Simple Servo Amplifiers

Introduction

The output stage of all servo amplifiers is an analog circuit. The analog circuit provides a means to allow the voltage and current for the motor to be adjusted to control position, velocity, and torque. The feedback and comparator stages can be any mixture of digital and analog devices. For example, if the feedback section uses a resolver, the output of this device is analog, so the section it works with is generally also analog. If the feedback device is an encoder, its output is digital, and the digital signal can be converted through a frequency-to-voltage converter so that the signal is usable in an analog circuit. Or it can be filtered and can use a digital value. The advent of microprocessors has allowed the digital values to be used through every part of the servo controller except the final output stage.

Figure 11-76 shows a diagram of the components in a typical servo linear amplifier. The circuit shows the motor winding connected to a set of transistors (TR1 and TR2). The transistors can control positive (+V) or negative (—V) voltage to make the motor turn in the clockwise or counterclockwise direction. The transistors can be pulsed on and off as in a pulse-width modulation (PWM) circuit, or they can ramped up and down as a simple linear circuit. The base of the transistors can be controlled by a controller section of an amplifier that is completely linear. Or the controller can be digital with a D/A converter to provide the analog control signal to the base of Q₁ and Q₂, or the controller can be completely digital and the base of transistors Q₁ and Q₂ can be pulsed directly by the digital controller. Typically IGBTs (insulated gate bipolar transistors) are used in modern servo amplifiers where PWM or other switching circuits are used. The IGBTs allow the transistors to be switched on and off at frequencies that limit the harmonic hum in a motor or amplifier. The high-pitched hum represents both audio and electronic noise that must be eliminated or controlled.


The amplifier circuits for DC servomotors are similar to the AC circuits used for pulse-width modulation or other switching systems. In fact the complete amplifier for AC servomotors will be similar to the variable-frequency drive amplifiers shown earlier in this chapter.

Early Amplifiers (Push-Pull Amps)

The design of amplifiers has changed rapidly over the last 15 years because transistors, triacs, and SCRs have become able to handle larger voltages and currents without damaging themselves. It is easy to see the advantages that the changes in these devices have brought to motor drive amplifiers, but you must keep in mind that the early amplifiers were built so well that you will run into them even today when you are asked to troubleshoot a drive system. For this reason it is prudent to leam their basic parts and functions so you will be able to troubleshoot and analyze them. It is also a good idea to understand their basic operation because this is what has been modified to make the newer drives more efficient and more powerful.

One of the earliest types of linear amplifiers is called a push-pull amplifier, which was designed so that two transistors switched on and off to share the current load for the motor. Figure 11 -77 shows an example of this type of amplifier, and you can see that Q₂and Q₃ are the power transistors. They are connected to the primary winding of transformer T₂. The servomotor winding is connected to the secondary winding of transformer T₂.

The operation of the push-pull amplifier begins with a sine wave signal that enters the input of the push-pull amplifier through capacitor C₁. Capacitor C₁ makes sure that the input signal is a pure sine wave with no DC bias. The base circuit of transistor Q₁ has a DC bias on it of approximately 13.5 volts. The base-emitter junction needs only 0.7 volt to turn it on, the rest of the DC bias voltage providing DC current through R₂ and R₃. This causes the sine-wave input signal to practically turn transistor Q₁ off at its minimum, causing Q₂ to be driven almost into saturation at the sine wave's maximum. This causes current to flow through the primary winding of transformer T₁. Notice the secondary of T₁ is center tapped to ground. When the positive half of the sine wave appears on the secondary, it appears across the entire secondary. Because of the center tap, only the upper portion of the secondary sees a positive voltage, and this forward bias transistor Q₂ allows it to conduct. Transistor Q₃ is turned off because it sees a negative voltage at its base. With Q₂ conducting, current flows up through the primary of T₂, providing a positive pulse to the secondary of T₂. When Q₃ conducts, a negative pulse is provided to the primary of T₂.

Another type of early amplifier for a servomotor is called the chopper amplifier (see Fig. 11-78). In this type of amplifier the positive rectangular DC pulses arrive at the input of the amplifier circuit at capacitor C₁. These pulses arrive at the base of Q₁ as narrow spikes, which momentarily turn Q₁ on. This in turn momentarily turns Q₂ on, which allows current to flow through the primary of transformer T₁. Now the primary of transformer T₁ is really an L-C tank circuit. (Remember that the primary winding of the transformer is actually a big inductor.) When this tank circuit is hit by a pulse, it will produce a cycle or two of pure sine wave. When hit, in other words, the tank circuit will ring like a bell. The amplifier circuit is the clapper that rings the bell. Notice the secondary of T₁ is center tapped to —60 Vp. The secondary of T₁ sees a pure AC sine wave, and to this AC signal, the —60 Vp appears as a ground. This means that for the positive half-cycle of the sine wave, Q₃ would see a positive pulse, and Q₄ would see a negative pulse. Both power transistors are NPN transistors, so a positive bias is needed at the base to cause them to conduct. As both bases are grounded , Q₄ would go into conduction because its emitter is lower than its base, giving it a forward base-emitter bias. The output of the tapped control winding would then be a sine wave. It should be noticed that the tapped control winding has +60 Vp on it, and the secondary of T₁ has —60 Vp on it. This means that the output of the tapped control winding is going to be a 120-Vp sine wave.

PurchaseIndustrial Electronics from Prentice Hall.

Related Links:
AC Servo Amplifiers
Amplifiers for DC Servomotors

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