We are currently designing a motorized zoom lens system to be integrated into an Acquisition, Tracking, and Pointing (ATP) bench setup. This lens has some challenging requirements, including, but not limited to the following:

• Entrance pupil is located far out in front of the zoom lens
• Over 7.5x zoom range is desired
• Diffraction Limited imaging over a very large wavelength range (>600nm)
• Boresight shift over the entire zoom range <0.5mrad
• Zoom speed over the entire range must be <2 seconds

We created a preliminary optical design for this system and it is shown below:

Our design above uses three lens groups, each of which much be moved separately during zooming. The design features relatively short lens movements, which is a critical feature in view of the very tight decenter and zoom speed requirements. The graph below shows the amount of movement required for each of the lens groups to achieve the entire range of zoom settings.

We see in the chart above that the largest and most massive lens group (Lens Group 1) has a total travel distance of approximately 35mm, while the lens group that is required to move the largest distance (Lens Group 3) has the smallest mass. Again, these are important features that allow our systems to meet very stringent design requirements for centration during zoom.

For the opto-mechanical design of this system, we followed the design and fabrication principles used for the cinematography and military industry, as referenced in various open-literature publications by Paul Yoder, Iain Neil, Ellis Betensky, and Jacques Angenieux. The design techniques used in the production of zoom lenses for high-demand applications utilize precision bearings (instead of bushings) for the rotating components: optical components ride on rollers that can be adjusted to remove excessive play throughout their motion. In our experience, we have found that these design techniques, when applied properly, yield a very robust system.

Each lens group is mounted on its own carriage that rides on spring loaded precision rollers inside the main housing. Although they are significantly more expensive than bushings, there are two compelling reasons for using rollers for this zoom lens system. First, rollers reduce "stiction" of the assembly thus requiring less power to drive. Second, the use of rollers reduces image jump (lens decenter or boresight drift) when changing zoom settings. In conventional consumer-grade zoom lens construction, this image jump is due to excessive clearances between the carriage OD and the housing ID. Our zoom lens design solves this problem with the use of carriages and rollers for each lens group.

The carriages are actuated by helical cam profiles cut in a cylinder. The profiles are unique to each lens group and are derived from the graphs shown above. The specific cam profiles were laid out in SolidWorks to produce a manufacturable part. Cam followers attached to the carriages ride in the profiles on the cylinder and are kept from rotating by slots in the main housing that are parallel to the optical axis. The cylindrical cam has a ring gear mounted to it, which is driven by a motor. The cylindrical cam also has a bearing at each end of the cylinder for reduced friction and more precise motion control. Contrast chose a motor of sufficient power to drive the zoom at very fast rates. Limit switches are placed at the minimum and maximum zoom positions for each group, in order to provide positive end-stops.

The electronics necessary to control the motion components in this system include the following:

• Switching power supply that accepts a specified voltage and creates a cleaned-up voltage needed to operate the various motors and controllers of the lens system 
• Multi-channel motor driver that provides a current-limited, pulse-width modulated (PWM) means of controlling the motors with the processor. 
• Encoder interface that allows the processor to monitor the encoder position so the motors can operate "closed-loop" 
• Digital communications that counts pulses sent to the motors in a logic-based (CPLD) layer, required for high-speed operation 
• Control processor that accepts serial commands from external control software, performs PID trajectory control and reports the system status. 

Learn more about Contrast's electronic and controller capabilities here.

 

 

SUMMARY

We have shown an example of one of the many zoom lens systems we have designed for various clients/applications. Contact us to learn more about how Contrast Optical can solve your optical design and opto-mechanical design problems.