- Closed Loop
- GPIO Pins
- Heat Sink
- Indirect Drive
- Ingress Protection
- Linear Servo
- Open Loop
- Pulse Amplitude
- PWM Signal
- Raspberry Pi
- S. Bus & M. Bus
- Servo Attachments
- Servo Connectors
- Servo Gears
- Servo Gear Sets
- Servo Horns
- Servo Mounts
- Servo Speed
- Servo Splines
- Servo Torque
An Arduino is a popular brand of microcontroller. While created by the Arduino team initially they are open source so clones and offshoots can be manufactured by anyone. They provide a much lower barrier-to-entry than what existed previously to begin programming electronics. They are great at controlling servos, LEDs, motor controllers, reading sensors and more. Newer versions come with WiFi and Bluetooth capabilities. There is a large ecosystem of boards designed to plug directly into the top of an Arduino, such a board is called an Arduino Shield.
The phrase closed loop means that the system has a sense of where it is, and therefore, can be told to go to a specific position. Closed loop systems are common within both motors and servos. Motors that have encoders can utilize the encoder reading to get a sense of location (whether relative or absolute) and position themselves accordingly. Servos typically use an absolute potentiometer for a position sensor and when the servo receives a signal from the servo controller, it is then able to check its current position and move to the position that corresponds to the signal received.
Feedback usually refers to some sort of signal or reading in a closed loop system. In the case of a servo, the feedback is the resistance coming from the potentiometer which is used to check the position of the servo. In the case of a motor, it's often times the signal coming from an encoder that is counting the number of rotations from a known start point.
GPIO Pins are General Purpose Input/Output Pins. They exist on microcontrollers like Arduinos, microcomputers like Raspberry Pis. They can be digital or analog and allow the user to write code which reads data from sensor or sends communication signals to output devices.
A heat sink is used to draw heat from something, in our case, typically a component on a circuit board. In most cases, the heat sink is constructed of a material that has a high thermal conductivity so that the heat is quickly pulled from the component on the board. Many times, the heat sink will be made in a way that maximizes the surface area so that it too can cool quickly rather than become heat-soaked.
Indirect drive means that the item creating motion is not fused or coupled in-line to the item that it is driving. For example, most servos have potentiometers inside for use of feedback. The potentiometer is typically directly below the splined output shaft of the servo and the shaft profile of the potentiometer matches the bore in the back of the final gear of the servo. This would be considered direct-drive. If, however the final gear of the servo had a gear on the bottom which mated to a gear on a potentiometer (no matter the ratio created by these two gears) the setup would be considered indirect drive.
The IP Code (or International Protection Rating, sometimes also interpreted as Ingress Protection Rating*) consists of the letters IP followed by two digits and an optional letter. As defined in international standard IEC 60529, it classifies the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in electrical enclosures. The standard aims to provide users more detailed information than vague marketing terms such as waterproof.
The digits (characteristic numerals) indicate conformity with the conditions summarized in the tables below. For example, an electrical socket rated IP22 is protected against insertion of fingers and will not be damaged or become unsafe during a specified test in which it is exposed to vertically or nearly vertically dripping water. IP22 or 2X are typical minimum requirements for the design of electrical accessories for indoor use.
IP First number - Protection against solid objects
0 No special protection
1 Protected against solid objects over 50 mm, e.g. accidental touch by person's hands
2 Protected against solid objects over 12 mm, e.g. person's fingers
3 Protected against solid objects over 2.5 mm (tools and wires)
4 Protected against solid objects over 1 mm (tools, wires, and small wires)
5 Protected against dust limited ingress (no harmful deposit)
6 Totally protected against dust
IP Second number - Protection against liquids
0 No protection
1 Protection against vertically falling drops of water e.g. condensation
2 Protection against direct sprays of water up to 15o from the vertical
3 Protected against direct sprays of water up to 60o from the vertical
4 Protection against water sprayed from all directions - limited ingress permitted
5 Protected against low pressure jets of water from all directions - limited ingress
6 Protected against temporary flooding of water, e.g. for use on ship decks - limited ingress permitted
7 Protected against the effect of immersion between 15 cm and 1 m
8 Protects against long periods of immersion under pressure
A linear servo is a motor that creates motion in a linear fashion (typically through driving a lead screw that extends and retracts a tube or shaft) and has feedback to make a closed loop system. The feedback is typically from either a potentiometer or an encoder. The feedback must be connected to a circuit board which utilizes the feedback to know the position of the extension tube or shaft.
A microcomputer, as the name implies, is a computer in a small form-factor. Raspberry Pis are the most common example of a micro-computer. Like a full-sized computer, a microcomputer runs an operating system and has I/O ports like USB and HDMI. Microcomputers like the Raspberry Pi will also have GPIO pins to output PWM signals and connect various other electronics.
A microcontroller is a programmable electronic board with GPIO pins. Unlike microcomputers they do not run an entire operating system, only the code that a user writes and loads onto them. Because of this they need much less overhead in terms of computing power.
A microsecond is a millionth of a second. While this unit of time is used for many things, it is often times used to describe electrical pulses sent to a servo.
The phrase open loop is used to describe a system that does not have a form of feedback. It's a common phrase when referring to pan and tilt and other camera control devices which have a controller designed to vary the speed of the device. In the case of a joystick controller, the further the stick is deflected, the faster the motor (or continuous rotation servo) will rotate.
A potentiometer is a variable resistor that is commonly used within servos to get a position reading. Potentiometers typically consist of 3 poles (two reference and one wiper). As the potentiometer shaft is turned, the resistance between the reference and wiper poles will change. Servos are able to utilize this reading and position themselves according to specific signals sent from the servo controller they are connected to.
Deadband is a term commonly associated with the positional accuracy of a servo. Servos are typically closed loop (minus a few exceptions which raises the question whether or not those rare cases should be considered servos or not) which means they can be told to go to a specific position. The deadband is essentially the size of the bullseye - how accurately must the servo position itself based on what the controller called out to satisfy the request. Servos with very small deadbands are shooting for extreme accuracy and are excellent for high precision applications whereas servos with wide deadbands are less picky about positional accuracy but are able to get to an idle state and pull less current once they get close to the desired position.
Electronic devices are many times controlled by short pulses of voltage (this creates a square wave signal). The amplitude of the pulse refers to the difference between the minimum and maximum voltage in the square wave signal. Because the minimum voltage is almost always 0V, the pulse amplitude is almost always the peak signal voltage. Hobby servos generally operate on a pulse amplitude between 3 and 5V.
Period is the frequency at which a pulsed signal is sent to an electronic device. For hobby servos, this period required is typically 20 milliseconds (ms) which means it requires that a signal be sent 1000 times per second. The period is also commonly measured in Hertz (Hz). Hz is cycles per second so a signal of 50Hz is 50 cycles per second which equates to 20 milliseconds.
A Pulse Width Modulation (PWM) Signal is used to control all kinds of electronic devices. A PWM signal is a short pulse of voltage which is typically repeated over and over in a very short amount of time, known as the period. Servos require a PWM signal, sent from a servo controller, to operate. The PWM signal sent to a servo is commonly quantified in microseconds (μsec). This range may vary from one servo to the next but most hobby servos will function properly on a PWM range of 1000-2000μsec. In hobby servos, the constants are generally the period (50Hz) and the pulse amplitude (3-5V). A change in PWM signal will cause the servo to rotate.
A Raspberry Pi is a popular microcomputer. GPIO pins allow them to interface with various other electronics, and many Raspberry Pi "Hats" (electronic boards that physically mount to the top of a Raspberry Pi) extend them with capabilities they otherwise would not have. While Raspberry Pis are capable of outputting a PWM signal, the lack of dedicated on-board timer for that signal makes them poor at controlling servos directly. However a servo hat resolves that issue as well as making it easier to properly power your servo.
S. Bus & M. Bus
M.Bus (aka S. Bus) is a communication protocol that allows you to use one physical port on the receiver to control multiple servos. Each servo must be M.Bus/S.Bus capable and programmed to be on a particular digital channel.
A Servo Attachment is any item that mounts to the output shaft of a servo. An attachment can be a horn or a hub that is part of a linkage (such as a steering system on an RC car) or it can be a gear or sprocket that transfers that motion to another shaft (such as the servo gear in a Servo Gearbox)
A ServoBlock is a patented servo mount which incorporates a bearing supported servo shaft which greatly increases the servo's load-bearing capability.
Servo Connectors can be found in several slight variances through history. Variances aside, they almost all are directly compatible with 0.1" (2.54mm) row pins used on circuit boards (including RC receivers). Outside of the hobby servo realm, this connector style is classified as a TJC8 connector. In an attempt to reduce confusion, we call out the gender of the housing and the contacts. For example, an MH-FC connector means that it has a Male Housing and Female Contacts.
"Servo gears" can refer to gears inside of the servo or a gear that goes on the output spline of the servo. Gears inside of the servo are driven by the motor and there are typically several stages that each act to reduce the speed and increase the torque. These gears come in many different materials, such as but not limited to nylon, Karbonite, aluminum, brass, steel and titanium. When the phrase 'servo gear' refers to a gear that goes onto the output spline of the servo, the gear will have a bore that needs to match the spline type of the servo it is to be installed on. This servo gear typically mates to a hub gear in order to create a ratio between the servo and the driven shaft to change the torque and the speed at that driven shaft.
Servo Gear Sets
Servos have several gears inside of the servo case which are driven by the pinion gear on the motor. There are typically several stages of gears that act to reduce the speed and increase the torque. These gears come in many different materials, such as but not limited to nylon, Karbonite, aluminum, brass, steel and titanium.
Servo horns are servo attachments that are commonly used to to transfer motion from the servo to an arm, push-rod or linkage. Horns take on many different shapes and can protrude in one or many directions from the center where it attaches to the servo.
Hobby servos typically have tabs (also known as "ears") that can be used to fasten the servo to your project, however, sometimes those tabs are not positioned in a way that makes them useful for your project. Servo Mounts come in many forms and fasten to the stock servo tabs, leaving you with holes (thru or threaded, depending on the one you choose) that make fastening the servo to your project much easier.
Servo speed refers to the transit time of the servo. It's usually captured with the number of seconds it takes for that servo to rotate 60 degrees. Since most servos have a finite amount of rotation before it reaches its mechanical stop(s), an abbreviated range was chosen so that servos, no matter the max amount of rotation, could be compared more easily. A typical servo speed would be 0.15sec/60°. This means that the servo will rotate 60 degrees in 0.15 seconds. If the servo has a max rotation of 180 degrees, you can easily derive the transit time for the maximum amount of rotation (180/60=3 therefore it will take 0.45 seconds (3 x 0.15 seconds). This unit does, on occasion, get transferred to continuous rotation servos even though RPM would be a more easily understandable unit of measurement for most. To convert from sec/60° to RPM, simply multiply the transit time by 6 (to get from seconds per 60 degrees to seconds per 360 degrees) and then divide 60 seconds by that number. In the case of a continuous rotation servo with a speed of 0.15sec/60°, 0.15 x 6 = 0.9 seconds per 360 degrees and 60 seconds / 0.9 seconds = 67RPM.
Servos have an output shaft that typically uses a splined profile in order to transfer the torque from the output shaft into the servo attachment that is fastened to it. This splined profile is typically classified by the number of teeth, however, there is potential for two very different servo splines to have the same tooth count as it doesn't capture the spline diameter or tooth profile. For example, A15T and D15T both have 15 teeth, however the D15T is a much larger spline size than the A15T. The graphics below are loose representations of the common spline sizes that are found on hobby servos. If you have a servo that we do not offer on the site, you can check to see if your servo has a spline that matches any of the below spline profiles by counting the teeth and measuring the distance across the spline.
Torque is a measurement of rotational force. For servos, the unit of measurement is usually oz-in though some manufacturers prefer to use kg.cm. In the case of oz-in, it is the number of ounces that servo can move if the load were applied at 1 inch away from the center of the servo spline. Let's consider an example where a servo has 200 oz-in. It is able to move 200oz if the load were applied at 1" from the center. This can be a 2" pulley (since the radius of a 2" diameter pulley is 1" and the string will always feed off the pulley at a 90 degree angle), a gear with a 2" pitch diameter (again, since radius is 1"), a sprocket with a PD of 2", a wheel with a diameter of 2", or an arm with a linkage attached 1" from the center of the servo spline. If, however you took that same servo and changed where the load were applied, the weight would therefore change. For example, a 1" pulley (which has a radius of half of the unit of measurement used) would allow the servo to move 400oz whereas a 4" pulley (which has a radius of twice the unit of measurement used) would allow the servo to move only 100oz. Servo torque is most commonly logged as stall torque, or the absolute max the servo can move. It's good practice to choose a servo with at least twice the torque that your project needs so that the motor inside of the servo is not running constantly at max capacity. Running a servo at or near stall torque will cause the servo to generate heat which can lead to premature failure.
TJC8 is a specific connector size and style. It is not specific to the number of contacts but does refer to the housing size and the spacing between contacts. TJC8 connectors are directly compatible with 0.1" (2.54mm) row pins and a slight variation of a 3 position TJC8 connector is used on hobby servos. To help protect against having servos plugged into servo extensions in the wrong orientation, many of the connector housing are molded with very slight clipped edges on one side to act as a key.