We’ve made a lot of progress on the design for the three degree-of-freedom simulator, and are hoping to order parts to start the build early next week.
This new design allows for continuous rotation on every axis, although it will require a lot of space to do so.
Rigidity might be tricky on this one. After running the numbers on some worst-case scenarios, we think that the frame itself will deflect less than 0.16″ on the inner axis. The axis bearings may cause more than that, though. And it’s difficult to know how the new drive train is going to perform in terms of backlash and rigidity.
But we’ll find out! The build process will start next week.
I get asked this all the time, and it’s an understandable question. Servo motors are the most expensive part of the machine, and you need a one for every axis. I’ve seen forums where people talk about disassembling treadmills to repurpose the high power DC motors. So… let’s look back and talk about the motors we’ve considered and used on this project! I’ll talk a little about gearboxes and other mechanical drive parts.
Clearpath motors are great, and I can’t recommend them enough! They aren’t the cheapest option out there, but they have a lot going for them:
Now, in retrospect, I ordered this motor too soon. It was actually underpowered, so I returned it. Within the last year, Teknic actually released a new line of motors that ramps up the maximum power significantly. Their higher power motors are more expensive, but are still a great deal compared to a lot of other options. There are a lot of different models available. If you can find one that fits your budget and power requirements, you can’t go wrong by using a Clearpath motor from Teknic.
We didn’t end up using Teknic on the first-rev. prototype because there were cheaper options, and at the time they didn’t have anything with high enough power output. Now they do, but they still aren’t the lowest cost.
I don’t know who makes these motors, but you can get them from a variety of places. We experimented with a 1.8 kW AC servo, model number 110ST-M06030, shown below:
This is a solid motor, and the price is unreal. Seriously, you can get one of these, including the driver and controller, for $322 (USD). We got them from FastToBuy.com. They ship from China, so expect to pay a ridiculous amount for shipping. And FastToBuy is an intermediary, so I don’t know who actually makes them.
Downsides? Documentation is poor and inaccessible – it’s just a bad Chinese-English translation. And expect to know something about PI gain tuning to get the servo operating like it should in your system. If you don’t know how to do that, don’t let it scare you – you can figure it out. But it isn’t going to be as simple as getting a Teknic motor set up.
Also, the driver needs 240V single phase, or 3-phase power (3-phase is better if it’s available. Less ripple on the internal rectified DC voltage used to drive the system). So you can’t just plug your system into a normal single-phase wall outlet.
But it’s hard to argue about the amount of power these offer at the price! These are actually significantly overpowered, but that’s OK. I can’t say a lot about longevity, since we haven’t run them for a long time. So far they are doing well.
The linear actuators on the Version One 2-DOF prototype actually use an even larger 2.3 kW servo motor from the same manufacturer and supplier: the hefty 130ST-M15015. It’s only $395 off the shelf, some $70 more expensive than its 1.8 kW counterpart. We thought, “Hey, at that price, we may as well upgrade!” These servos don’t even warm up on the 2-DOF prototype. They run cold. We haven’t even begun to make them work hard.
Caveat: We haven’t run these on a dynamometer, and can’t say that they are really 1.8 kW or 2.3 kW. If I had to guess, I’d say “no, they aren’t that powerful.” They are clearly more than enough for this application, though.
On the current linear actuators, we are using the 130ST-M15015 servo motor with a 10:1 NMRV063 worm gearbox, also from FastToBuy. Had to get the speed down and the torque up! The gearbox goes to a ~3″ diameter timing belt pulley, which moves the assembly. It isn’t clear on FastToBuy’s website, but there are actually a lot of different gearbox sizes and ratios available, and the factory (whoever/wherever it is) can do custom shaft sizes, input and output flanges, etc. FastToBuy sells smaller and larger gearboxes of the same type as well. They can work with you to get the one you need for your application.
Interestingly, this exact gearbox model, possibly from the same manufacturer, is also available from Automation Direct. I know it’s the same because I just bought one from them. 🙂 You’ll pay more from Automation Direct, but it ships from the US, and it actually has specs available on the website. You know, output torque, max speed, input torque, etc., etc. Important stuff like that. It’s a lot easier to work with, but there are fewer options when it comes to input and output shafts, flanges, and so on.
We are using 1.5″ H-series timing belts and pulleys. You can get them all over the place (McMaster, for example). But you can get them for cheap from shop.polybelt.com. They have lots of pulley and belt sizes, belt types, and so on. It’ll be way cheaper there than from McMaster.
We’ll most likely be switching motors for the next revision. Why?
Here’s what we are looking at for the next version:
This is a 3/4 HP, 90V brushed DC motor from Automation Direct (ironically… made in China, but at least Automation Direct handles the overseas issues). The math says that these motors should be fine. The gearbox is a NMRV model basically just like the ones we currently use, but the ratio is much higher, and it’s available from Automation Direct, sized specifically for the DC motor.
The motor doesn’t have an encoder, and of course it doesn’t include a driver or controller. That will drive up the price, so it might end up being a little more expensive than what we are currently using. We’ll find out! Encoder and servo drive are on the way, and I already know how we are going to get the encoder set up.
I’ve been working on a new frame design that allows for three axes of movement instead of two. Adding that final yaw axis with any large amount of motion freedom significantly increases the frame size, but it might be worth it!
I have two design variants right now, one that uses axis yokes supported on two ends (like the current design), and one where every axis is cantilevered, “hanging” off of the new bearing design. Here are some images of the cantilevered design concept:
And here are some images of the “supported” design:
The cantilevered design may be tricky, but if we can get it working, it would be amazing! The primary issue is that because every axis connects at a single point, there is a lot of torque on every joint, with the potential to cause a lot of flex. And because all the axes are connected serially, deflection on one axis is felt by every axis to follow. So the frame and bearings have to be rigid, or it won’t work well.
And because the axes are all axially driven (as opposed to the actuator-driven design), backlash on the gearboxes and chain sprockets will be really noticeable.
Finally, the distance between the user and the moving frame is actually fairly small. You could easily reach out and touch the frame. Or you could reach out at the wrong time and (*gulp*) chop your fingers off. So we’ll need to build protections into the design to prevent accidental dismemberment…
Altogether, I really like this design concept and I’m eager to try it out.
When we started this project, we wanted to build a full 6-DOF motion simulator, allowing for rotation and translation in every direction. In the interest of keeping costs low and simplifying the initial prototype, we started with a 2-DOF version, which only allows for pitch and roll rotation. We used linear actuators in the design so that we could use the same actuator design in the full 6-DOF version down the road.
As I design the second-revision 2-DOF frame, it is becoming increasingly clear that linear actuators may not be the best option. Here are the problems:
Don’t get me wrong, I’m glad that we have the actuator design! We’ll definitely use it on more capable motion platforms down the road!
But I’m fairly confident that an axis-driven design would help solve all of these issues. By spinning each rotation axis directly, instead of lifting the edge of the axis with a linear actuator, we increase the allowable rotation angles significantly (although we may have to increase the frame size to allow for the necessary space). Without linear rails and idler pulley assembles, the transmission becomes more simple. We can size the motors appropriately without having to lift a load. Driving the rotation axis directly will require less space. And finally, my initial estimates suggest that I can shave between $400 and $600 off the cost of each axis with the more simple design.
A potential downside is that we’ve never attempted a design of this type, so there will probably be some hiccups along the way. Backlash might be a big problem with the cheap gearboxes we are using. Without some kind of shaft coupler, high peak torques could damage the gearbox. And so on. We can solve problems as they arrive, I’m sure.
The verdict? An axis-driven design is at *least worth exploring seriously. I’m hammering out details of the design now.