Beckhoff EL7037 User Manual download pdf (Page 25)

Product overview

Requirement

• Encoder with sufficiently high resolution required (minimum 4000 [INC/360°])

• Slightly higher parametrisation effort required (speed controller)

• Commutation determination at startup (due to incremental encoder)

• Only possible with stepper motors from Beckhoff Automation (AS10xx)

2.3.4 Sensorless operation

Because the default operation of a stepper motor with a constant load-independent current is not energy-

efficient and leads to a permanently high thermal load, efforts are made to reduce this load.

Function

By analyzing the speed-proportional induced countervoltage, it is possible to control the stator current

depending on the load with the aid of a machine model (without sensor/encoder), thereby significantly

increasing the efficiency.

Since this operating mode requires a minimum amplitude of the magnitude of the induced countervoltage,

sensorless control only works in the medium and upper speed range. In the lower speed range the motor is

operated in standard mode. The changeover to sensorless operation take place via a programmable, motor-

dependent switching speed. The switching speed is usually in the range between half and three revolutions

per second (crossover velocity 1).

When sensorless control is activated, the transient phenomenon results in a slight mechanical jerk of the

shaft, which is proportional to the load acting on the shaft.

Fig.16: Influence of the crossover velocity thresholds (1,2,3) on sensorless control

After switching on, the control current remains constant up to a second configurable speed and is reduced to

a third parameterizable speed via a linear ramp.

A long control current ramp leads to a stronger stabilization of the transient phenomenon of the control.

However, it also leads to a longer flowing constant motor current and therefore slightly higher losses.

Motor dependency

Due to the fact that the control is strongly dependent on the motor parameters, the controller parameters and

motor behaviour itself, sensorless operation is limited to Beckhoff motors. This mode is not supported for

motors from other manufacturers.

Parameterisation

Compared to the other operating modes, the parameterisation effort is relatively high. However, all the

required necessary parameters are pre-specified via a startup list for the respective motor types. All that is

required during commissioning is an adjustment of the speed control parameters, due to the given mass

inertia ratios of the connected loads in the mechanical system.

For the speed controller, in principle the same dependence on the mass moment of inertia and the torque

constant applies as in the Extended Operation Modes. Thanks to the lower-level sensorless control it is,

however, possible to achieve a better overall result through different parameterisation.

EL70x7 25Version 1.0

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Beckhoff EL7037 User Manual Page 25