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  • This video is about homebuild linear drives.

  • The first example is from an old printer:

  • Its the drive of the printhead.

  • A brushed DC motor pulls the carriage along the round guide bar using a toothed belt.

  • while the head moves, a transmissive optical sensor scans the fine lines printed on a stripe of plastics.

  • The two sensor outputs are connected to an Arduino Uno and through an H bridge the microcontroller can command the motor in either direction using two output pins.

  • The Arduino counts the number of scanned lines and compares the actual value with the setpoint that is transmitted from a host computer through the USB interface.

  • This type of linear drive is fast:

  • Less than two seconds are needed to move the printhead from left to right which equals a distance of appriximately 30 cm.

  • The motor consumes around 1 W of electric power when operated at 5V.

  • High speed means low torque - the carriage can be stopped by hand easily.

  • Even with the fine pattern on the plastics stripe you still can notice the carriage moving for a single step when having a close look.

  • 7000 steps are needed to move the printhead for 30cm, thus one step equals 0.04mm.

  • With the linear sensor, the microcontroller can detect movement relative to the plastics stripe.

  • If the carriage is stopped by hand, the motor is kept energized...

  • ...until the mechanism is released and the setpoint is reached.

  • When deflecting the carriage manually, the microcontroller compensates the movement of the print head caused by the side load.

  • In doing so, backlash or slip are partially balanced by the control loop.

  • The microcontroller detects movement in relation to the line pattern on the sensor stripe.

  • When removing the tab from the mechanics, the carriage is controlled in such a way, that the print head follows the movement of the plastics stripe.

  • Ensure that the sensor stripe is mounted tightly which is done by a spring steel plate at this mechanics.

  • Here, the toothed belt is replaced by a simple cord.

  • Even while the cord slips along the motor shaft, the microcontroller still can direct the printhead to the given setpoint reliably.

  • Spindles are often used to transform rotational movement into linear movement.

  • Here, a 6mm threaded rod is used.

  • Whenever the tread is turned by the motor...

  • ...the carriage moves along the aluminum square tubes, guided by ball bearings.

  • The brushed DC motor is from an old printer and the gear ratio of the plastics wheels is 12:1.

  • The pitch of the thread is 1mm per turn.

  • The motor must make 12 turns in order to move the carriage for 1mm which is a large overall transmission.

  • The speed of the linear drive is approximately 1mm per second, thus it is a rather slow...

  • ...but powerful movement.

  • The sensor disc from the printer, mounted at the plastic gear on the treaded rod, is scanned by two transmissive optical sensors.

  • Both sensor outputs are connected to the Arduino Uno which can command the DC motor to spin in either direction through an H bridge.

  • The working principle of this type of rotary encoders has been treated in a previous video.

  • The fine line pattern on the sensor disc results in 3000 pulses per revolution.

  • With the given pitch of 1mm per turn, the tread transforms one step into a linear movement of 0.3 micrometers - at least in theory.

  • In practice, backlash is usually higher than the academical step width.

  • The microcontroller can't detect the backlash, thus it can't compensate that kind of movement.

  • Another drawback of that fine line pattern is the large number of pulses generated by the optical sensors with each revolution which limits the rotational speed - remember the low clock speed of the microcontroller.

  • With two transmissive sensors and a toothed sensor disc having 4 teeth, we get 16 steps per revolution.

  • With each step the carriage moves 0.06mm - a resolution that is sufficient for many applications.

  • We can also use the rotational sensor composed of resistors in the control circuit.

  • Instead of using a second wiper, the washer side of the sensor is pulled to ground through the ball bearings whose resistance is obviously low enough.

  • Contact bounce is a huge drawback of this type of electromechanical encoder limiting the maximum revolution speed,

  • What about using an optical computer mouse as shown in a previous video to create another linear sensor?

  • As you can see, it is indeed a practical motion sensor for our control circuit.

  • Only the change in movement along the Y axis is processed by the microcontroller.

  • The motion sensing is done relative to underlaying surface.

  • When moving the sheet of paper by hand, the carriage follows that movement.

  • Even when interrupting the movement by hand, the motor stays energized until the setpoint is reached.

  • Deviation caused by sideload is counterbalanced by the control circuit.

  • I will do more investigations about using an optical mouse as linear sensor in a future video.

  • When connecting a bipolar stepper motor to the threaded rod, the linear drive works even without sensor feedback.

  • This motor is commanded by a Raspberry Pi.

  • Without feedback, the Raspberry Pi can't detect if the setpoint is reached.

  • When locking the mechanism by hand, the linear drive doesn't reach the designated point.

  • When using a linear drive without feedback loop, the torque generated by the stepper motor must be sufficiently high, so that the movement of the mechanism isn't interrupted unforseen.

  • Finally I am using two linear drives to build a computer controlled machine:

  • The printhead drive is mounted perpendicularly to the linear drive composed of a DC motor and a threaded rod.

  • A relay with a tiny lever moves a wire up and down, transferring ink from the reservoir to an underlaying sheet of paper.

  • The working principle is similar to that of dot matrix printers.

  • With no more than one needle, it is inevitably a monochrome printer, producing large dots with low speed.

  • Nonetheless that's why this machine is an illustrative way of explaining how printers work or how bitmap graphics are drawn on a computer screen.

  • Each dot has a diameter of approximately 1mm, thus the resulting resolution is 25 dots per inch.

  • Around one hour passed by until the 300 times 318 pixel graphic, resulting in 11000 black dots was transferred to the paper sheet.

  • Read more about this machine and linear drives on the project page.

  • Thanks for watching and: "I'll be back!"

This video is about homebuild linear drives.

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B2 中高級 英國腔

DIY打印機和線性驅動器 (DIY printer and linear drives)

  • 84 6
    samko5sam 發佈於 2021 年 01 月 14 日
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