Beckhoff EL7037 User Manual download pdf (Page 22)

Product overview

2. Determine mass m and moment of inertia (J) of all parts to be moved

3. Calculate the acceleration resulting from the temporal requirements of the moved mass.

4. Calculate the forces from mass, moment of inertia, and the respective accelerations.

5. Convert the forces and velocities to the rotor axis, taking account of efficiencies, moments of friction

and mechanical parameters such as gear ratio. It is often best to start the calculation from the last

component, usually the load. Each further element transfers a force and velocity and leads to further

forces or torques due to friction. During positioning, the sum of all forces and torques acts on the

motor shaft. The result is a velocity/torque curve that the motor has to provide.

6. Using the characteristic torque curve, select a motor that meets these minimum requirements. The

moment of inertia of the motor has to be added to the complete drive. Verify your selection. In order to

provide an adequate safety margin, the torque should be oversized by 20% to 30%. The optimisation

is different if the acceleration is mainly required for the rotor inertia. In this case, the motor should be

as small as possible.

7. Test the motor under actual application conditions: Monitor the housing temperatures during

continuous operation. If the test results do not confirm the calculations, check the assumed

parameters and boundary conditions. It is important to also check side effects such as resonance,

mechanical play, settings for the maximum operation frequency and the ramp slope.

8. Different measures are available for optimising the performance of the drive: using lighter materials or

hollow instead of solid body, reducing mechanical mass. The control system can also have significant

influence on the behaviour of the drive. The Bus Terminal enables operation with different supply

voltages. The characteristic torque curve can be extended by increasing the voltage. In this case, a

current increase factor can supply a higher torque at the crucial moment, while a general reduction of

the current can significantly reduce the motor temperature. For specific applications, it may be

advisable to use a specially adapted motor winding.

2.3.2 Standard mode

Stepper motors were originally operated with very simple output stages, which were only able to switch the

voltage of the motor phases separately (nowadays current control takes place via PWM with pulse-width

modulation as standard). Initially the motor phases there were controlled individually in turn. A switching

sequence in the positive direction of rotation corresponds to the switching sequence (+A, +B, -A, -B).

Sequential switching results in rather irregular operation in this mode. In order to make the operation

smoother, so-called microstepping was introduced later, in which the four set voltages were extended by

intermediate values (e.g. from a stored sine table). These days, microstepping based on 64 steps is

commonly used.

Fig.11: Control structure of a standard stepper motor drive

Neglecting the sampling resulting from the microstepping, the motor current I as function of the electrical

angle φe and of the magnitude of the motor current I

ABS

(when using a current controller) can be described as

follows:

I(φ

) = I

+ jI

= I

ABS

cos(φ

) + jI

ABS

sin(φ

)

EL70x722 Version 1.0

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Beckhoff EL7037 User Manual Page 22

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