Camera position can make or break a shot. With users ranging from bloggers to entertainment channels, to scientists and engineers, the video market has need for accurate and repeatable positioning, but at a fraction of the typical budget. Camera support system specialist Kessler Crane puts the focus on developing affordable positioning systems that still give users the power to create the shots they need.
The positioning systems of Kessler Crane camers must be able to make quick 180 degree pans or slowly turn from 15 to 20 degrees.
The positioning system needs to make lightning quick 180 degree pans to track a hockey puck speeding over the ice, or to slowly turn 15 to 20 degrees over a row of plants. It has to provide a broad range of motion while operating with the lowest possible audible noise—and at the lowest possible price.
To balance these demands, the company offers two types of motion systems for video cameras: a pan-tilt unit called the Revolution and a narrow-rail automatic-dolly system known as the CineSlider.
The pan-tilt unit consists of a two-axis system powered by M32P gear motors from MicroMo’s Micro-Drives line. With a 516:1 reduction ratio, the gear motors deliver 4 Nm of torque. To achieve the extreme range of motion, the engineers built the unit with a hollow axel, running the wires up through it. That eliminated windup and allowed the Revolution to pan beyond 360 degrees in each direction.
Velocity varies from one revolution in 3.8 s to one revolution in 10 min 40 s. The slider features a 4 to 5 in. wide rail system that mounts on top of a tripod and typically runs 3 to 5 ft. The automatic version combines another M32P gear motor with Kessler Crane’s Oracle controller to command the unit in real time by joystick. Velocity varies from 12 in./s to 12 in./3+ years in time-lapse mode.
The initial design goal was to produce a system capable of ultralow-speed motion. The problem is that at very low speeds, most motors experience cogging. After looking around at their options, the team chose a Micromo motor with a five-segment commutator rather than a three-segment commutator design, which helped minimize the effects of cogging. That took care of low-end velocity, but the same motor also needed to follow a speeding hockey puck. Getting sufficiently high-velocity operation remained a challenge until engineers tried to briefly and intermittently overdrive the 12 V motor at 24 V. Because the system gets a boost for only 10 or 15 seconds at a time, and only when the load is already moving, the technique does not significantly degrade motor lifetime, especially for the kind of low duty cycles of video applications. “We tested to make sure they would survive,” says the design engineer. “We ran one motor at 36 V for an hour. That’s equivalent to years of use in this product.”
Another key requirement was backlash-free motion. Instead of a single expensive, high-performance motor, they chose a pair of MicroMo’s dc micro motors set to run slightly opposed in a master-slave configuration. When properly coupled together, with the aid of fairly sophisticated software, the slave motor pulls the slack from the system.
The other benefit of the dual-motor design is that it offers complete lockup braking power. Even small video cameras are heavy, especially if the camera isn’t placed over the mount’s center of gravity. When the system is powered down, the weight of the camera could cause it to drift. With the dual-motor design, the same software that drives the slave motor to pull the slack from the primary can direct it to help the primary motor to resist the load.
Robust, consistent dc gearmotors (the black cylinders) let the Revolution pan-tilt head record and repeat camera moves without an encoder.
Photo courtesy of Kessler Crane Inc.
Another challenge was how to get repeatable motion without an encoder. The engineering team decided to try to leverage the effect of energized motor windings turning the motor a specific amount to achieve recordable motion. In order for the approach to be predictable, though, the relationship between energy in and displacement out had to be consistent. In tests, the Kessler Crane group discovered that while the low-end motors varied from unit to unit, for a given motor and load, response was quite repeatable.
The next challenge was to make the system work for time-lapse recording. This time, the team focused on the correlation between the number of times they pulsed the motor and how far it turned. Using a calibration routine, they eliminated variation introduced by load, camera positioning, and environmental factors. Here, again, motor repeatability provided a crucial benefit. “If you give the motor an amount of energy, an amount of work comes out,” says the engineer. “We found that to be pretty consistent.”
Future plans for Kessler Crane include interfacing external motor drives with the camera focus rings to allow automatic focus, simplifying one more aspect of the video filming process. They’ll be using MicroMo motors on those systems, as well.
Filed Under: Motion control • motor controls, Motors (gearmotors), Vision • machine vision • cameras + lenses • frame grabbers • optical filters
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