Faster and cheaper processors currently being turned out by the silicon foundries are making it easier to justify digital drives instead of traditional analog drives. But do digital drives offer a clear advantage for more applications?
Compare the two approaches by first analyzing the classic analog drive shown in the diagram. In this case, the drive is configured as a torque amplifier. An external controller generates a ±10-Vdc command signal. The drive monitors the motor winding current and estimates the mechanical torque. A compensator takes the difference and generates a voltage command signal. Next, a pulse-width-modulated signal is applied to an inverter, which switches the correct voltages to the motor.
Contrast this to the equivalent digital drive system as illustrated. Most of the analog circuitry (summing amplifier, scaling amplifier, compensator, and PWM generator) is hidden in the large block called calculations, while the power inverter is identical. Because the calculations are all done in software, different concepts, such as new compensation-scheme algorithms, can be programmed on the fly and evaluated much faster than with an analog system (minutes instead of hours). This helps drive companies develop new, advanced features more quickly.
From a user's perspective, the main difference is in how the signals are obtained. Both the current and torque signals need to be converted to digital representations. This is done by a sample and hold (S/H) circuit and an analog-to-digital converter (ADC). The sample and hold circuit takes the value of the analog signal at a specific instant in time and freezes it. The frequency at which this is done is known as the sampling rate, for example, 4 kHz. The frozen signal is then converted into a digital value with a specific number of bits. The more bits that are used, the more accurate the digital representation. For example, a 12-bit converter provides approximately 5 mV of resolution on the torque command. This quantization effect needs to be considered when selecting a drive.
One of the main advantages of a digital drive system is the ease with which new features can be added. If it has sufficient processing power, additional functions can be added for only the cost of development. For example, consider the active damping feature first introduced on steppers. The analog drive circuitry required more than 100 components. Obviously, this had to be accounted for early in the design. When the active damping feature was implemented in the digital drive, it merely consisted of several lines of code. With sufficient processing power, features such as encoderless stall detection, which would be nearly impossible to do in analog, can be economically added to the drive.
Another advantage of a digital drive is the ease with which it is field upgradable. Using flash memory, new software can be posted on a Web site or e-mailed to customers. This lets new features and bug fixes be easily delivered to the field without sending the drive back to the factory. Custom software specifically tailored to a single customer's application is also possible.
Digital systems do have some shortcomings relative to analog ones, however. One major issue concerns the sampling delay. The controller sends out a torque command, but by the time the drive control algorithm receives the digital value, several hundred microseconds have passed. This sampling delay can reduce the overall bandwidth of a motion system. The sampling delay in the torque command also affects the current feedback. Digital systems use pulse-by-pulse current control, while the current is continuously monitored in an analog compensator. When the current reaches the maximum threshold, the voltage immediately shuts off. The duty cycle for a digital system is invariant and could allow the current to exceed the maximum value as shown in the figure. This can be a significant problem in low inductance systems where large current ripple exists and the rate of current change is approximately the bus voltage divided by the inductance.
Although analog drives are typically less expensive than digital ones, the controller often is integrated into the digital drive, which can reduce overall system cost. Analog drives are also easier to set up, with DIP switches being set rather than having to program the parameters so initial motion profiles can be generated quickly. However, digital drives offer more parameter variations, which lets system designers have more options.
This article was contributed by Scott Ellerthorpe, Senior Engineer, Parker Compumotor Corp., 5500 Business Park Dr., Rohnert Park, CA 94928, (800) 358-9068, www.parker.com