Being a fan of all things having to do with robotics and animatronics, you’ve picked up a lot over the years. Some of it came easy, some didn’t. Understanding servo torque ratings and how to pick the right servo for your motorized prop or robot arm might just fall into the latter category, it certainly did for me.
For most of us, the first servo we’re introduced to is the SG90 micro servo. As soon as I learned how to control a servo with Arduino, it was my go-to servo for all my initial projects. At the time, I never paid attention to datasheets or servo specs because they always worked for adding movement to my cardboard characters.
It wasn’t until I tried to make a motorized spider prop with a motion-activated silly string shooter that I realized that my micro servo needed to hit the gym. Do you even lift bro?
Upon detecting motion from the PIR sensor, the micro servo was supposed to pull a string that depressed the nozzle from a can of silly string, dousing the victim in green webbing. But after waving my hand in front of the motion sensor, I heard a click of the servo arm and then nothing. It looks like I helped it into an early retirement.
Selecting the right servo can make or break your project’s success. Whether you’re engineering a lifelike animatronic for a theatrical performance or designing a nimble robot, the strength and precision of movement hinge largely on the torque of the servo motor. But how does one interpret torque ratings, and how do these numbers translate to the actual weight a servo can handle? Come along with me as we demystify servo torque ratings, providing you with the tools to make informed decisions and ensure your next project moves just the way you envision.
In this Servo Torque Tutorial:
What is Servo Torque?
Torque is a measurement of rotational force that’s used to quantify the strength of a servo. The official unit of torque is the Newton-meter (Nm) but datasheets for servo motors will often specify torque ratings as ounce inches (oz-in) or kilogram centimeters (kg-cm). You’ll notice that all these units are a combination of both weight and distance.
A servo’s torque rating determines the maximum amount of force it can apply at a right angle (perpendicularly) to a lever (servo arm). The larger the number, the more force the servo can exert.
How to Determine What Torque You Need for Your Servo
Think you need to be a math whiz to crack the torque code? Unlike the Da Vinci Code, it’s not difficult to figure out the torque you’ll need for your servos. As a bonus, there’ll be no convoluted plots or chase sequences to survive either. To calculate torque, use the following mathematical formula:
Torque (τ) = Force (F) × Distance (d)
There’s no calculus or weird mathematical symbols to trip you. And to prove it, let’s look at a few real-world examples.
Example 1: How much weight can a servo move?
The Tower Pro SG90 is probably one of the most popular micro servos on the market and the one most of us start out with. Let’s take a look at what the datasheet says about its abilities.
At minimum, servo datasheets will have information about dimensions, operating voltage, weight, torque and speed. We can see that at an operating voltage of 4.8, we can expect to get a torque of 25.0 oz-in, or 1.80 kg-cm out of the SG90 micro servo.
So what exactly does 25.0 oz-in or 1.80 kg-cm of torque mean? It translates to the fact that this SG90 micro servo can move a weight of 25 oz when it’s attached to the servo arm at 1 inch from the center of rotation, and/or it can move a weight of 1.80 kg when it’s attached at 1 cm from the center of rotation. This is also called the maximum servo stall force or maximum holding force.
Okay, so we know how much weight the SG90 micro servo can move so long as it’s attached to the servo arm at a specific distance from the center of rotation. But what if I need to attach the load at a different distance? How will this affect the servo’s torque?
Let’s say I need to move a weight that’s attached 2 cm from the servo arm’s center of rotation. This is double the distance specified in the datasheet. How heavy could this weight be before the micro servo stalls?
In order to determine the weight, we need to solve for F using our torque formula: T = F x d
Solving for F gives us: F = T / d
F = 1.8 kg-cm / 2 cm
F = .9 kg
By doubling the distance of attachment on our servo arm, we’ve halved the amount of weight it can lift. What if we bring the weight closer to the center of rotation, say at .5 cm?
F = 1.8 kg-cm / .5 cm
F = 3.6 kg
By cutting the rated distance in half where the weight is attached on the servo arm, it allows us to double the weight the servo can lift.
With these examples, you can see that the weight a servo can move depends on the hole you choose on the servo arm. Half the lever length and you double the force. Double the lever length and you half the force.
Example 2: What torque rating do I need to move a known weight?
In this example, you’re in the process of trying to pick the right servo for your motorized prop project. The payload weighs about 15 kg (33 lbs) and you plan to attach it to a servo using a short linkage, no more than 2 cm from the center of rotation. What torque rating will your servo need?
Let’s plug in what we know into the torque formula:
T = F x d
T = 15 kg x 2cm
T = 30 kg-cm
You’ll need to find a servo motor with a torque rating of at least 30 kg-cm for your motorized prop to work. I recommend that you pick a servo with a higher torque rating than you need just in case you need to make adjustments to attachment points on the servo arm.
Weight of Links: To better illustrate the relationship between distance, force and torque I left out any additional weight the servo arm adds to the equation. When performing these calculations, remember to add the weights of any links, push rods, etc. to the payload for the total force.
Metric vs. Imperial Measurements: These examples have all been in metric measurements (kg-cm) but if you’re more comfortable using imperial (oz-inch), the same formula and calculation method applies.
How the Holes on a Servo Arm Affect Travel
Where you connect your payload along the servo arm not only affects the amount of weight it can move, but it also affects the amount of travel or movement you get. The farther the hole you choose on the servo arm, the more travel and movement you get but at the cost of decreased weight. The closer the hole you chose on the servo arm, the less movement you get but you can move more weight.
If you’re not sure what your mechanism layout is yet or what hole to use to give you the movement you want, try mocking up your mechanism with an existing servo as a placeholder. Once you have your push rods or linkages giving you the movement you want, measure the length between your servo’s center of rotation and the hole on the servo arm you plan to use. Using this distance and the weight of your payload, you can calculate the torque rating you need.
How Operating Voltage Affects Servo Torque
Servo manufacturers often provide specs for both speed and torque specifications in two common voltages, 4.8 volts and 6.0 volts. More powerful servos typically include higher voltage options. A servo’s operating voltage has a direct impact on its torque, speed, and overall performance. Generally, when you increase the voltage to a servo (without exceeding its limits), you also increase the torque it can deliver. To get as close to the official torque rating as possible, use a power supply that provides the maximum voltage listed in the servo’s datasheet.
Going Beyond Servo Torque Ratings
Although a servo’s torque rating is one of the most important pieces of information I use to pick the right servo for my electronics project, there are other factors that go beyond the scope of this tutorial that affect servo performance.
What about projects like robot arms that consist of multiple servos, links and payloads? The torque required at each joint will be different. To accurately calculate torque you also need to take into account factors like angular acceleration (torque required for a joint to move) and moment of inertia (resistance an object has to any change in its velocity), and as you can imagine, the math gets pretty involved. Luckily, there are various robot arm torque calculators to help you get the torque values you need. Then as a rule of thumb, go a bit higher with the torque ratings just to be safe.
In a future servo tutorial I’ll take you through how to calculate the torque required for each joint of a robot arm in a more beginner-friendly manner but for now, you have more than enough to choose the right servo to animate your props.
Remember, it’s not merely about choosing any servo, but the one that aligns with the weight and movement needs of your specific project. Armed with a clear understanding of torque and its implications, you’re well-equipped to ensure that your creations move with the grace, strength, and reliability you’ve always imagined. Let your projects be a testament to the power of informed choices and meticulous planning.