AUTOMATIC BLIND

The idea of this project is to create a blind that automatically lowers or raises depending upon the light conditions. In the offices at MakingThings several of the windows face south, which is very pleasant, but during winter in particular for a number of months the sun comes in so brightly that we can't see our monitors. The windows don't have blinds, so we have taken to putting large pieces of cardboard up for parts of the day until the sun angle changes. What we thought would be helpful would be a blind that automatically came down when the sun is at its most severe. Our solution was to use a stepper motor and a couple of Blocks to turn the blind shaft to the two positions, raised and lowered, at the appropriate light levels.

The design starts with the choice of a stepper motor, rather than any other kind of motor, to move the blind. Steppers are excellent for positioning tasks since they can hold their position very strongly and provided they are not over-worked, they can be relied upon to rotate to particular positions on demand. A controller must be used to sequence the stepper motor coils to step forwards or backwards, which it does sometimes up to thousands of steps per second. A good controller will send the step configurations slowly at first, then speed up as the mechanism gains momentum.

In our blind raising and lowering application, there are two important positions for the blind - raised and lowered. The stepper motor needs to be able to go from one position to the other, perhaps hundreds or thousands of steps away. The Stepper Block can control any unipolar stepper motor that draws less than 2A of current. Conveniently, it has a mode (Scaled Mode) which will allow the position to be specified as an analog voltage from 0V to 5V. This is translated into a step position by multiplying it by a scale. The scale is a voltage set by the trimpot on the Stepper Block. If the scale value is very low, there may only a few steps between either extreme, if it's very large, there might be millions of steps from one end to the other. For our application we'll want the 0V position to correspond to the blind all the way up and the 5V position to correspond to the blind all the way down. Part of our setup will be determining what the right scale is for the particular window we need to shade.

One issue that will affect nearly all users of stepper motors is determining the position of the mechanism (in this case, the blind) at power-up. In our case, the power may have gone off while the blind was all the way up, all the way down, or somewhere in between. Often this problem is solved with limit switches, which are activated when the mechanism reaches the extreme ends of its movement. Both the Scaled mode and Automatic mode of the Stepper Block can use limit switches to locate the mechanism. Upon power-up in Scaled mode, the Stepper Block moves the mechanism back towards the 0V position, stopping either when a limit switch is activated, or when the maximum number of steps have been taken, as indicated by the scale. In the former case (limit switch activated) the mechanism tells the Block that it's at the zero position. In the latter case, by running backwards for the maximum number of steps, the mechanism must be in its final position.

It's this latter technique that we'll use for this application since a) it is hard to design a limit switch that will work for all different types of blind and b) there are no bad consequences for the blind winding up too far - it just spins around a few times. So when the power is applied to the Automatic Blind, the stepper runs in reverse for the number of steps that would take the blind from lowered to raised. If the blind happened to have been raised when the power was resumed, the blind will flap around several times. If the blind had been partially lowered, the blind will wind up and roll around a few times. If the blind were fully lowered it would roll all the way up and stop. In all cases, the blind would be completely rolled up so the Stepper Block and the blind mechanism would have a common point of comparison.

So in outline, we now know how to make the blind go up and down in response to a voltage. The question is how to get the Photocell to cause 0V or 5V to appear at the Stepper Block? The answer is to use a Poly Block to compare the output of the Photocell to a reference voltage and to output a "Greater" signal when the light levels are higher than the target. So the Photocell is connected to the Poly Block's X (I0) input, the Poly Block is set up to run in Compare Mode and it outputs 5V on its Greater output (O1) when the light levels are greater than the reference setting. The trimpot on the Poly Block is used to set the exact level of lighting required to trigger the blind.

This is kind of a trick, since the Stepper Block is expecting an analog value between 0V and 5V and what we're giving it is a digital voltage of either 0V or 5V, but it works since 0V and 5V are still legitimate analog values, representing the two extremes of motion.

In summary, the various components function as follows in the two different light conditions:

Component	Low Light	High Light
Photocell	Low voltage from signal output. Sends this signal to the Input X of the Poly Block.	High voltage from signal output. Sends this signal to the Input X of the Poly Block.
Poly Block (Compare Function)	Input X (I0) voltage is less than trimpot reference voltage. Compare function outputs a logical "1" on the Less (O0) output and a logical "0" on Greater (O1) output. Sends the Greater (O1) signal, a "0" or 0V, to the Stepper Block.	Input X (I0) voltage is greater than trimpot reference voltage. Compare function outputs a logical "1" on the Less (O0) output and a logical "0" on Greater (O1) output. Sends the Greater (O1) signal, a "1" or 5V, to the Stepper Block.
Scale Trimpot	The Scale Trimpot is adjusted to the point where the correct number of steps are taken from one extreme position to the other exactly corresponds to the blind up to blind down distance.
Stepper Block (Scaled Mode)	Receives 0V on it's Target Position (I0) Input. The current position is compared with 0, and the stepper motor is moved accordingly (and proportionally to the Scale input value).	Receives 5V on it's Target Position (I0) Input. The current position is compared with the largest position, and the stepper motor is moved accordingly (and proportionally to the Scale input value).

The motor for this application needs several key characteristics: 1) it must not draw more than 2A, 2) it must be easily mountable (some motors require exotic mounting mechanisms), 3) it must have sufficiently high torque to run the blind mechanism, 4) it must have a shaft that is easily connected to. Conveniently, one of the MakingThings motors fits this specification exactly.

Possibly the most difficult part of this project is determining how to mount the motor and how to connect it to the blind. We had an old blind lying around but connecting it to one of our stepper motors didn't seem too easy - on one end there is a tiny (1/8"?) shaft and on the other a flat strip connected to the spring inside the blind. Neither of these seemed very convenient to attach a motor to and expect it to be secure. We decided, after a trip to the hardware store to use some 1/2" plumbing pipe and a brass 1/2" plug. A plastic sleave connected the 1/2" pipe and the 1/2" brass plug both threaded externally. To secure the shaft we drilled a 1/4" hole into the plug and drilled and tapped a set screw hole. Our final mechanical addition was a flexible coupling (purchased from a mechanical parts store - SDP/SI) that permits up to 16 degrees of shaft misalignment while still connecting motor to shaft.

The Motor is connected to the wall via a piece of T-section aluminum drilled in the appropriate places to permit the motor to be screwed to it.

The other end of the pipe is connected to a thick spacer screwed into another piece of T-section. The pipe, when inserted over the spacer can turn freely.

The shaft coupler could have been omited, by connecting the motor shaft directly to the end of the modified pipe plug, but we felt that having the additional flexibility to permit slight mis-alignment in the way the two ends are secured to the wall.

Step 2 - Connecting the Stepper Block

The next thing to do is to connect the Stepper block to the stepper motor. The Stepper Block has four outputs. It is designed to connect these to the four stepper phase coils. The common wires from the coils are connected to the Stepper Block's V+ connection. The power supply has to be chosen to fit the stepper motor and the Stepper Block. This means that the voltage must match the Stepper Motor's specified voltage and that it must be between 7V and 24V, as required by the Block power guidelines. In addition, the power supply must be able to supply sufficient power for the stepper motor. Our standard 3A 12V power supply and our large stepper motor are good matches for this.

It is a good idea to run the stepper motor to see how it manages with the blind mechanism you've built. This can be achieved by connecting the stepper motor and by adding a couple of potentiometers, one for the scale input and another for the positon input which will soon be replaced by the output of the Poly Block. Unipolar Stepper motors have two independent coils. Not always well labelled, sometimes a multimeter is required to find out which wire connects to which coil.

Before powering the system up, confirm the block is in "Scale" mode by making sure that the function switch is set to function 2 : "10".

When you connect the power, if you have connected the Stepper properly, it will start to step in reverse seeking the zero position. The number of steps it takes will depend on the voltage the scale potentiometer presents to the "Scale" (I2) input. Once the stepper has travelled the appropriate distance, it will stop, and then attempt to find the position the "Position" (I0) input specifies. Try adjusting the "Scale" and "Postion" voltages to confirm that the motor is performing the way you expect. Conceivably the motor will need to be rewired to make it turn in the right direction.

Step 3 - Connecting the Poly Block

Assuming the Stepper Block functioned correctly in the previous step, it is a good time to add the Poly Block. The Poly Block will need to get power (V+) and ground (0V) from the Stepper Block. Don't connect the 5V lines of the two blocks together as this will increase the likelihood of power supply noise and will cause one of the regulators shoulder the load for both devices which is not good.

The Photocell is connected to the Poly Block in the regular way - connecting it to the 5V and 0V lines and connecting the Signal line to the "X" (I0) terminal on the Poly Block. The Poly Block is set to perform the Compare Function by setting the switch to 0110.

The "Greater" (I1) output of the Poly Block is connected to the Stepper Block's "Position" (I0) input. If you feel the need to confirm the operation of the Poly Block, connect the output "Less" (O0) to the anode of an LED, connect the other end of the LED through a small value resistor (say 300 ohms) to 0V. Then when the light is less than the reference value the LED will be on. Note that this is an extra step and not necessary for the function of the system.