Advantages and disadvantages of the turret structure of the placement machine
(1) Advantages and disadvantages of the turret structure
· Up to 6 nozzles of different sizes are suitable for any placement head on the turret, and there is no need to change the nozzle during work. In other words, any nozzle can pick up components at any time, eliminating excessive and unnecessary operating steps.
· The visual system is installed between the pick-up point and the placement point of the turret placement machine, and image processing can be performed “on the fly”.
· The simple mechanical gear-driven turret design can keep the power supply cable, servo system, encoder, sensor and camera stationary, so that there will be no electrical problems caused by the constant bending stress generated by the movement, which may cause the circuit to break (for example, when the stage is constantly moving from the feeder to the camera, the circuit board may malfunction during operation).
· The placement speed of the turret placement machine can now reach more than 54,500 points/h (such as HT122).
The best single-head gantry placement machine can only mount about 15,000 points/h. In order to achieve the same speed as a bench-type placement machine, multiple machines are required, which means that more parts will need maintenance, lubrication, calibration and repair.
·Using a turret design, the feeder is only installed on one side of the placement machine. In this way, the front and rear width of the placement machine can be reduced by about 1 m. For this reason, there is no need to install additional feeders and component storage devices on the other side of the production line, thereby reducing the area of the entire operating area.
(2) Disadvantages of the turret structure
·Due to the limitations of the mechanical structure, its placement speed has reached its limit and cannot be greatly improved. In addition, it occupies too much space and produces a lot of noise.
·The turret placement machine can only mount components in tape packaging or bulk packaging, while tube and disk materials cannot be mounted.
·The placement head position of the turret placement machine is fixed relative to the PCB placement position.
The PCB is accurately positioned at the specified placement position by running along the X-Y direction. Due to the vibration caused by high-speed movement, the already mounted components will be offset, which will ultimately affect the placement accuracy. ·The range of components that can be mounted is limited to chip components, and it is not suitable for mounting IC devices.
(3) Limitations of the turret structure
When the turret structure is in motion, the mounting speed is limited due to the maximum allowable value of the radial acceleration on the vacuum nozzle. The device on the nozzle is subjected to the acceleration vector force. This vector is the resultant force of the centrifugal force and the tangential force. The resultant force acting on the device is balanced with the friction between the device and the nozzle end face. This friction is restricted by the suction force of the nozzle end face and the friction coefficient between the device and the nozzle.
According to the calculation of mechanical motion engineering mechanics, the maximum allowable acceleration between the device and the nozzle is usually no more than 50 m/s2.
Therefore, when high mounting output is required, the diameter of the turret should be reduced. Another factor that restricts the mounting speed of the turret structure placement machine is: the maximum acceleration allowed for the first device on the PCB. Usually this value is 7 to 10 m/s2.
There are three time determination cycles for the turret time: turret shift motion acceleration limit, PCB motion acceleration limit and feeder shift motion limit. Among these three time cycles, the longest cycle is the limiting factor of the turret structure mounting output. In addition, if the distance between two mounting positions on the PCB exceeds 30 mm, the mounting output will be reduced by 50%, and the change time between the two feeders will also affect the production capacity. Large-sized packages and heavy devices will also reduce the mounting output. This is because the mounting of such devices is carried out at the end of the mounting cycle after all small-sized devices have been mounted.
Therefore, turret mounters are mostly used in occasions with many resistive and capacitive components and high assembly density, such as computer boards, mobile phones, and home appliances.