Cruise control

There are various means of achieving linear displacement, with most of today’s major developments being in electro-mechanical actuators.

This method involves the conversion of rotary motion into equivalent linear displacement. Typically, a rotary driver, such as an electric or hydraulic motor, is mechanically connected to a long shaft — the screw.

The rotary motion of the electric motor will rotate the screw shaft which has a continuous helical thread machined on its circumference running along the length. These are called ball grooves because there are spherical balls that roll in this groove. The screw is inserted into another mechanical body, the ball-nut.

This also has similar helical thread machined on its inside diameter, plus a built in system to re-circulate the balls. These are trapped in the mating ball grooves on the screw shaft and ball-nut and convert the rotary motion of the screw shaft into linear displacement of the ball-nut.

When a major cruise ship line contacted Macron Dynamics about designing and installing a telescoping stage aboard two of its ships’ nightclubs, the company’s engineers were sure they could design a system to meet the specifications. There was, however, a catch — to eliminate downtime, the system had to be installed while the ships were at sea.

The nightclubs were to house a dance floor across which a telescoping stage would move, carrying performers and musical equipment. In addition, the dance floor had to remain flat and free from obstructions that could interfere with its normal use.

‘This project presented a number of challenges,’ said Macron’s national sales manager Joe Baird. ‘The logistics were the biggest hurdle our engineers had to overcome.

‘Of course, the added perk of performing the installation while the ship was bound for a sunny Pacific destination didn’t hurt the enthusiasm of our engineers.’

Even before the installation, they were faced with the challenge of designing the system without ever having set foot aboard either of the ships. ‘Since the vessels were continually operating cruises out of ports on America’s west coast, we weren’t able to see the nightclub space first-hand until boarding the first ship to perform the installation,’ said Baird.

Working with a general contractor who was hired by the cruise line to carry out the electrical installations and necessary on-site construction, Macron’s engineers began designing a system to conform to the required specifications at its Pennsylvania facility in the US.

The contractor was able to board the ships prior to the design process, gather general information, and relay that information to the engineers. It was decided to design the motion components of the stage using a modified version of its 14-Z belt-driven linear actuator — a telescoping actuator that uses a stationary motor, and is usually called on for high-speed vertical motion and medium load applications.

The flat belt, which is routed through the actuator’s drive system, would allow the system to be installed flush with the dance floor without interfering with the safety of the guests. Baird said that the idea was to modify the actuator’s drive system to accomplish two goals.

The first was to eliminate the continuous loop belt system. ‘By modifying the actuator to use the timing belt — which has one smooth side and another with teeth — in a pulley-type fashion the belt could be flush with the dance floor,’ explained Baird.

The second goal was to route the belt through the actuator, which was to be concealed under the stage. ‘To accomplish this, a special mechanism was designed to lift the belt out of the floor as the stage passed over that section of belt, feed it through the actuator drive system, and then place the belt back into the floor as the stage continued on,’ said Baird.

With the design complete, Macron began to assemble as many components as possible in Pennsylvania. When the time came to install the system on the telescoping stage, the assembled components and tools necessary to carry out the work were shipped to the west coast ports and the waiting vessels. Once aboard, the week-long installation went smoothly.

Meanwhile, in a different development, SMAC has developed what is believed to be the world’s fastest and most precise three-axis pick-and-place system. The technology, developed for a multinational Japanese manufacturer of luxury goods, was required to both significantly increase product output and improve the flexibility of the manufacturing process.

And while the need for speed was paramount, the requirement for precision, accuracy and repeatability was even more demanding.

The speed, position and force are fully and independently programmable — all at the same time. The customer required the picking and placing of a number of different-sized, high-value micro components. The weight of the components is only a few grams and they are extremely delicate, making them difficult to handle. The X axis of the system has a stroke of 50mm, while the Z axis 30mm and the Theta (rotary) axis of 360º.

The system is required to use the Z axis actuator to pick up the micro component with vacuum using SMAC’s patented Softland function before releasing the vacuum and lifting off to ensure the component has not been damaged during pick-up. Then it is required to rotate the component 180º to ensure it is in the correct position for placing.

The X axis then moves 100 mm into position and the Z axis places the component — again using the Softland function.

Both the start and target positions are claimed to guarantee the correct placement of the part. The system then returns to the start position and repeats, with a complete cycle time of under 300ms. Integrating force and position sensors into a single direct drive device represents a major step forward in terms of speed and performance and has set new standards in high-speed pick-and-place factory automation.

The shaft run-out on the z axis actuator was required to be under 10 microns, as was the overshoot on this axis as well as the settlement (to pick the component).

The same demanding specification was also required of the return cycle. On the Y axis actuator (50mm stroke) overshoot was allowed and an accuracy of 10 microns on the settlement of the actuator. The rotary or Theta axis integrated into the z axis actuator had to be accurate to two counts on an encoder with a resolution of micron. This was necessary for settlement when flipping the component 180º.

One of the key features of the system is that by design it has inherent positioning and feedback capability — a closed loop system that ensures no external sensors or switches are required. This reduces the amount of external wiring required and facilitates both quick installation and 100 per cent diagnostic feedback and data.

The pick-and-place system integrated two Multipole Moving Coil Actuators (MCA) one of which was a two-axis rotary unit. The Y axis unit has a stroke of 50mm, the Z unit a stroke of 30mm and there is an in-built rotary axis of 180º.

The other main method of linear displacement is by using hydraulic actuators. These typically involve a hollow cylinder into which a piston is inserted. The two sides of the piston are alternately pressurised and de-pressurised to achieve controlled precise linear displacement of the piston, and whatever is connected to it. The physical linear displacement is only along the piston/cylinder axis.