PDP Engineering Science, ADZ5100 Assignment – Computer Controlled Crane.Posted: May 1, 2014
We started the project by researching cranes in Cardiff bay, finding a rail based, boat lifting crane particularly useful. This crane had a lifting arm which was mounted on a rotating bed about half way up the tower (see blog for images). We also observed the workings of traditional construction cranes. From this point we were able to start sketching some basic ideas about how the mechanism could work, realising that it was likely we would require three motors to move the hook through three different planes. We agreed that the crane didn’t need to be unnecessarily tall, as we were lifting things along the same plane. Unnecessary height would only unstable the design.
At this stage we developed our mechanisms with some further sketching and model making. We established some basic guidelines, including the minimum arm length necessary to reach the lifting and dropping area. We followed this up with some 2D cad modelling, keeping the forms simple and a little larger than necessary, knowing that we would need to modify and adjust mechanisms and structures later. The key here was to give us the best platform to apply our iterative design skills to building the crane. We chose to remove circular pieces from the panels, as this would give us further material to build motor mounts, but without the detriment of losing strength in the structure.
Among the most challenging parts for us was developing a working code for the Arduino. Our original idea was to use H-Bridges to power motors which would then run gearboxes that we had laser cut ourselves. We found and modified codes, but struggled to understand how we would be able to power three H-Bridges with one Arduino. We then came across the idea of using three way switches, which we intended to use for two motors, leaving one H-Bridge (with superior control and a potentiometer link) to power the final motor. At this stage, we considered using a combination of gearboxes and rubber pulleys to ratio the drive.
As it transpired, this idea developed again. Later, when attempting to test our gearboxes, it became apparent that plastic cogs had warped during laser cutting which was causing jamming in the gearbox. Now at home, and without access to a laser cutter, I felt the best option was to switch to servo motors. I replaced all the gearboxes with servos which gave better torque and control. I developed a code and established the wiring. For a short period this worked, but after a period of testing the crane started to vibrate. I consulted the internet and found that this was either due to bad wiring, or a lack of power. As a result I used capacitors and reduced the number of servos to two, with the intention of using a second Arduino to power the final servo.
We were now back in Cardiff, and we carried out some testing. At this stage we realised that a 180 degree servo did not provide enough rotation for the winch. We decided at this stage to apply a recycled dynamo-radio gearbox and a standard motor to the body of the crane, to serve as a winch. We applied a drum directly to the mechanism to take in the winch line. This worked very effectively with a three way switch, and was strong enough to lift the crane arm alone.
All components on the crane have either been made from scratch using the laser cutter, or have been recycled from other products. The baring between our arm cabin and base are taken from a recycled drawing board. The pulley system which rotates this cabin from the base uses recycled copper wire and recycled lollypop sticks which work very effectively. The gearboxes have all been recycled from various mechanical products. The base has a recycled ice-cream box filled with gravel, which simulates small rocks considering the scale. This could be replaced with water or sand to provide balancing weight. We chose to use plywood as this didn’t warp as badly on the bed of the laser cutter and provided good strength. The arm was made of two identical pieces with spacers to provide a gap for the winch line.
Having got to a stage where our crane was working, following a fair amount of testing, we then found ourselves with some major issues with one of our servos. I had decided the arm should be mounted directly onto the servo, however this servo was now having issues and as a result was failing to lift. We removed the component and tested it externally and it still had the same issues. We considered how we might be able to modify this component and agreed that we would remove this servo mechanism and replace it. At this stage we considered whether we might be better developing a system more similar to the construction cranes with a stationary arm, as opposed to a lifting arm, however, due to the limited time remaining and a lack of knowledge of this mechanism we decided to stick with our lifting arm idea.
Instead, we designed a spool on the side of the cabin of the crane, and positioned a servo lower on the cabin with a large lollipop arm. To the end of this arm, we attached a cable which then ran around the spool and then back around and attached to the tip of the arm. This was effective, as it removed the continual stress from the servo as it had originally when the arm had been attached directly to the servo. We also decided to use two Arduinos as we had the knowledge to wire these and ensure that enough power could be applied to operate both.
Throughout the development period of the crane, we faced issues with friction and weight. Overcoming these by removing material where necessary. If I were to work on a project like this again, I would definitely spend more time planning the process, developing 3D CAD models to better develop the arrangement of the structure. During this project I’ve learnt many new things which I feel I can apply to my product design practice. I’ve learnt how to use an Arduino to make working models and prototypes, I’ve gained experience in using basic electronics, I’ve learnt some principles of mechanical design which I can apply in future, these include developing gears, pulleys and winches.