1 Introduction
DC servo motors are characterized by fast response, good low-speed stability, and wide speed range, making them commonly used in servo systems for precise speed and position control. They are widely applied in industrial, defense, and civilian fields, particularly in artillery stabilization systems, shipboard platforms, radar antennas, and robot control. Although AC servo motors have developed rapidly, they are still difficult to replace DC servo motors in these areas.
Traditional DC speed control systems consist of two feedback loops: a speed loop and a current loop. They employ a tachometer, current sensors (Hall effect devices), and analog electronic circuitry to achieve closed-loop speed control. Modern digital DC servo control, on the other hand, uses a high-speed digital signal processor (DSP) to directly sample speed and current signals. Software-based functions such as digital comparison, digital adjustment calculations (digital filtering), and digital pulse width modulation are then used to achieve precise speed control. Compared to analog speed control systems, which are simpler in structure, lower in cost, and more reliable, analog speed control systems are more complex to debug because modifications to circuit parameters often require hardware changes. Digital speed control systems, while more complex and more expensive, offer high speed control accuracy, are easier to debug, and their performance can be controlled by software.
This paper introduces a method that falls between analog and digital speed control: using a programmable analog device (ispPAC10) to implement an analog speed control system. The system's circuit parameters can be adjusted via software, and the established system model can be simulated. This method was used to improve an existing DC speed controller for an automatic zoom system of a CCD camera, achieving excellent results.
2. Composition and Working Principle of Analog DC Speed Control System
Analog speed control systems typically consist of two closed loops: a speed loop and a current loop. To ensure their coordination and effective operation, two regulators are incorporated into the system to regulate the speed and current, respectively. These two feedback loops are arranged in a nested structure, known as a dual-loop speed control system. This system offers advantages such as fast dynamic response and strong anti-interference capabilities, leading to its widespread application. Figure 1 shows the system's block diagram, where ASR and ACR are the speed and current regulators, typically constructed from analog operational amplifiers forming PI or PID circuits. Signal conditioning primarily involves filtering and amplifying the feedback signal. Considering the mathematical model of a DC motor, the dynamic transfer function relationship of the analog speed control system is shown in Figure 2.
Taking the speed regulator ASR as an example, its circuit principle is shown in Figure 3(a), where Zin(S) represents the complex impedance of the input network and Zf(S) represents the complex impedance of the feedback network.
That is, the transfer function of the regulator is equal to the ratio of the complex impedance of the feedback network to that of the input network. Therefore, by changing Zf(S) and Zin(S), the required transfer function can be obtained to meet the needs of dynamic system correction. The PI regulator shown in Figure 3(b) has the dynamic structure shown in Figure 4.
in:
During the debugging of analog speed control systems, the parameters of the motor or the mechanical characteristics of the load often differ significantly from the theoretical values. This frequently necessitates replacing components such as resistors (R) and capacitors (C) to modify circuit parameters and achieve the desired dynamic performance. This process is very cumbersome. However, if a programmable analog device is used to construct the regulator circuit, system parameters such as gain, bandwidth, and even the circuit structure can be modified via software, making debugging much more convenient. The following example, using the PI regulator shown in Figure 3, illustrates how to implement an analog regulator circuit using the programmable analog device - ispQAC10.
3 Implementation Methods
3.1 Introduction to ispPAC10
The ispPAC10 is an in-system programmable analog device manufactured by Lattice. It utilizes non-volatile E2CMOS technology, and its internal analog component blocks (PACblocks) can replace traditional analog circuits such as operational amplifiers and filters without the need for external resistors, capacitors, or other components. Through software programming, circuit design and modification can be achieved, significantly shortening the development and debugging cycle and offering a high performance-to-price ratio. Lattice provides the powerful and user-friendly PACDesigner integrated software package for developing the ispPAC10, which can be downloaded online. The ispPAC10 contains four analog component blocks—its internal structure is shown in Figure 5.
The circuit schematic of PACblock is shown in Figure 6(a), and Figure 6(b) is the representation of PACblock in the PAC-Designer software package.
Its transfer function relationship is as follows:
Thus, equation (3) can also be written in the following form:
Equations (4), (5) and Figure 6(b) show that the PACblock module has basic operation functions such as scaling, summation, integration and filtering. One ispPAC10 contains four PACblock modules, each with two sets of differential inputs and one differential output.
By properly connecting these four parts, a more complex analog circuit can be formed.
3.2 Implementing a regulator circuit using ispPAC10
Taking the specific circuit shown in Figure (3) as an example, let R0 = 10kΩ, C0 = 0.15μF, Rf = 40kΩ, Cf = 0.5μF, and its transfer function is shown in Figure 7.
To implement the above structure using ispPAC10, it needs to be transformed into the form shown in Figure 8.
The above regulator can now be implemented directly using ispPAC10. The specific circuit is shown in Figure 9. The gain of the op-amp and the value of the capacitor are set by the software PAC-Designer.
4 Conclusion
Programmable analog devices make it easy to design and implement analog circuits. When designing analog speed control circuits, the circuit parameters and structure can be adjusted via software, greatly simplifying the debugging process. It's important to note that these devices generally operate at no more than 5V; the ispPAC10 operates at +5V. Therefore, the input signal cannot be too large, and currently, they can only be used in small-signal analog circuits. Nevertheless, their development prospects remain very promising.
The above regulator can now be implemented directly using ispPAC10. The specific circuit is shown in Figure 9. The gain of the op-amp and the value of the capacitor are set by the software PAC-Designer.
