Servos

I know I said I was going to focus on the image processing part of the problem first, but I couldn't help look ahead a little bit to the robot building phase. It's better to know now if this project is feasible, how much it will cost, and any design constraints that might be mitigated by image processing. And it's also fun, and mixes up my day a little.

So, never having built a robot before, I've been doing some reading and looking at some stores. The fundamental building block for robots is almost always servos. In construction, they are an electric motor, a set of gears to strengthen-but-slow the spinning of the motor, a feedback mechanism that can tell what angle the motor is currently pointed, and some electronics to use that feedback to control the spinning of the motor. From a functional perspective, you tell them where to point, and they point there and hold their position until you tell it to point somewhere else. I'm sure there are many nuances, but I can't be bothered with such petty details.

Important Specs of a Servo

There are two main specifications of servos that interest me: rotational speed and torque. Rotational speed is how fast the servo can change positions. Since my robot will have to move fast enough to hit a ball, speed matters. This is usually expressed either in rpm or in seconds-per-60-degree-rotation. They are easily convertible:

sec per 60 degrees = 10 / rpm
rpm = 10 / sec per 60 degrees

Torque is the turning strength of the motor. It is expressed in any of Nm (the metric version), oz-inches (the US version), or kg-cm (a bastardization of the two). If we consider a servo with 20 oz-inches of torque, it can hold a 20 oz mass against gravity at the end of a 1 inch arm from the servo. Or it could support a 1 oz mass against gravity at the end of a 20 inch arm. So you multiply the mass times the length of the arm to get the required torque. The metric version of Nm removes gravity from the interpretation and explicitly measures the force it can apply at the end of an arm (yay metric for making sense!). Now keep in mind that this is the limit of what the servo can support. If you have a 20 oz-inch servo supporting 10 oz at 2 inches, it won't actually be able to move the arm -- but it will keep the arm suspended, just fighting off gravity. Any more weight or length, and the arm will fall under the load. My point is that if you want the arm to actually move against gravity, you have to supply more torque than that. And, conversely, you can use less torque when moving the arm down, as gravity is pushing that way anyway.

Types of Servos

I see two types of servos: analog and digital. Analog is the most common and the most affordable. They tend to use PWM (pulse width modulation) as the way you tell it what angle you want. For example, hobby servos used in model airplanes typically turn to 0 degrees with a 1000us signal, to 90 degrees with a 1500us signal, and to 180 degrees with a 2000us signal. It is analog because the length of the pulse is translated into the position of the servo. These types of servos need relatively simple control electronics to create the pulses, but it's not so simple that you can just do it from your computer directly. Some sort of servo controller is necessary to produce these pulses, and then your computer can talk to the controller to choose the pulse width.

Digital servos are more expensive, but seem to be preferred for robotics. I'm not really sure why yet, but there is probably a reason. Instead of taking PWM pulses over a signal wire, they take some form of digital communication over a signal wire or wires, and the on-board electronics read the message, extract the degrees you wanted, and move the servo accordingly. This means that the servo controller is different: it has to be able to speak in the appropriate serial protocol, instead of a simple on-off pulse.

Dynamixel

There is a dominant brand in the robotics servo market: Dynamixel, made by Robotis. They make a variety of servos with different speed and torque specifications to suit your particular application. They communicate over a TTL serial line or a RS-485 serial line (honestly I'm not sure why they have two protocols as from a high-level they seem equivalent).

I'm going to list a few of the servos in the Dynamixel line, to give you an idea of the specs available. This is taken from the Trossen Robotics store, which seems to be a good resource.

Model	Speed (s/60 deg)	Torque (kg-cm)	Price (USD)
AX-12A	0.196	16.5	45
RX-24F	0.079	26	140
EX-106	0.143	107	500

This covers the three corners of specifications: the AX is cheap, the RX is fast, the EX is strong. Well, none of these are cheap. Here is a typical hobby servo for model airplanes and helicopters:

Model	Speed (s/60 deg)	Torque (kg-cm)	Price (USD)
Hitec HS-322HD	0.15	3.7	10

Much cheaper. But it also illustrates why Dynamixels are preferred: torque. The AX is 5 times more expensive but 5 times more torque. The EX is 50 times more expensive, but 25 times more torque.

Honestly there really aren't other brands of robot servos available. You can either try to use servos intended for a different application (like these model airplane servos), or you can use the Dynamixel line, intended for robotics, or you can have a fancy lab and build your own. My inclination is to stick with what other roboticists have decided makes sense, and use the Dynamixel line.

Since there isn't a one-size fits all servo, each joint will need to be evaluated to determine what speed and what torque is desired, and then I can choose the most economical servo to accommodate that. It also puts upper limits on the the speed (about 0.079 s/60) and torque (about 107 kg-cm). Well... torque can be improved by using two servos in the place of one servo. There are even pre-fabricated brackets to team two EX-106 servos together, effectively doubling the torque (at more than double the cost!). Speed is a little more fussy to multiply, but in theory it can be done by adding your own gears, sacrificing strength for speed. Honestly I don't want to do that if it can be avoided.

Conclusion

So it looks like I'm headed towards using Dynamixel servos. I don't own any yet, and I would rather know which specs I need before I run out and buy some. So in a future post I might work on a bit of the math to determine how fast and strong I need the arm to be.

Epilogue
I've ordered an RX-24F to play with. Sometime next week?