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.

Simultaneous-Sampling ADCs The current drawn by a motor can be split into two components--one produces torque, and the other produces magnetic flux. For optimal performance of the motor, these two components should be controlled independently.

In conventional methods of controlling a three-phase motor, the current (or voltage) supplied to the motor, and the frequency of the drive, are the basic control variables. However, both the torque and flux are functions of current (or voltage) and frequency. This coupling effect can reduce the performance of the motor. For example, if the torque is increased by increasing the frequency, the flux tends to decrease.

Vector control of an ac motor involves controlling phase in addition to drive and current frequency. Controlling the phase of the motor requires feedback information on the position of the rotor relative to the rotating magnetic field in the motor. Using this information, a vector controller mathematically transforms the three-phase drive currents into separate torque and flux components.

A typical dual-channel, simultaneous-sampling ADC (AD7862) for vector-controller applications is shown in circuit (Fig. 2). Along with simultaneous-sampling capability, the high-speed, low-power, 12-bit ADC has two 4-µs successive-approximation ADCs and two track/hold amplifiers.

There are four analog inputs that are grouped into two channels (A and B). Each channel has two inputs (VA1 and VA2, and VB1 and VB2) that can be sampled simultaneously, thus preserving the relative phase information of the signals on both analog inputs. The critical track/hold specifications for preserving the relative phase information are the -3-dB small-signal bandwidth (in this case, 3 MHz), aperture delay (20 ns), aperture jitter (100 ps), and the aperture delay matching (200 ps). Designed for motor-control applications, the ADC under discussion also accepts bipolar analog input ranges of ±10 V, ±2.5 V, and 0 to 2.5 V, while operating from a single 5-V supply.

The position of the field is derived by determining the current in each phase of the motor. Only two phase currents need to be measured as the third can be calculated if two phases are known. Simultaneous sampling inputs VA1 and VA2 of the ADC are used to digitize this information.

Simultaneous sampling is critical if the relative phase information between the two channels is to be maintained. A current-sensing isolation amplifier, transformer or Hall-effect sensor is used between the motor and the ADC. Rotor information is estimated from voltage measurements from two motor windings. The other two simultaneous inputs of the ADC, VB1 and VB2, are used to obtain this information. Once again, the relative phase of the two channels is important. A DSP microprocessor is then used to perform the mathematical transformations and control-loop calculations on the information fed back by the ADC.

Sigma-Delta (Δ-Σ) ADCs The Δ-Σ converters discussed in this section are part of a one-chip motor control solution, the ADMC300, variations of which are available from a number of manufacturers. Δ-Σ converter technology is based on an oversampling technique. The input signal is typically sampled with a one-bit converter (a modulator) at high frequencies, typically 1 to 2 MHz (Fig. 3). The resulting high-frequency bit stream is digitally filtered and decimated (down sampled) to the effective sample rate.

This converter technology is well-suited to a bulk CMOS process, and is also very conducive to mixed-signal integration. The Δ-Σ converter does, however, introduce additional system-level design constraints that will be discussed later on in this article.

Typically, the sampling of a current waveform is synchronized with the pulse-wave-modulated (PWM) waveform to reduce the effect of PWM ripple on the sampled data. In doing so, each of the sideband harmonics are symmetric about the PWM switching-frequency harmonic. By sampling at the PWM switching frequency, all the sideband harmonics sum to dc. By sampling at the zero mean point of the current ripple, the sideband harmonics sum to zero. This point aligns with the synchronization pulse of a center-based PWM system (Fig. 4). The frequency spectrum of a current waveform for a single-phase system is shown. The figure depicts the fundamental, first, second, third, and fourth PWM ripple harmonics.

With the Δ-Σ converter, the data is modulated (sampled) at 1 to 2 MHz. Therefore, multiple harmonics of the PWM ripple are sampled. The Δ-Σ converter architecture that is part of the ADMC300 one-chip motor control solution was analyzed to generate the spectrum of the current waveform as well as the spectrum of the remodulated current waveform, respectively.

It is apparent that the PWM harmonics are indeed sampled. However, by choosing a modulation frequency equal to the decimation factor times the PWM frequency, the sampled PWM harmonics are aliased to dc. As is the case in the successive-approximation approach, the PWM harmonic content of the original current waveform are not present in the sampled spectrum.

The synchronizing of the modulation frequency with the PWM sample frequency is key to the effective use of a Δ-Σ in a motor application. Not doing so would require the use of antialiasing filters to remove the harmonic content before the current information could be sampled. Typically, this synchronizing is not an option with discrete implementations. However, with the integration of a Δ-Σ converter and a PWM onto the one device (ADMC300), the necessary sampling conditions can be satisfied.

A successive-approximation ADC is typically modeled as a finite delay that is attributed to the conversion time of the particular device. A Δ-Σ converter, on the other hand, is a dynamic system and must be modeled as such.

In a motor-control application, these dynamic characteristics have to be accounted for in the design of the control system. Failing to account for the dynamics of the Δ-Σ converter in the feedback path of a closed-loop system will impact the robustness of the resulting controller. Both the modulator and decimator blocks exhibit a dynamic behavior. The dynamics of the decimator portion, however, dominates the dynamic response of the converter. The decimation filter is described as Figure 5.

where D denotes the decimation factor and G(z) is the transfer function of the filter. The value "z" corresponds to a sampling delay (i.e. for a set data samples x(0) ...x(n) ...where x(x) is the most recent sample, then z x x(n) = x(n-1).

In the case of the ADMC300, the decimation factor is 64 and the modulation frequency is 64 x PWM frequency. The phase response of the decimation filter for a 10-kHz PWM switching frequency is shown. Typically, the PWM switching frequency is a decade above the closed-loop bandwidth; therefore, the large-signal harmonic will be typically less than l kHz.

For example, a 600-Hz-fundamental current waveform would have a phase delay in the order of 30°. In some applications, this phase delay may be unacceptable, in which case the Δ-Σ can be configured to sample the current waveform at a multiple of the PWM frequency, and the phase delay can be reduced by the oversampling factor. However, oversampling the current waveform at a multiple of the PWM switching frequency will introduce additional PWM harmonics into the passband. These harmonics can be removed by a DSP with a notch filter.

Indeed, the integration of multiple Δ-Σ converters, a center-based PWM scheme, a 25-MIP DSP core, and other peripherals such as encoder interfaces and event timers, provides the motor-controller designer with a very powerful platform to implement sophisticated control schemes which traditionally would not have been at their disposal because of cost and design constraints.

References

1. Murray, Aengus; Kettle, Paul; Moynihan, Finbarr; Margio, Alessandra, Advances in DSP Based Motor Control Solutions, Intelligent Motion, Proceedings, June 1997.

2. AD7862 Datasheet, 10/96, Analog Devices Inc., Norwood, Mass., pp 14 to 15.