Building an Actuator Molecube
This page will guide you through the assembly of an actuator Molecube.
- 1 Step 1. Installing Thermoplastic Inserts into 3D-Printed ABS Parts
- 2 Step 2.Installing Main Axis Potentiometer
- 3 Step 3. Installing the MOOG Electrical Collector
- 4 Step 4. Coupling the Potentiometer to the Motor, Installing the Bearing
- 5 Step 5. Modification of the Plastic Case of AX-12 Servo Motor
- 6 Step 6. Installing AX-12 Servo Motor into the Actuator Molecube
- 7 Step 7. Installing Main AVR Controller PCBs into the Actuator Molecube
- 8 Step 8. Installing Servo Motor Connector and Hall Sensor
- 9 Step 9. Final Assembly: Side Boards, Angle Sensor, and Shells
Step 1. Installing Thermoplastic Inserts into 3D-Printed ABS Parts
|The very first assembly step is obtaining the structural parts of the actuator Molecube, as shown on the left. In the STL files that we made available for download from Sourceforge, we have tuned design tolerances for all module parts in such a way that best precision is obtained by 3D-printing parts with any of Stratasys line of modelers. For example, we use Stratasys Dimension SST.
As you can also see on the left, we printed the internal module parts and the external shells with two different colors: black and blue. Printing parts from different colors requires executing as many 3D-printing jobs, as you would like to have different colors, because you will need to print parts selectively, as opposed to printing out one full robot kit at a time. This is why, it is more time efficient to print all module parts with one color. Moreover, the choice of color for the internal parts is, in fact, not critical, as they are fully covered by the shells, and normally are not visible during robot operation.
If you like, you can choose another color of ABS plastic from which your Molecubes will be printed: Stratasys offers ABS plastic cartriges for FDM printing in a variety of colors.
|We like to drive inserts into plastic using a heated soldering iron. Iron temperature anywhere around 600-650 F is plenty enough for this task. You will notice that the iron becomes hot enough to drive inserts into their designated locations within several seconds after being turned on. It's better not to use very sharp and pointed iron tips, as the insert may somehow get stuck on the iron tip and will pull back from the plastic when you try to withdraw the iron. To make your iron tip live longer, remember to clean any plastic residue off the tip in between individual insert installations. Try to install all inserts flush with the surface or a hair deeper.|
|Make sure all inserts are in place: there should be a total of 6 inserts in both the north half and the internal gear, 3 inserts in the south half, and 4 inserts in the motor cap.|
Step 2.Installing Main Axis Potentiometer
|Piher potentiometer is used as a module angle sensor. It is installed in the only place inside of a Molecube where we were able to mechanically connect its body and shaft to the two mutually rotating module halves. Specifically, this location is between the servo motor and the electrical collector. The volume between these two parts is so limited that the potentiometer can only fit into this space if its leads are trimmed or bent - so, before installing the pot, we have to shorten its leads by half, as shown in Fig. 2.1.|
|We used multi-colored AWG 28 wires for marking the potentiometer terminals. To avoid confusion, we recommend to adhere to our suggested (or similar) color coding scheme. In our case, the red and blue wires are attached to the potentiometer ground and power terminals, and the green lead is attached to the sliding contact.
These wires are not expected to carry any mechanical load, or to move after the Molecube is assembled. However, there is very little space available inside of the module volume for wire routing. In this way, we only use a very small amount of solder to attach the very tip of the potentiometer lead to the tip of the wire, so that the resulting bead has approximately the same diameter as the potentiometer lead. (To better see our this, have a look at the large size version of Fig. 2.2.)
|Anywhere between 6" and 8" long leads should be plenty sufficient.|
|Sensor wires should be fished through the designated wire holes before the potentiometer is pushed into its slot. Make sure that you have machced the wires to holes correctly (as shown in Fig. 2.4.) and that you are not fishing the wires through the fastening holes or thermoplastic inserts - if you do so, the wires will get cut when a screw is inserted.|
|Once the wires are properly inserted, pull on them slightly, and the potentiometer should slide into its location. You may need to gently push it in to fully insert it.|
|The angle sensor installation is completed by bending and placing the potentiometer leads into the wire routing channel as shown in Figure 2.6.|
Step 3. Installing the MOOG Electrical Collector
|The slip ring is currently the most expensive off the shelf element of the module, so we recommend exercising reasonable caution with it. The slip rings are shipped with 12" leads attached. It is possible to leave wire trimming until the very end of assembly, however, we recommend to cut them shorter early enough and thus make them more manageable for routing through the robot. 3" of wires length left attached to the collector body is good enough.|
|Recommended length of wires left attached to the rotating shaft is 8".|
|Here we approach the next fine detail of the angle sensor installation, which might give you a taste of what a hack means for a mechanical engineer. In a side view of the angle sensor (see Fig. 3.3), we can identify a larger body part - which contains the electrical slip rings - and also a small dark collar covering the bundle of wires exiting the main collector body on its top. This collar is a plastic mold around the wire bundle and they rotate together.
In our search for an absolute angle sensor usable in the Molecube actuator, we have only been able to identify one device that would be both small enough to fit inside of the 7.5 mm clearance between the servo and the slip ring, mechanicaly suitable for continuous rotation applications, and also have a sufficiently wide range of angle measurement (340 degrees). This potentiometer is Piher N-15, and we have gone through its installation is the previous section.
On the other hand, MOOG slip ring is the only device we found capaple of long term current load of 1A per channel, providing Molecubes with overall power bus current carrying capacity of 4A. Under these conditions, the geometric constraints on the internal module component placement are such that the only way to fit the potentiometer into the 7.5 mm clearance is by having the rotating bundle of slip ring wires go through the main shaft coupling hole of the potentiometer.
This naturally requires that the wire bundle fits through the potentiometer shaft hole. Guess what? It does not fit through the hole. Unless the plastic collar is removed. And this is exactly what we will have to do in the next step.
|This step requires the use of a strong and steady hand. With a sharp cardboard knife, very carefully remove the collar by gradually peeling plastic from the top of the collar to its bottom. Be very careful with the knife: try not to hurt yourself - or the wires... If you have any ideas on how this can be done better or safer, or if you know of any replacement components requiring fewer mechanical adjustments - let us know.|
|Here you can see how the slip ring looks after wire collar outside of the collector body has been removed.|
|Moving on to the next trick: here you can see how the stator part of the slip ring body looks originally. Inside of the Molecube actuator, the rear end of the slip ring stator is located in intimate proximity against the Molecube outer shell. This results in a possibility of jamming of the wires exiting the slip ring between the sharp edge of its body and the flat inner surface of the Molecube shell. Any such mechanical contact is highly undesirable due to the potential danger of damaging insulation, and resulting electrical shorts.|
|To prevent this from happening, we found out that it is sufficient to remove ~2 mm off the top of the cylindrical slip ring body immediately underneath the locations where wires exit the collector (see Figure 3.7.): after this alteration, the upper edge of the slip ring body will come in contact with the inside of the Molecube shell before the wires thus protecting them from mechanical damage.|
|This is the final adjustment of the slip ring body aimed to prevent signal wire damage. The "before" and "after" images are shown in Figures 3.8 and 3.9.|
Step 4. Coupling the Potentiometer to the Motor, Installing the Bearing
|Inset motor cap and bearing into the plastic housing of the Molecube north half. Separate wires into two sets of six and bend them sideways away from the hole and into the wire gutters. Now you should be able to see the notch on the potentiometer shaft. It is very important that the notch is aligned with the symmetry axis of the motor cap, try your best to achieve this as precisely as possible. Minor inaccuracies due to manual alignment will be taken care of during the automatic angle measurement calibration process. However, do not count on this process if your final misalignment is above +/- 5 degrees.
The notch is very small, about 1 mm in size - you can only see it in Figure 4.4 between the wires if you view it at full resolution.
|We used M3 DP-100 epoxy with work life of 4 minutes - remember to mix it well and promptly apply it as a void filler between the wires in the motor cap hole.|
|Epoxy should soak through the voids between the wires and bond the wires, potentiometer shaft, and the motor cap together. M3 DP-100 epoxy will start solidification in 4 minutes after mixing, and will become rather solid in 20-30 minutes.|
|Once epoxy has solidified (in about 20-30 minutes after application), it is a good idea to verify its alignment with respect to the north half housing. To do this, connect ohmmeter to the red and green potentiometer leads (as shown in Figure 4.8.) and rotate the motor cap around the module axis until you notice that the potentiometer readout sharply changes from 0 to ~10-11 kOhm or the other way.|
|When you find this location (where the potentiometer switches from min to max readout and back) notice the orientation of the motor cap relative to the north half enclosure. You should see the switch occur around the motor cap positions shown in Figures 4.8 and 4.9.|
Step 5. Modification of the Plastic Case of AX-12 Servo Motor
|This section will guide you through the modification of AX-12 servo before it can be installed into the actuator module. Modification by removing excessive parts of the servo body is required due to low space availability inside of the actuator module. Figure 5.1 shows how AX-12 looks when just pulled out of the box.|
|First, we take the servo apart and remove all internal gears. We need to make modifications only to the main servo body and the gearbox cover. We will not use all of the original parts of AX-12: the lower protective cover, coupling flange, and the long bolts (parts on the left side of Figure 5.2) will not be used.|
|Modifications to the main servo body consist in fully removing its front wall and the 0.4" of the front parts of its side walls. The front part of the gearbox cover is also removed. We used a band saw to cut these parts, remember to carefully clear the cut parts from the plastic debris after cutting, as they may interfere with the operation of the gearbox. The gearbox cover is modified by removing the mounting flanges flush with the servo side walls, as well as its front part beyond the output shaft mounting hole to avoid interference with the Molecube bearing.|
|Shown modifications allow us to fit the servo inside of a thin section ball bearing and engage its output shaft with the large printed internal gear on attached to the north half housing.|
|The corners of the plastic servo body protrude too far outside of the Molecube south half housing, which prohibits correct placement of the interconnection PCBs. On the other hand, these corners are only made of plastic and can be easily removed by cutting them away (with either band saw or a knife) as shown in Figure 5.5.|
Step 6. Installing AX-12 Servo Motor into the Actuator Molecube
|We start by inserting the gearbox cover into the servo cap which is suspended in the thin section bearing. Next, install the inner plastic bearing which will be connected to the north half housing with six bolts right outside of the bearing.|
|Correct alignment of the servo with regards to the main robot potentiometer is very important. Molecubes are designed so that they can determine their absolute angle of rotation at any time. For this, they use the readings of two potentiometers with superimposed sensing ranges, which helps to avoid angle insensitivity. Alignment of the angle sensors is established at this stage of assembly.
Attach an ohmmeter to the Red and Green leads of the main Molecube potentiometer (as sown in Figure 6.2), and keep rotating the motor cap until you find the potentiometer's insensitivity region. This region is detected by abrupt change in the potentiometer readout from its minimum to its maximum value or otherwise.
|When you find this region, you are prepared to insert the servo into its designated location. Before inserting the servo, check that the notch on its output shaft is aligned with the middle-of-the range notch on the gearbox cover (see Figure 6.4 below).|
|You may need to pull the servo out of the housing and re-insert it one or two times, so be careful not to lose any gears or damage them with excessive force. When the servo is installed and aligned correctly, the notches on its output shaft and gearbox cover should be aligned with one another and with the lower corner of the diamond-shaped opening in the plastic housing, as shown in Figure 6.4.|
|Attach the Red and Green leeds to an ohmmeter and very gently rotate the installed servo about 5 degrees in each direction about the alignment position. You should start feeling resistance from the servo gearbox: use caution to avoid damaging the gears with excessive force. If the servo has been installed and aligned correctly, you should still notice the abrupt change in the measured potentiometer resistance when the servo is rotated for ~5 degrees clockwise and counterclockwise away from the alignment position.|
|By swiveling the actuator halves relative to one another in alternating directions about the position of initial alignment, you should notice the potentiometer resistance change abruptly from nearly 0 to 10 kOhm. If you can see that, motor has been installed correctly.
Note: to minimize any unintentional assembly errors, set the Molecube into the center-aligned position per Fig.6.4, and avoid swiveling the halves relative to each other until the assembly is complete.
|Next, pull the wires that have been routed from the electrical collector through the Piher potentiometer and around the servo motor through the large opening in the printed motor holder (the inner structural part of the actuator Molecube's southern half).|
|Set the motor holder in its position. Take care while routing the wires and installing the motor holder to avoid any wire loops, over-twisting, or kinking.|
|Visually inspect the alignment of the mechanical actuator components by looking at the module from the side, as shown in Fig.6.9. The adjacent surfaces of the north and south components of the actuator Molecube should be parallel to each other and there should be a small air gap between them. Each half should only be in firm mechanical contact with its respective race of the thin section bearing. Apart from the contact with the bearing races, there should be no direct mechanical contact between the plastic components of the north and south half.|
|Secure the servo motor and the inner race of the thin section bearing between the printed motor holder and motor cap by installing the four nylon bolts as shown in Fig.6.10.|
|Secure the outer race of the thin section bearing between the printed inner gear and the printed gear holder by installing the six nylon bolts as shown in Fig.6.11.|
Step 7. Installing Main AVR Controller PCBs into the Actuator Molecube
Design and construction of the Molecube controller and interface printed circuit boards is discussed here.
|Note that each actuator Molecube uses two sets of PCBs (one in South half and one in North half), however only has one servo motor (controlled by the South half). As a result, each Molecube only requires one servo motor power regulator. The power regulator circuitry resides on the Right Board (see Fig. 7.1., for example), and only one of every pair of right boards needs it. So we only populate 50% of all actuator Molecube PCBs with motor power regulator components (C10, C11, C12, D1, IC10, L2, R17, R18).
Before installing the PCBs into the Molecubes, visually inspect the assembled printed circuit boards for the quality of soldering and to ensure absence of any mechanical fabrication defects. Only one of the two PCB sets will require the servo motor power regulator components installed.
|Make sure to inspect the circuit boards on both sides.|
|Carefully break off the excess PCB material along the perforation lines. Take care to avoid any unintended damage to the assembled boards in this process.|
|In this section, we will focus on the installation of the two main AVR controller boards (those that are marked as "Main Board", with Atmel processors and two FFC cable connectors). The pairs of left and right boards will be installed in the following sections.|
|Trim the slip ring wires off the Molecube north half leaving them ~2" long.|
|Re-route the slip ring wires off the Molecube south half via a smaller case opening. Take care to avoid wire over-twisting or kinking.|
|Trim the slip ring wires off the Molecube south half leaving ~2" of length beyond the casing.|
|Set your wire stripper to gauge 21.|
|Carefully strip ~1/8" lengths of isolation from all slip ring wires, taking care to avoid damaging the conductors.|
|Solder the slip ring conductors to the main boards as shown in Fig.7.12. Take care to observe the correct electrical connectivity by following the color code example. Here, we use 4 conductors for each of the two power lines and, and 2 conductors for each of the two communications lines.|
Step 8. Installing Servo Motor Connector and Hall Sensor
As we discussed in the previous section, each actuator Molecube includes two sets of PCBs, however, only one of them (South Half) controls the servo motor. Therefore, the circuit boards on the North Half don't need the motor connector. Hall Sensor is used by the actuator Molecube to determine if its current angle of rotation is close to the insensitivity region of the main axle potentiometer. This input is combined with the AX-12 servo's embedded angle sensor input in the actuator Molecube's controller absolute angular position calculation algorithm.
|Unpack the servo motor connecting cable. Measure and cut ~1.5" length of the cable with connector.|
|Use gauge 21 wire stripper to clear ~1/16" of isolation from all three connector leads.|
|Before soldering the servo motor lead wires, check to make sure that you will be soldering them onto the Right board which already has the servo motor power regulator components (C10, C11, C12, D1, IC10, L2, R17, R18) installed. Solder the motor lead wires to the PCB observing the wire connectivity as shown in Fig.8.3.|
|These are the Hall sensor and auxiliary passive components we used in the Hall Sensor circuit.|
|Install the Hall sensor as shown in Fig.8.7, while ensuring the correct sensor package orientation relative to the PCB and correct sensor lead connectivity with the circuit board.
Note: there's an error in the PCB layout requiring the sensor package leads to cross over. We used a short length of stripped wire isolation to protect the leads from unintended electric contact.
Step 9. Final Assembly: Side Boards, Angle Sensor, and Shells
|Use two 50-conductor flat flex cables to connect the Left and Right boards to the South Half's Main board. Make sure to use the Right board with the servo motor power regulator components, Hall sensor, and servo motor lead wire installed.|
|Use two 50-conductor flat flex cables to connect the Left and Right boards to the North Half's Main board. Due to physical space constraints, the current mechanical design of the North half enclosure cannot accommodate the Right board with the servo motor power regulator components installed.|
|Trim the potentiometer leads to approximately 3" length off the actuator plastic enclosure. Strip ~1/16" of isolation off each wire lead.|
|Route the potentiometer lead wires around the flat flex cable as shown in Figure 9.4 and solder them to the Right board terminals observing collect electric connectivity as shown.|
|Bend both flat flex cables of the North half circuit board assembly to approximately 90 degree angles as shown. Insert the PCB assembly inside of the actuator Molecube exterior shell. Push the exterior electric connector packages installed on each board through the openings in the Molecube shell to ensure their full insertion and snug mechanical fit.|
|Very carefully install the cover shell on the North half, taking great care to avoid any unintended wire jams. If the installation is done correctly, all wires will settle into their dedicated channels and openings within the plastic enclosure.|
|Attach a ~3/4" x ~ 3/16" piece of magnetic tape to the exterior surface of the 3D printed internal plastic gear as shown in Fig.9.7.|
|Pug the servo motor connector wire into the socket on the servo motor. For optimal wire routing, push the Main board into its designated position on the plastic enclosure until the slip ring wires form a loop as shown in Fig. 9.8. Plug the servo motor connector through this loop.|
|Carefully install the Molecube shell on the South half taking great care to avoid any wire jamming. Assuming that the halves of the actuator Molecube have not been swiveled about their axle relative to each other after the potentiometer alignment (see Fig.6.4), check to make sure that the Hall sensor is placed over approximately mid-length of the magnetic tape strip.|
|Secure the shell with bolts.|
|Assembly is complete.|