Closed Loop Position Control

It is possible to image an entire coronary tree in less than 10 seconds, enabling routine coronary artery imaging to be performed for the first time.Original equipment manufacturers (OEMs) of computational tomography (CT) instruments have made dramatic advances over the past few decades, such as increases in detector breadth that enable larger areas of the body to be imaged in less time. At the same time, OEMs are trying to increase rotational speed of the gantries that carry the detectors in order to reduce imaging time.

Increasing rotational speed increases the importance of accurately controlling the position of the gantry, which needs to move at a constant speed so that capturing of images is at precisely spaced intervals around the patient. Manufacturers of CT equipment are achieving excellent image quality by moving to closed loop position control that continuously corrects the position of the sensor for velocity and positional errors. The latest generation of servo drives provides integrated control loops and versatile filters that make it possible to achieve higher rotational speeds while improving positional accuracy.


Advances in Technology
In early CT scanners, high voltage cables wrapped around rotating drums while pullets powered X-ray detectors. The rotating gantry made one slow revolution and then stopped, while the patient’s table moved forward by the slice distance. The gantry then rotated in the opposite direction for the next slice to avoid fouling the cables.

The next generation of CT scanners utilized power slip rings that allow the gantry to rotate continuously at higher speeds. The spiral or helical CT scanner moves the gantry, carrying the detectors in a circular path while the patient’s table moves axially, making it possible to image a longer cross-section of the patient’s body. In recent years, improvements have focused on increasing the number of slices, which refers to the number of detectors arrayed in a row to capture the image. Increases in the number of slices and in rotational speed reduce the time required for image acquisition – enabling users to get the same image, but with reduced X-ray dosage.

For example, a typical CT scanner covers the entire lung with 0.5mm slices in about 13 seconds, a period during which most patients are able to hold their breath and avoid moving. Quadrupling the slice technology makes it possible to cover a larger distance. Combined with higher rotational speeds, this makes it possible to image an entire coronary tree in less than 10 seconds, enabling routine coronary artery imaging to be performed for the first time.


The latest generation of servo drives, such as Kollmorgen’s AKD Servo Drive family, make it possible to achieve higher rotational speeds and improved positional accuracy.Positional Accuracy
The substantial increases in rotational speed required by the most recent generations of CT scanner technology have created major challenges for OEMs. Faster rotational speeds mean that acquisition of a complete series of images requires much less time than with previous generations of machines. Yet, the position at which each image acquired needs controlled to a higher level of accuracy than in the past in order to maintain the quality of the 3D picture of the organ after assembly of the individual images. Poor positional accuracy can lead to fuzziness or artifacts that can seriously degrade the quality of CT images, sometimes to the point of making them diagnostically unusable, or requiring an extensive amount of reconstruction with image enhancement programs. Users cannot re-expose patients to get better images.

The first few generations of CT scanners used AC induction motors to position imaging gantries. These motors have the advantage that their high inertia reduces the mismatch between the motor and the load. However, as OEMs increase the rotational speed and image quality of their instruments, they often face performance limitations of these motors. AC induction motors’ high inertia make them resist changes in velocity, which in turn makes it difficult to meet the positional accuracy requirements of the recent generations of CT scanners.

In a number of CT scanners, use of AC induction motors with velocity control is in an effort to improve positional accuracy. A velocity controller has a control scheme that mathematically determines the position from velocity data. Nevertheless, deterministic errors in the equations create the potential for the system to overshoot or undershoot the commanded trajectory.


Resolving Problems
Closing the position loop around the velocity loop resolves these problems through its far superior ability to control the position-time relationship. A permanent magnet servomotor replaces the AC induction motor, providing very high peak and continuous torques. The higher torques allow for greater acceleration, which enables the CT scan gantry to acquire images significantly faster. A key advantage of these motors is that torque is directly proportional to input current while speed links to input voltage.

With low inertia construction being inherent to the design of most permanent magnet servomotors, users must account for the mismatches between the high inertial loads of imaging gantries and the low loads of the motor. Tuning of servomotor control systems to handle inertia mismatches that are inherent in medical imaging systems is possible, but the coupling methods (often belt drives) used in these devices cause compliance or lost motion between the motor and load. Optimizing the gain in the servo amplifiers will help maximize response while avoiding instability and oscillations.

Considered out of control is a control system where the gain is -3dB or less, or the output phase is -45° or less from the control signal, or -135° relative to a reference from the motor. Closed loop stability problems can be predicted using the phase margin (PM) and gain margin (GM). PM is the difference of -180° and the phase of the open loop at the frequency where the gain is 0dB. GM is the negative of the gain of the open loop at the frequency where the phase crosses through -180°. The greater the variability of the load, the higher GM and PM need to be to ensure the stability of the control system.


Improvements
For example, when the resonant frequency is well below the first phase crossover (270Hz) the effect of the compliant load is to reduce the GM. If the inertia mismatch is five, the reduction of GM will be six or about 16dB. This means the gain of the compliantly coupled system would have to be reduced by 16dB, resulting in much poorer command and disturbance response. In recent years, servo control systems have provided far superior ability to compensate for inertial mismatches and compliant loads. These improvements have been because compliant mechanical systems typically have just a few resonant points prone to oscillations. The traditional approach is to use low-pass, band-pass, and high-pass filters to eliminate the unwanted frequencies. The problem with this approach is that the multiple filters that are required to eliminate the resonances introduce calculation delays and phase shifts, which have a tendency to throw the system out of control.

Recently, achievement of substantial improvements in performance is with biquadratic filters, which can emulate nearly any combination of simpler filters without introducing significant delays. The biquadratic filter tunes out problematic frequencies, making it possible to increase the PM and GM to optimize servo system performance. For example, if the mechanical system has a 200Hz resonance, the biquadratic filter configures to remove 200Hz while providing high levels of gain at the much lower control frequencies.

It is important to note that large belt-drive gantries have a strong physical roll-off that makes them act as a low-pass filter that cuts off everything above approximately 10Hz. By cutting the gain at 10Hz, while passing the velocity loop between 30Hz and 40Hz, the biquadratic filter makes it possible to increase the gain at the critical control frequencies in the 2Hz to 4Hz region.

The latest generation of servo drives, such as Kollmorgen’s AKD Servo Drive family, can maintain the position, velocity, and current loops within the drive while using fieldbus protocols for communication, eliminating the need for a separate motion controller in many applications, with multiple feedback devices supported. Sine encoders provide the highest level of accuracy, incremental encoders provide good accuracy at an economical price, and smart feedback devices provide a high level of ruggedness and smooth velocity transitions. These new servo drives provide plug-and-play operation with servomotors and feedback devices, making it possible to get optimized systems up and running quickly.

Overall, the use of drives with integrated control loops and biquadratic filters enable higher rotational speeds that reduce imaging time, in turn reducing the radiation exposure as well as the possibility of patient motion.


Kollmorgen
Radford, VA
kollmorgen.com