3.4 Interface between NC and servo amplifier
Generally the NC controls the servo motors in the following control modes:
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(1)Torque control mode
(2)Velocity control mode (including torque)
(3)Position control mode (including velocity and torque)
Normally mode (1) requires a very high speed calculation and transmission rate for NC, but it enables full control of motors. On the other hand, mode (3) requires rather low transmission rate, but requires some intelligence for the servo amplifier. One of these modes is selected in accordance with the purpose or characteristics of the machining. To control the servo motors, NC should send (and receive) certain types of data. These data are roughly divided into two types.
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(i) Dynamic data, which is transmitted periodically.
(ii) Static data, which is transmitted non-periodically.
Dynamic data (i) will be transmitted periodically and used to control the condition of the servo motors. Most of the static data (ii) will be transmitted in the first initialization cycles and will not be changed. But some static data will be changed in order to alter the modes or parameter tunings, etc. Typical examples of these data are listed below.
(i)
- Torque command value (NC -> Servo)
- Velocity command value (NC -> Servo )
- Velocity feedback value (NC <- Servo)
- Position command value (NC -> Servo )
- Position feedback value (NC <- Servo)
- Combination of these data
(ii)
- Some limit values (Position, velocity, torque)
- Scaling parameters
- Gain parameters
- Action time parameters
- Amp and motor characteristic parameters
- Status information (Error, warning etc.)
- Option parameters
For interface between NC and servo, the selectability of the modes mentioned above, variable cycle time and the ability to control the spindle motor are required. The SERCOS interface has sufficient ability to realize these requirements, but because the SERCOS interface emphasizes flexibility of vendor implementation, it is not accepted as a good standard interface.
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