The OP amplifier mentioned above is an aspect of the application of the amplifier in the field of robotics. The specific implementation method of the controller is described below. In a specific circuit, the OP amplifier is used for information processing, that is, converting the voltage into usable information. From the point of view of efficient use of energy, the current flowing through the circuit should be controlled to a minimum. The same problem is also faced when the computer performs information processing.
However, when inputting the control amount determined by the OP amplifier circuit or computer processing into the servo, it is necessary to add a circuit with a certain output impedance in order to generate enough current and apply enough voltage to the driver. Playing this role is the power amplifier.
1. Power OP amplifier
As mentioned earlier, the output impedance of an ideal OP amplifier is zero. This means that no matter how much current is required at the output, it can do it. However, the output end of the OP amplifier is connected to the input end of other OP amplifiers in most cases, and it can basically be considered that the OP amplifier is connected to the high input impedance during design. Therefore, its actual output impedance is not very large. Correspondingly, considering the purpose of connecting with a low-impedance load, the OP amplifier can also be designed according to a large output current, which is generally called a power amplifier.
The difference between the power amplifier and the general OP amplifier is only the part that outputs a large current, other than that, there is almost no difference between the two. The biggest issues that must be taken care of when dealing with large currents are the allowable losses inside the OP amplifier and its heat release measures. Depending on the working state, the power consumed by the OP amplifier also varies greatly, and the power loss is quite large in areas with poor energy efficiency, so a reasonable heat dissipation design is required.
The actual situation is that most of the power amplifier products on the market have a maximum current of about 1A, and of course there are also amplifiers with an output current of about 3-10A. The shape of the amplifier should also adapt to the requirements of heat dissipation, for example, equipped with a cooling fan, installing a cooling plate, etc.
The advantage of the power OP amplifier as a power amplifier is that the amplifier itself has a controller, so the design and installation are very simple. Therefore, it is suitable for various experiments. However, there is a problem from an energy efficiency point of view that it has to back up two power sources. Especially autonomous mobile robots have strict restrictions on the power supply and high energy efficiency requirements, so it is difficult to say that a power amplifier is a more suitable choice.
2. Linear Power Amplifier
The power amplifier composed of current amplifying transistors (power transistors) has a wide range of performance specifications and is sold in large quantities in the market, which is very convenient to form a power amplifier that can meet various requirements. A typical example is the linear power amplifier, the basic principle of which is shown in Figure 3.35.
The basic idea of this circuit is to divide the power supply voltage with the transistor and the load to adjust the voltage applied to the load. As mentioned above, the parameter h of the transistor itself is highly dispersed during manufacture and is greatly affected by temperature, so it is difficult to obtain stable operation without taking measures. Another problem is that the current flowing through the load in Figure 3.35 only flows in a certain direction. In the field of robotics, most of the objects driven by power amplifiers are motors, and they need to be driven in both directions, and the unidirectional flow of current is obviously not suitable.
Based on the above practical problems, the linear power amplifier circuit is changed as shown in Figure 3.36.
In the circuit of Figure 3.36, the OP amplifier is used to return the feedback to the voltage applied to the load, which helps eliminate the effects of transistor parameter variations. By using a composite transistor pair with two power supplies, the load current can flow in the direction of the sign of the input signal.
As mentioned above, the circuit uses the load and the transistor to divide the voltage, which plays the role of adjusting the load voltage. The energy dissipated by the transistor is equal to the product of the voltage applied across the transistor and the load current. Therefore, it is necessary to fully consider the selection of the output transistor and its heat dissipation countermeasures.
There are many components that make up this circuit. In fact, especially in the typical application occasion of a power amplifier such as a motor driver, a motor driver product with all the required components and components can be purchased in the market. When the motor capacity is small, it is more convenient to use a dedicated IC.
3. PWM amplifier
H-bridge power amplifiers based on PWM (Pulse Width Modulation) amplifier drive methods are often used in tens of watt motor drive circuits for robots.
As mentioned in the aforementioned linear power amplifiers, the transistors at the output poles generate power dissipation. Usually, this part of the power dissipates heat through the heat sink, so it has the disadvantage of considerable energy loss. However, in this way, there is no energy loss if the transistor is completely on or completely off. The reason for this is that the voltage applied to the transistor when the transistor is on, and the current flowing through the transistor when it is off is 0.
Based on this principle, the on and off pulse signal (PWM signal) is repeatedly and alternately applied to the transistor to reduce the energy loss. This method is called the PWM drive method. However, although on and off are repeated at this time, in order not to cause the motor to vibrate, the pulse period of the signal (frequency of the pulse signal) is required to be set smaller than the electrical time constant of the motor (generally on the order of several milliseconds). As for the control of the speed, it is realized according to the ratio of the on and off time, that is, the duty cycle.
Figure 3.37 shows the basic circuit of the PWM drive method often used in motor drivers. In this circuit, the motor is equipped with four transistors arranged in an H shape, which can realize the forward and reverse driving of the motor powered by a single power supply. This method is called the H bridge method. On this basis, braking can be achieved by turning on the two transistors of the next stage at the same time to cause a short circuit between the motor terminals. The corresponding combinations of the four states of the transistor and the four actions of forward rotation, reverse rotation, braking, and stop are shown in Table 3.8.
In this circuit, the diode connected in parallel with the transistor is called a freewheeling diode. Due to its function, during the off period of the pulse input, the energy stored in the motor can maintain the current of the motor, thus achieving the effect of improving efficiency.
If the command issued to the motor is an analog voltage, it is necessary to generate a PWM duty cycle signal proportional to the analog voltage value. The most commonly used method at this time is to prepare a certain period of triangular wave in advance, and input it and the analog signal into the operation comparator at the same time. However, if the command value is output through a computer, since there are already ready-made ICs that generate bus interface PWM waves on the market, they can be purchased and used directly. In the single-chip MPU, most of them are embedded with the function of generating PWM waves, which is very convenient to use.