MECHANICAL DESIGN OF THREE AXIS MOTION (SMT PICK AND PLACE ASSEMBLY)
· SCARA ROBOTIC ARM.
In SCARA robotic arm the robotic arm is used for the placements of the SMT components. Picture shows a robotic arm pick and place machine.
· 3-Axis movement machine.
In 3-axis machine a nozzle is attached to a plotter-like device to allow the nozzle head to be accurately manipulated in three dimensions. Additionally, nozzle can
be rotated independently. Figure 2.3 shown a belt driven 3-Axis pick and place machine. In some machines lead screws are also used instead of belts.
The length and breadth of the machine is limited due to the bending of the rail rods. The rods get deflected due to the weight of head (pick and place nozzle assembly). When the length and breadth of the machine is increased the deflection of the structure in downward direction also increases. Therefore they are not increased beyond a certain limit. For the same reason, the diameter of the rail rods is also kept at a certain value and is not decreased as for larger diameter of the rods weight also increases.
We choose belt driven mechanism to drive movement of assembly. Belt greatly increases the speed of the machine but it decreases its precision. Increasing the diameter of the pulleys do increases the speed of the machine but it decreases its precision. Due to this reason, the diameter of the driving pulleys is also kept small.
Head (Pick and Place Assembly)
It is most vital part of Pick and Place machine from speed and accuracy prospect. Speed of machine is in direct relation with the speed of head. It is actually speed with which machine can pick and place components and it is usually measured in parts per hours (PPH).
Pick and Place heads can be classified on the basis of following factor,
1. Number of nozzles on head
Most industrial machine has several nozzles on head. They pick up several components during pick up run and place these components during place run. In some kinds of machines components are feed into multiple heads (like bullets in a chamber), these components are then fired one by one on PCB. Picture below shows commercial multiple nozzle head.
Speed can be increased by multiple factor by using multiple heads on a single machine or multiple nozzles on single head. Small projects or research based Pick and Place machines usually have single head with single nozzle. These machines have speed around 1000pph to 4000pph.
Drive mechanism(motor driven or pneumatic driven)
Most common mechanism for driving head up and down is motor driven. Motor can be connected to lead screw system to drive nozzle up and down. In order to attain high speed belt driven system is used. picture shows a pneumatic driven head.
Many industrial Pick and Place machines use pneumatic drives to move nozzles up and down. Selection of drive mechanism depends on availability and price of component. In our machine we will be using motion based on belt driven mechanism due to the low price of DC servo motor and ease of control.
Common features that typical Pick and Place head have are:
· Shock absorption
· 180 degree component rotation angle Controlled suction
· Small pick place transient overshoots
Most prominent limitations on Pick and Place head design are:
Maximum size of head
Increasing the size of head decreases the work area for Pick and Place machine. Small head causes the designer to compromise on many quality factors. Designers are carved to use small low torque motors, small diameter guiding rods and small number of linear bearings.
Maximum speed of head
Increasing the speed to pick and place components cause the danger of dropping the component from vacuum nozzle. Head need a minimum amount of time for settling before pulling up and putting down component. Also heads speed is limited by control speed of vacuum pump and time delay of image processing processor
1. Important Considerations
There is always risk of slight shock to the nozzle while picking or placing components on PCB. These shocks if not avoided can cause nozzle to bent from top causing uneven suction distribution over component surface. This uneven distribution of suction will cause frequent dropping of SMT components.
b. Nozzle Changer
SMT components are available in different sizes. Same nozzle cannot be used to pick and place SMT components of different sizes. Nozzles of specific gauge are used to pick place components of different sizes. In order to cope with this limitation head needs to be designed such that nozzles can be changed either manually or automatically
c. Availability of Nozzles
There is a large industry dealing with manufacturing nozzles for Pick and Place machines. These companies make nozzles for almost every type of component. But because of non-availability of nozzles in our region, nozzles are needed to be fabricated.
As SMDs are itself special components because of their size and packing. So getting these component ready to be picked needs special devices called as feeders. Feeders are specially designed to feed the components to the machine.
These are specially designed feeders for the components packed in the form of long tapes. Following online links were examined:
Tapes are such that they have holes on their side. These holes are of standard size and are distant equally. As the figure 2.6 below shows the driving gear for the tape for its motion on the feeder as the gear moves it pulls the tape with its teeth.
Consulting patent of different feeders we see they are mostly belt driven with driving element i.e. motor on side and driver on the other end. Figure 2.7 shows the gear for reel movement at 55 and being driven by 57 at the other end which can be driven by a motor.
From the complexity of feeders coping with tapes inside them was bit difficult to design so the having the tape completely isolated from the inside of feeder was considered most practical design.
As components are covered with protective tape on the top so this tape should be removed before component could be accessed. For this following simple design figure 2.8 below was considered. It is a commercial feeder by PCB unlimited for SMT components. It shows the tape removing pulley at the top and main components being moved forward.
For the position of each feeder on the machine we considered a product by LPKF called proto place. It is a semi-automatic machine for SMT components pick and place as shown in picture.
Proposed Specs of Machine
For any type of SMT/THT specification of Machine according to your requirements and Company setup, Please consult to our Engineering department at Shenzhen Southern machinery Sales and Services Co; Ltd.