controller has three ports that bring signals in or send signals out of the controller. The power supply, servomotor, and tachometer are connected to port P3 at the bottom of the controller. You can see that the supply voltage is 115-volt AC single phase. A main disconnect is connected in series with the LI wire. The LI and N lines supply power to an isolation step-down transformer. The secondary voltage of the trans-former can be any voltage between 20 and 85 volts. The controller is grounded at terminal 8. You should remember that the ground at this point is only used to provide protection against short circuits for all metal parts in the system.
The servomotor is connected to the controller at terminals 4 and 5. Terminal 5 is + and terminal 4 is —. Terminal 3 provides a ground for the shield of the wires that connect the motor and the controller. The tachometer is connected to terminals 1 and 2. Terminal 2 is + and terminal 1 is —. The shield for this cable is grounded to the motor case. The wires connected to this port will be larger than wires connected to the other ports, since they must be capable of carrying the larger motor current. If the motor uses an external cooling fan, it will be connected through this port. In most cases the cooling fan will be powered by single-phase or three-phase AC voltage that remains at a constant level, such as 110 volts AC or 240 volts AC.
FIGURE 11-86 (a) Tran-sistors connected to the three windings of the brushless servomotor. (b) Waveforms of the three separate voltages that are used to power the three motor wind-ings. (c) Waveforms of the signals used to control the transistor se-quence that provides the waveforms for the previous diagram, (d) Waveform of the overall back EMF.
FIGURE 11-87 Example servomotors and ampli-fiers.
FIGURE 11-88 Diagram of a servo controller. This diagram shows the digi-tal (on-off) signals and the analog signals that are sent to the controller, and the signals the con-troller sends back to the host controller or PLC.
The command signal is sent to the controller through port PI. The terminals for the command signal are 1 and 2. Terminal 1 is + and terminal 2 is —. This signal is a type signal, which means that it is not grounded or does not share a ground potential with any other part of the circuit. Several additional auxiliary signals are also connected through port 1. These signals include inhibit (INH), which is used to disable the drive from an ex-ternal controller, and forward and reverse commands (FAC and RAC), which tell the con-troller to send the voltage to the motor so that it will rotate in the forward or reverse direc-tion. In some applications, the forward maximum travel limit switch and reverse maximum travel limit switch are connected so that if the machine travel moves to the extreme posi-tion so that it touches the overtravel limit switch, it will automatically energize the drive to begin travel in the opposite direction.
Port PI also provides several digital output signals that can be used to send fault signals or other information such as \for all digital (on-off) signals.
Port P2 is the interface for analog (0-max) signals. Typical signals on this bus include motor current and motor velocity signals that are sent from the servo controller back to the host or PLC where they can be used in verification logic to ensure the con-troller is sending the correct information to the motor. Input signals from the host or PLC can also be sent to the controller to set maximum current and velocity for the drive. In newer digital drives, these values are controlled by drive parameters that are programmed into the drive.
PWM Servo Amplifier
The PWM servo amplifier is used on small-size servo applications that use DC brush-type servomotors. Figure 11-89 shows a diagram for this type of amplifier. From the diagram you can see that
single-phase AC power is provided to the amplifier as the supply at the lower left part of the diagram. The AC voltage is rectified and sent to the output section of the drive that is shown in the top right comer of the diagram. The output section of the drive uses four IGBTs to create the pulse-width modulation waveform. The IGBTs are con-nected so that they provide 30-120 volts DC and up to 30 A to the brush-type DC servo-motor. The polarity of the motor is indicated in the diagram.
The remaining circuits show a variety of fault circuits in the middle of the diagram that originate from the fault logic board and provide an output signal at the bottom of the diagram. You should notice that the fault output signals include overvoltage, overtempera-ture, and overcurrent. A fourth signal is identified as SSO (system status output), which in-dicates the status of the system as faulted anytime a fault has occurred. A jumper is used to set the SSO signal as an open collector output with a logic level \ The input terminals at the bottom right part of the diagram are used to enable or inhibit the drive, and to select forward amplifier clamp (FAC) or reverse amplifier clamp (RAC). The inhibit signal is used as a control signal, since it inhibits the output stage of the amplifier if it is high. The FAC and RAC signals limit the current in the opposite direction to 5%.
The input signals are shown in the diagram at the upper left side. The VCS (velocity command signal) requires a +VCS and a -VCS signal to provide the differential signal.
Applications for Servo Amplifiers and Motors
You will get a better idea of how servomotors and amplifiers operate if you see some typical applications. Figure 11-90 shows an example of a servomotor used to control a press feed. In this application sheet material is fed into a press where it is cut off to length with a knife blade or sheer. The sheet material may have a logo or other advertisement that must line up registration marks with the cut-off point. In this application the speed and po-sition of the sheet material must be synchronized with the correct cut-off point. The feed-back sensor could be an encoder or resolver that is coupled with a photoelectric sensor to determine the location of the registration mark. An operator panel is provided so that the operator can jog the system for maintenance to the blades, or when loading a new roll of material. The operator panel could also be used to call up parameters for the drive that
cor-respond to each type of material that is used. The system could also be integrated with a programmable controller or other type of controller and the operator panel could be used to select the correct cutoff points for each type of material or product that is run.
FIGURE 11-89 Diagram of a pulse-width modulator (PWM) amplifier with a brush-type DC servomotor.