplished in around 400ms.

The motor contactors can either be electromechanical or solid state, but if electromechanical, require 24V coils. The position feedback system is based on the digital part of the Heidenhain EnDat protocol. Since no analogue signals are involved, it is highly immune to noise. Further integrity is ensured by the EnDat protocol incorporating checks for broken wires and making cyclic redundancy checks.

The system is suitable for use with Stober servo motors rated at up to 48kW. Martin Preece, sales director at the company, says: "It is best to use motors running within a 2.5 ratio between the highest and lowest nominal currents in reference to the nominal current of the inverter, in order to achieve optimal closed loop control of any axis."

The system works best with Stober motors, which store data internally about current ratings, torque and encoder data, which can be communicated automatically to the drive's automatic recognition system.

All signal lines are plug-in and prefabricated to simplify wiring, save installation times and reduce costs. Additionally, Stober's Posidyn SDS 5000 inverter can work with the PosiSwitch and incorporates other novel features, including a self configuring integrated bus (IGB).

To configure an IGB network with several inverters requires only cable connections between the units. When the inverters are switched on, the network configures itself automatically. Up to 32 axes can be operated in an IGB network, and every inverter can be accessed for network diagnosis. Possible applications include bending machines and machine tool changers where each requires multiple servo axes.

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