ADCs Lend Flexibility To Vector Motor-Control Applications

The performance of high-end servo drives is quickly approaching its limits due to the physical constraints imposed by high-resolution position acquisition. These limits are set by the need to use analog position-feedback signals to obtain the higher resolution needed. Furthermore, the motor-control industry in general is demanding higher levels of system integration from their silicon providers.

Responding to the challenge, these providers are pushing the limit of mixed-signal integration of high performance analog-to-digital converters (ADCs) on the same substrate as a high-performance digital signal processor (DSP). This push has led to the introduction of single-chip motorcontrol solutions that integrate successive approximation (SAR) or sigma-delta (Δ-Σ) converters, which in turn are only part of a range of single-chip DSP microcontrollers for ac motor-control applications.

However, before any comparative discussion of discrete simultaneous-sampling, SAR ADC solutions and an integrated Δ-Σ solution can begin, it is important to review what is required of a typical ac motor-control system. The discrete SAR and integrated Δ-Σ solutions, both of which demonstrate the versatility of ADCs, can then be discussed in the context of applications issues that arise when choosing one solution over the other in high-performance, variable-speed, motor-drive systems.

Types Of ADCs Used
A servo-control application will typically demand synchronous sampling of at least two motor currents. At the system level, this usually requires the use of sample-and-hold amplifiers, the outputs of which feed a multiplexer, which in turn is sampled with a single SAR ADC. In a discrete realization of such a system, the analog and digital portions can be isolated by using separate ground planes and separate supplies. With mixed-signal integration, the analog and digital portion must coexist on the same substrate. As a result, the high-frequency clocks, which emanate from the processor portion of the die, will couple into the analog portion of the device. There they will corrupt the information content of the measurement signal. The sample-and-hold amplifier, which is in essence a large capacitor (in die area, not value), is particularly susceptible to this substrate noise.

One solution to this mixed-signal integration problem is to remove the sample-and-hold amplifier and still satisfy the system requirement for simultaneous sampling by providing two SAR ADCs converting in parallel. A second alternative would be to reduce the conversion time of the ADC, say to less than 1 µs. By doing so, the hold time, and hence the capacitor size, can be reduced. This, in turn, will reduce the susceptibility of the analog portion to substrate noise.

A third approach is to use a fast ADC with a conversion time of less than 1 µs. Then, sample the current information serially, without using a sample-and-hold amplifier. Although this doesn't satisfy the system-level requirements for simultaneous sampling, many applications could tolerate the error associated with a delay of l µs between samples. The limitation here is the provision of greater than 11 bits of accuracy in less that 1 µs--for a reasonable cost. Today, this is no longer as big an issue due to the availability of cost-effective, 12-bit devices that operate at upwards of 60 Msamples/s.

A fourth approach is to use two ΔΣ converters in parallel. A successive-approximation ADC, by design, has a sample-and-hold amplifier on the input channel. As in the case of the system-level sample-and-hold amplifier, it too is susceptible to substrate noise. In this case, however, the hold time, and hence the size, of the capacitor is smaller. Nevertheless, this sample-and-hold amplifier is susceptible to substrate noise.

With a Δ-Σ converter, the sample-and-hold amplifier is an order of magnitude smaller again. Indeed, the size of the analog logic portion of the converter is an order of magnitude smaller than that of its successive-approximation counterpart. Thus, the overall susceptibility of a Δ-Σ converter to substrate noise is significantly lower than that of a successive-approximation converter. This article will focus on the two most common ADC architectures mentioned, SAR ADCs, and the Δ-Σ-ADC approach.

The Ac Motor-Control System
The typical motor-control signal chain requires a processor core and a generic set of peripheral function blocks to interface between the digital processor and the "real world" signals (Fig. 1). The basic blocks to interface to an ac motor power converter are a PWM generator and an analog-to-digital conversion system. There also are other peripherals required for real-time embedded control systems, such as a parallel digital I/O block, a serial communication interface, a watchdog timer, and event timers. The DSP microcontroller combines the powerful DSP core with the set of peripherals to complete the signal chain.

The control system has two loops--the motion loop handles the mechanical load and maintains rotary position and velocity, while the current loop handles the dynamics of the motor electrical system and controls torque production. Motion-control loops in variable-speed and servo-drive systems typically have bandwidths of the order of 20 or 30 Hz, with sample rates of 500 Hz to 3 kHz. This bandwidth can be handled by 8- or 16-bit microcontrollers. Typically in these systems, the current loops are implemented in the analog domain and the input signals to this domain are generated using digital-to-analog converters (DACs).

Recently introduced to the world of motion control, DSPs are high-speed microcomputers developed originally for such applications as telecommunications and speech processing. The high-speed signal-processing capability of these devices makes them well-suited for ac motor-control applications.

There are many reasons for moving toward a completely digital control system with DSPs at the core. The primary reason is that a digital system offers the most flexible control-system architecture. The switching signals for the three-phase power converter are digital rather than analog in nature. Though they may be produced by using analog comparators, they are just as easily generated using digital timing functions, which eliminates the requirement for a DAC for each current phase. The realization of completely digital control also reduces the susceptibility of the system to the noise sources associated with the power converter.

Typically, current-loop bandwidths are of the order of 1 to 2 kHz, requiring sample rates up to 20 kHz. This must be matched to the power-converter frequency, so high processing speeds are required. Although DSPs have the computing power to control high-bandwidth current loops, they require additional peripheral hardware to implement some of the motor-control peripheral functions--unlike some conventional microcontrollers. Often these functions have been implemented using standard components such as ADCs, and by using gate arrays or application-specific ICs (ASICs). However, depending on the application and the feedback resolution required, this may not be a cost-effective solution.

The performance of high-end servo drives is quickly approaching its limits due to the physical constraints imposed by high-resolution position acquisition. These limits are set by the need to use analog position-feedback signals to obtain the higher resolution needed. Furthermore, the motor-control industry in general is demanding higher levels of system integration from their silicon providers.

Responding to the challenge, these providers are pushing the limit of mixed-signal integration of high performance analog-to-digital converters (ADCs) on the same substrate as a high-performance digital signal processor (DSP). This push has led to the introduction of single-chip motorcontrol solutions that integrate successive approximation (SAR) or sigma-delta (Δ-Σ) converters, which in turn are only part of a range of single-chip DSP microcontrollers for ac motor-control applications.

However, before any comparative discussion of discrete simultaneous-sampling, SAR ADC solutions and an integrated Δ-Σ solution can begin, it is important to review what is required of a typical ac motor-control system. The discrete SAR and integrated Δ-Σ solutions, both of which demonstrate the versatility of ADCs, can then be discussed in the context of applications issues that arise when choosing one solution over the other in high-performance, variable-speed, motor-drive systems.

Types Of ADCs Used
A servo-control application will typically demand synchronous sampling of at least two motor currents. At the system level, this usually requires the use of sample-and-hold amplifiers, the outputs of which feed a multiplexer, which in turn is sampled with a single SAR ADC. In a discrete realization of such a system, the analog and digital portions can be isolated by using separate ground planes and separate supplies. With mixed-signal integration, the analog and digital portion must coexist on the same substrate. As a result, the high-frequency clocks, which emanate from the processor portion of the die, will couple into the analog portion of the device. There they will corrupt the information content of the measurement signal. The sample-and-hold amplifier, which is in essence a large capacitor (in die area, not value), is particularly susceptible to this substrate noise.

One solution to this mixed-signal integration problem is to remove the sample-and-hold amplifier and still satisfy the system requirement for simultaneous sampling by providing two SAR ADCs converting in parallel. A second alternative would be to reduce the conversion time of the ADC, say to less than 1 µs. By doing so, the hold time, and hence the capacitor size, can be reduced. This, in turn, will reduce the susceptibility of the analog portion to substrate noise.

A third approach is to use a fast ADC with a conversion time of less than 1 µs. Then, sample the current information serially, without using a sample-and-hold amplifier. Although this doesn't satisfy the system-level requirements for simultaneous sampling, many applications could tolerate the error associated with a delay of l µs between samples. The limitation here is the provision of greater than 11 bits of accuracy in less that 1 µs--for a reasonable cost. Today, this is no longer as big an issue due to the availability of cost-effective, 12-bit devices that operate at upwards of 60 Msamples/s.

A fourth approach is to use two ΔΣ converters in parallel. A successive-approximation ADC, by design, has a sample-and-hold amplifier on the input channel. As in the case of the system-level sample-and-hold amplifier, it too is susceptible to substrate noise. In this case, however, the hold time, and hence the size, of the capacitor is smaller. Nevertheless, this sample-and-hold amplifier is susceptible to substrate noise.

With a Δ-Σ converter, the sample-and-hold amplifier is an order of magnitude smaller again. Indeed, the size of the analog logic portion of the converter is an order of magnitude smaller than that of its successive-approximation counterpart. Thus, the overall susceptibility of a Δ-Σ converter to substrate noise is significantly lower than that of a successive-approximation converter. This article will focus on the two most common ADC architectures mentioned, SAR ADCs, and the Δ-Σ-ADC approach.

The Ac Motor-Control System
The typical motor-control signal chain requires a processor core and a generic set of peripheral function blocks to interface between the digital processor and the "real world" signals (Fig. 1). The basic blocks to interface to an ac motor power converter are a PWM generator and an analog-to-digital conversion system. There also are other peripherals required for real-time embedded control systems, such as a parallel digital I/O block, a serial communication interface, a watchdog timer, and event timers. The DSP microcontroller combines the powerful DSP core with the set of peripherals to complete the signal chain.

The control system has two loops--the motion loop handles the mechanical load and maintains rotary position and velocity, while the current loop handles the dynamics of the motor electrical system and controls torque production. Motion-control loops in variable-speed and servo-drive systems typically have bandwidths of the order of 20 or 30 Hz, with sample rates of 500 Hz to 3 kHz. This bandwidth can be handled by 8- or 16-bit microcontrollers. Typically in these systems, the current loops are implemented in the analog domain and the input signals to this domain are generated using digital-to-analog converters (DACs).

Recently introduced to the world of motion control, DSPs are high-speed microcomputers developed originally for such applications as telecommunications and speech processing. The high-speed signal-processing capability of these devices makes them well-suited for ac motor-control applications.

There are many reasons for moving toward a completely digital control system with DSPs at the core. The primary reason is that a digital system offers the most flexible control-system architecture. The switching signals for the three-phase power converter are digital rather than analog in nature. Though they may be produced by using analog comparators, they are just as easily generated using digital timing functions, which eliminates the requirement for a DAC for each current phase. The realization of completely digital control also reduces the susceptibility of the system to the noise sources associated with the power converter.

Typically, current-loop bandwidths are of the order of 1 to 2 kHz, requiring sample rates up to 20 kHz. This must be matched to the power-converter frequency, so high processing speeds are required. Although DSPs have the computing power to control high-bandwidth current loops, they require additional peripheral hardware to implement some of the motor-control peripheral functions--unlike some conventional microcontrollers. Often these functions have been implemented using standard components such as ADCs, and by using gate arrays or application-specific ICs (ASICs). However, depending on the application and the feedback resolution required, this may not be a cost-effective solution.