Motor mechanisms

This mechanism was designed around a single aluminium plate chassis which is secured in application by fasteners from the underside.

The mechanism moves on an off-the-shelf miniature linear slide rail. This was approach chosen primarily to reduce costs, but only after a careful round of engineering validation to prove that it had sufficient tolerance for the application needs.

In this design the rack and pinion provides motion to the Z-axis. A 0.5 module gear design is used, with a press fit for the pinion onto the output shaft of our standard 12mm spur gear motor design.

Meanwhile, the rack component is designed to serve other purposes such as a chassis for the backlash spring, as well as a light interrupting flag for the homing sensor on one end.

The mod 0.5 gear teeth are a compromise between additional precision and gearing offered by smaller mods, and the strength and robustness offered by larger mods.

In this mechanism both the X and Z axis have homing sensors, but the X-axis is most visible.

This is needed to create an repeatable positional zero point / datum for the motion controllers. In certain circumstances (e.g. power supply disruptions leading to unpredictable instrument shutdown / brown-outs), the current known position in a motion actuation may become corrupted.

Therefore upon start-up of the instrument, this mechanism will seek home and re-establish a safe motion control datum for all further motion control actuations. This internal calibration can also be commanded at any time from the instrument’s host processor / control panel.

In this design, back lash compensation in the form of a simple spring, was a very cost effective way to increase the mechanical positional reliability of the linear stage. By exerting a constant force on the sliding stage, backlash is 99% eliminated in 99% of usage scenarios.

This enables this design to use lower tolerance components which are cheaper to manufacture.

Motion controllers tell the motors when to move and by how much. As such, they need to know how far the motor has already moved, and how fast it is currently moving. This information or feedback, is provided by an integrated motor encoder.

In some examples we use light encoders, and in others we use magnetic ones. In this design, we have an simple cost effective AB hall sensor based magnetic encoder with 16 counts per revolution. With a 100:1 gearbox stage, that’s 1600 counts per rotation of the gearbox output shaft, which in this case is coupled directly to the rack pinion gear.  That’s more than enough accuracy to deliver 0.025mm accuracy (half the width of a human hair) to the X stage at a very reasonable price!

The ‘brain’ of the mechanism are these integrated motion controllers. Microcontrollers run optimised motion control code, with various control loop algorithms, safeguards and motion profilers.

We have a number of reference designs that we can use as a basis for projects like this, which delivers cost effective proven solutions, with a short development time.

In this example, the different circuit boards are inter-connected by flat flexible FFC cables.

In this mechanism, once positioned on the X-axis, the Z-axis is used to open a valve on a medical test cartridge. It must extend by a precise amount, and then retract, all with minimum amount of force, and within a specified speed.

In our design the fold-back spur gearbox drives a lead screw which then extends a sprung loaded tip. The distance traveled and speed of actuation are measured by an integrated motion controller, which feeds into the Z-axis motion controller circuitry.

The Z-axis motion controller is similar in design to the X-axis motion controller, albeit with different configuration parameters to enable smooth and accurate movement.

The Z-axis linear stage is assembled as a stand-alone unit which is then secured to the mechanism chassis with 3 fasteners.

The fold-back spur gearbox is a very useful design feature, to create a dimensionally compact linear actuator. In this case that style of motor is used for the Z-axis in this mechanism.

Using the fold-back gearbox reduces the height of this mechanism by 35%, compared to a straight line gearbox, which was critical for this particular application.

On top of the spur gearbox we have designed an application facing connector, which is another flat flexible FFC cable. This delivers power and SPI communication signals between the onboard motion controller processors, and the host application processor.

Our mechanism was fitted with a reliable AB optical encoder, to provide speed and positional feedback signals back to the motion controller. This design had no obvious index point, since the hub has to make multiple rotations. Therefore, a positional calibration technique was adopted where the motor was run slowly, and current measurements were used to detect an end of travel stall point.

We developed a 52mm coreless motor for this application, offering a high torque in a compact package. This design is also able to dissipate any motor heating that might arise from extended use of the lift.

The motor was designed with a double ended shaft. One end is fitted with a mod 1 pinion gear which meshes with the winding drum, and the other end connects to the electromagnetic brake and motion encoder.

The rear shaft from the motor is manufactured with a flat, creating a D-shaft that connects with the electromagnetic brake, fitted to the rear of the motor. This brake requires power to release, meaning that in the case of a power failure, the brake will engage automatically.

This is added to allow a failsafe mechanism, so that patients aren’t accidentally left to fall in the case of a power cut.

Connecting our assembly to the customer’s application is done electrically by the use of a heavy duty 8-way connector. The arrangement of pins within the connector was chosen carefully to minimise EMI cross-talk from the motor power lines to the encoder signals.

The front shaft of the motor is fitted with a mod1 steel pinion gear which interfaces with a large steel gear bolted to the side of the wining drum.

Running at target speed, and with appropriate lubricant (added during final assembly by the customer in this project) the resulting geartrain is efficient, quiet, and reliable. It will not need to be serviced again during the lifetime of the product.

The belt winding drum is manufactured from 3 pieces of turned aluminium and revolves around two internal bearings that sit on a steel shaft. The steel shaft is bolted to the enclosure / frame adding structural rigidity.

A mod1 gear is bolted to the side, and the belt is wound on, anchored in place with a bolt.

The belt runs over a plastic tube bearing sitting on another steel shaft which adds further structure to the enclosure / frame.

At the start of the project, the enclosure envelope was defined, and fixing point geometry agreed.

Through the development phase of project, the customer requested a series of features to be integrated into the enclosure frame. Most of these were locating features to aid final assembly of the lift, and a few others were used as points for routing the customers wiring looms.