Controlling Movement with Reciprocating Motion
Daniel Olbrys
ECE 480 – Design Team 5
ABSTRACT
Most sources of motion are rotational, meaning they revolve along an axis. There are commercially produced linear motion, however they are typically expensive and are limited in distance. Since there are several applications in which linear motion is desired and commercially available options are not suited for the particular application, it is necessary to convert rotational motion into linear motion. This process is called reciprocation motion.
KEYWORDS Servo, Rack, Pinion, Reciprocating Motion, Lead Screw,
INTRODUCTION
Servo motors are fairly cheap in comparison to competing stepper motors and linear actuators. They also offer additional freedom of movement that cannot be easily achieved with commercially available linear actuators. They tend to operate faster than linear actuators and are a suitable candidate if the rotational motion is converted into linear motion.
Our application requires extremely precise movement in one direction. The distance travelled must be consistent each time, otherwise difficulties will arise with raising pins such as the wrong pin being raise or even missing the pin completely. This motion must be converted so that the change in motion is a linear relationship from the rotation distance of the servo motor to the linear motion. For this two types of reciprocating motion were select: rack and pinion, and lead screw design.
RACK AND PINION
There is a rather simple approach to converting the rotational motion called a rack and pinion. This design uses a gear of a specific pitch that is attached to the end of the motor and rotates along an axis. This gear mates with a gear rack, which is a long substrate with the same gear teeth on one edge. Since the motor and gear are fixed to one location, when the gear turns, the rack moves instead and slides along a tangential axis to the gear. This design is extremely useful since its speed can be adjusted by the size of the gear and the length travelled can be customized by adjusting the size of the rack.
Benefits:
· Versatile, gear racks are easily adjustable to make them both longer and shorter
· Easy to implement, less moving parts leads to a more robust design.
Disadvantages:
· Requires a lot of space, inflexible gear rack may be difficult to design around
· Finding the perfect gear takes a lot of time and estimation, also becomes difficult to design around if too large.
LEAD SCREW
To enable precise movement along an axis that must carry a large load, research was performed and a lead screw design was chosen. A lead screw is a long threaded rod that is rotated to move a fixed threaded nut. This nut is attached to whatever is being transported and can be supported by guide rails to ensure it doesn’t turn with the screw. As the screw turns, the nut slides down the rod and moves along with the apparatus down the rail. By increasing the threads on the rod, the accuracy also increases since it requires more turns to move the nut the same distance as the one with less threads.
Benefits:
· Very precise, accurate for positioning
· Easy to implement, not many moving parts
Disadvantages
· Not energy efficient, lots of friction leads to loss in energy
· Tends to be very slow due to large number of turns
CONCLUSIONS
Both designs are very easy to implement and scale very well with distance. They are thoroughly proven to be very robust for applications that require very precise movement that needs to go a long distance, while also keeping the cost to be low and very doable. Overall, studies have proven this to be the optimal design for those kinds of applications.
REFERENCES
http://www.mekanizmalar.com/rack_and_pinion.html
http://en.wikipedia.org/wiki/Leadscrew#Advantages_.26_disadvantages
http://iescjmechanisms.wikispaces.com/file/view/rack_n_pinion.jpg/312890764/rack_n_pinion.jpg
http://rayharvey.org/wp-content/uploads/2010/03/splitpinion.jpg
http://www.adafruit.com/images/medium/1143_MED.jpg
http://www.leadscrew.co.in/images/lead-screw-india.jpg