Shades/Blinds Servo Controlled
Some would say this project was really for someone very lazy! I took it as a challenge to try and solve the issues around mini-blinds that needed to be remote controlled because of the difficult location and general "cool-ness" factor.
But first a little background. Some where along the way I acquired a continuous rotation servo typically used in radio controlled cars, planes or robot drive motors. These types of servos are great for applying a powerful rotation to a shaft with minimal electrical power. They are not fast, but they are easy to adjust for speed and direction.
But first a little background. Some where along the way I acquired a continuous rotation servo typically used in radio controlled cars, planes or robot drive motors. These types of servos are great for applying a powerful rotation to a shaft with minimal electrical power. They are not fast, but they are easy to adjust for speed and direction.
Analog RC servos require power, typically 6 volts, and a pulse train to command their rotational position. The pulse train depicted above shows the typical signaling required for a servo to decode the electrical pulses and mechanically rotate to a desired position. Non-continuous servos track their output rotation(0-120 degrees) to the width of the positioning pulse. If the pulse is 1.0 milliseconds wide, the servo rotates to one end of its rotation of zero degrees. A pulse with of 2.0 milliseconds would rotate the output position to 120 degrees. A pulse width of 1.5 milliseconds would rotate the servo output to 60 degrees. Since the pulse width can be varied, the position of the servo output would track each pulse as long as new pulses came in to update the servo on a regular basis. This is why the update rate of 50 milliseconds is important. The servo can update its current position 20 times a second. The limiting factors on precise positioning are in the speed and load requirements of the servo mechanism itself.
The continuous rotation servo operates with similar signaling as depicted above, however 1.0 millisecond pulse widths continuously rotate the servo one way while 2.0 millisecond pulse widths continuously rotate the other way. At some point, presumably 1.5 millisecond pulse width, the servo should stop and hold its current position.
I have found that in this servo controlled blinds project, that the servo does not need to continually hold the blinds in position, so I decided to remove the power to the servo using a transistor switch. This also keeps the system from buzzing when it is not actively moving the blinds.
One of the features necessary to make the remote controlled blinds functional is the ability to sense when the blinds are open and closed. There are numerous methods to do this with switches and timers and pulse counters, etc... however I wanted an easy solution where I could choose any position without a bunch of extra effort.
The blinds open and close to exact positioning by the use of a multi-axis accelerometer (SEN10955) that I mounted to one of the upper slats. The force of gravity can be used to measure the tilt angle of one axis. Reading the digital data on the X axis while the accelerometer is attached to the slat provides the exact pitch of the entire window shade. The microcontroller reads and learns what angle positions I have selected and once programmed, the servo can rotate the shaft that adjusts the opening and closing of the blinds. Once the desired position is sensed by the accelerometer, the microcontroller stops the servo and awaits the next command.
Below are some images of the servo mounted to the wall and connected to the blinds via a twist shaft.
The continuous rotation servo operates with similar signaling as depicted above, however 1.0 millisecond pulse widths continuously rotate the servo one way while 2.0 millisecond pulse widths continuously rotate the other way. At some point, presumably 1.5 millisecond pulse width, the servo should stop and hold its current position.
I have found that in this servo controlled blinds project, that the servo does not need to continually hold the blinds in position, so I decided to remove the power to the servo using a transistor switch. This also keeps the system from buzzing when it is not actively moving the blinds.
One of the features necessary to make the remote controlled blinds functional is the ability to sense when the blinds are open and closed. There are numerous methods to do this with switches and timers and pulse counters, etc... however I wanted an easy solution where I could choose any position without a bunch of extra effort.
The blinds open and close to exact positioning by the use of a multi-axis accelerometer (SEN10955) that I mounted to one of the upper slats. The force of gravity can be used to measure the tilt angle of one axis. Reading the digital data on the X axis while the accelerometer is attached to the slat provides the exact pitch of the entire window shade. The microcontroller reads and learns what angle positions I have selected and once programmed, the servo can rotate the shaft that adjusts the opening and closing of the blinds. Once the desired position is sensed by the accelerometer, the microcontroller stops the servo and awaits the next command.
Below are some images of the servo mounted to the wall and connected to the blinds via a twist shaft.
There are a few features that make this servo controlled blinds system workable. To allow manual operation, I created a hexagonal sleeve that slides up and down about half an inch to permit easy engagement of the servo drive with the blinds twist shaft. As seen in the first diagram, the hex sleeve has a small pin that is locked into the twist shaft, and takes care of rotating and sliding up and down when manual operation is required.
A small speed reduction was required to allow the servo to maximize power and not overshoot the programmed slat tilt angle. The servo output shaft has a small pinion pulley which couples to a larger drive pulley through a trio of o-rings that I had handy. This provides excellent pulley traction which never slips.
And finally, the entire assembly mounts to the wall via an adjustable yoke which matches the angle of the blinds twist shaft and provides a smooth bind free motion for manual and servo remote control.
And finally, the entire assembly mounts to the wall via an adjustable yoke which matches the angle of the blinds twist shaft and provides a smooth bind free motion for manual and servo remote control.
