In last month's “Camera lens image stabilization: Part 1” article, we examined the principles of Canon's variable-angle prism image-stabilization (VAP-IS) technology. The basis of the technology is to restore the image — in as close to real time as possible — to its correct spatial location on the camera image sensor system, thus delivering sharp images in portable HD lenses. This month, we take a more in-depth look at VAP-IS and its use in Canon's HJ15ex8.5B HDTV portable lens.
The Canon implementation of VAP-IS technology places a variable-wedge prism at the optical input port of the lens system — directly in front of the focusing element group. Thus, the prism directly intercepts all of the light rays passing through the “entry pupil” of the lens.
Figure 1 shows a simplified rendition of the construction of the VAP-IS system. Three key attributes constitute the heart of the prism being made variable in two dimensions:
- Two pieces of precisely flat circular glass joined by a flexible bellows
- A high refractive index liquid that is hermetically sealed within the enclosure
- The two rigid circular frames supporting the flexible bellows
Two mechanical actuating systems grip the VAP-IS circular metal support, and they, in turn, are controlled by fast-acting yaw and pitch actuators. This provides the appropriate squeezing of the prism in the horizontal and vertical direction that implements the requisite prism angle. See Figure 2.
Technology of the vari-angle prism
Key technologies in the VAP-IS system are:
- The special liquid hermetically sealed within the enclosure having the requisite high-refractive index. It maintains that optical functionality over a broad lens operating temperature range of -22° F to +176° F.
- A proprietary means of filling the prism with the liquid and its subsequent hermetic sealing.
- Special multilayer plastic material used for the prism bellows. This also sustains full operability over that temperature range while maintaining full pliability over the tens of millions of operational cycles anticipated over the normal life of a broadcast lens.
The effectiveness of the correction is obviously dependent upon the rapidity with which the VAP-IS system intercepts and redirects incoming light rays. Real-time control is the ultimate goal.
Three core miniature components inside the lens itself are central to the control system:
- Motion/vibration sensors
- Rotary actuators. Rotary actuators that physically manipulate the variable-angle prism are a magneto-electrical-mechanical system. They use miniature Voice Coil Motor (VCM) technology. These are direct-drive devices based on current-carrying coil windings lying within a permanent magnet field that produces a physical force that is directly proportional to the applied current. The consequent movement of the coil drives the two actuators that squeeze the prism either horizontally, vertically or both — depending upon the microcomputer control decisions.
- Microcomputer and associated specialized algorithms
Control loop for the VAP-IS system
Figure 3 shows a simplified representation of the stabilization control loop. Two motion detectors — one for horizontal movement and a second for vertical movements — are positioned within the body of the lens. Their electronic outputs are fed to the system microcomputer, where appropriate lens-motion analysis is made. The microcomputer computes a correcting control signal. That signal feeds driver circuits that in turn manipulate two actuator systems that physically alter the variable wedge prism in either a horizontal or vertical direction, or one that combines both. These actuators employ VCM technology.
The position sensor that monitors the prism movement sends feedback signals to the microcomputer, thus closing a feedback loop having high speed and a high degree of precision.
Technology of the VCM actuator
The coil is mounted on a non-magnetic arm and is free to move within the surrounding magnetic housing. A special wedge-shaped coil having an armature that pivots will rotate the VAP-IS's supporting mechanical unit. One VCM applies yaw rotation, and the second applies pitch rotation. When the direction of the current is switched, the direction of the coil's movement will also change. Using a coil of low inductance, this makes possible cycle times that are typically an order of magnitude faster than solenoid devices. This is essential to dealing with lens vibrational disturbances.
The key advantages of this VCM control are:
- Higher force compared to stepper or servomotor systems
- Higher acceleration rates than stepper or servomotors
- Direct drive. The absence of gears and cogs precludes backlash issue.
- Zero hysteresis. This is advantageous when rapid change in direction is required.
- Low acoustic noise
- Low heat generation. What little heat is produced is a factor of the resistance of the coil and a small amount due to friction.
A central part of the control loop design was development of a very fast-acting system for flexing the VAP-IS system. The principles of the mechanical actuators that do this are shown in Figure 4.
Digital feedback control loop
When Canon developed the first broadcast standard-definition portable lenses with VAP-IS in the mid-1990s, it employed a sophisticated analog feedback control loop. This worked well. Over the next decade, digital and microcomputer technologies advanced that facilitated the development of a far more sophisticated all-digital feedback control system for the new HJ15ex8.5B high-definition lens. The basics of this digital control system are outlined in Figure 5. This digital feedback control loop achieves a 20-fold increase in correction speed over that of its analog predecessor.
Figure 6 shows the measured performance achieved within the HJ15ex8.5B portable lens. This graph assumes physical disturbances (jolts, hand tremors or vibrations) that, uncorrected, would produce an image shift that is normalized to 100 percent. The plotted curve represents measured residual image shift following VAP-IS correction. As illustrated, over most of the frequency range, the level of correction is in the neighborhood of 100:1.
The frequency range shown encompasses handheld and shoulder-mount operations with the camera operator walking or running (generally in the 1Hz to 4Hz range), tripod mounted on an unstable platforms or under high-wind conditions (typically in the 3Hz to 6Hz region), and shooting from a car, motorcycle pillion seat, helicopter or boat (typically 5Hz to 12Hz).
The HJ15ex8.5B is the first HDTV portable production lens having a built-in optical stabilization based upon Canon's VAP-IS technology. It is a compact lens weighing only 4.4lbs. Advances on a number of core technological fronts have steered various refinements, producing a fast-acting and precision control feedback loop for a novel optical stabilization technology.
Larry Thorpe is the national marketing executive of the broadcast and communications division of Canon.