Silent Sentinel - CCTV industry embraces robotic technologies

CCTV industry embraces robotic technologies

As lens and camera performance improve the distance a CCTV system can perform over becomes further and further. However, the limiting factor was the PTZ unit and its ability to move the camera precisely without bounce, jolts or any other form of backlash when tracking a target at long distance.

As lens and camera performance improve the distance a CCTV system can perform over becomes further and further. However, the limiting factor was the PTZ unit and its ability to move the camera precisely without bounce, jolts or any other form of backlash when tracking a target at long distance. This problem has now been overcome with the adoption of Harmonic Drive gear mechanisms taken straight from the robotics industry in units such as the Osiris Harmonic from Silent Sentinel.

But what is a Harmonic drive – also known as Strain Wave Gearing?

Harmonic Drives are a special type of mechanical gear system that can improve certain characteristics compared to traditional gearing systems such as Helical Gears or Planetary Gears. The advantages include: no backlash, compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and excellent repeatability (linear representation) when repositioning inertial loads, high torque capability, and coaxial input and output shaft. They are used widely in industrial motion control, machine tool, printing machine, robotics and aerospace.

The strain wave gearing theory is based on elastic dynamics and utilises the flexibility of metal. The mechanism has three basic components: a wave generator, a flex spline, and a circular spline. More complex versions have a fourth component normally used to shorten the overall length or to increase the gear reduction within a smaller diameter, but still follow the same basic principles.

The wave generator is made up of two separate parts: an elliptical disk called a wave generator plug and an outer ball bearing. The gear plug is inserted into the bearing, giving the bearing an elliptical shape as well. The flex spline is like a shallow cup. The sides of the spline are very thin, but the bottom is thick and rigid. This results in significant flexibility of the walls at the open end due to the thin wall, but in the closed side being quite rigid and able to be tightly secured (to a shaft, for example). Teeth are positioned radially around the outside of the flex spline. The flex spline fits tightly over the wave generator plug rotates, the flex spline teeth which are meshed with those of the circular spline change. The major axis of the flex spline actually rotates with wave generator, so the points where the teeth mesh revolve around the centre point at the same rate as the wave generator. The key to the design of the harmonic drive is that there are fewer teeth (for example, two fewer) on the flex spline than there are on the circular spline. This means that for every full rotation of the wave generator, the flex spline would be required to rotate a slight amount (two teeth, for example) backward relative to the circular spline. Thus the rotation action of the wave generator results in a much slower rotation of the flex spline in the opposite direction. For a strain wave gearing mechanism, the gearing reduction ratio can be calculated from the number of teeth on each gear. For example, if there are 202 teeth on the circular spline and 200 on the flex spline, the reduction ratio is (200 − 202)/200 = −0.01 Thus the flex spline spins at 1/100 the speed of the wave generator plug and in the opposite direction.

This allows different reduction ratios to be set without changing the mechanism’s shape, increasing its weight, or adding stages. The range of possible gear ratios is limited by tooth size limits for a given configuration. That is the technical explanation although Paul Elsey, Managing Director of Silent Sentinel, is quick to point out “As engineers we always like to have a good understanding of how things work, although it is not necessary to have any specialist knowledge to use our new Harmonic Drive versions in the Osiris range. Everything comes fully configured so it can be installed as easily as the standard geared versions and support the same broad range of protocols and camera assembly options. But the performance improvements are instantly noticeable.”