what causes a servo to buzz when holding position

A servo buzzes when holding its position because its internal control loop is continuously applying electrical power to the motor to resist an external force or to reach a target position. As explained in Game Manual 0, the control system uses a built-in control loop with the servo's potentiometer to drive the servo to a target position and hold it. The official FTC Docs state that a servo is designed to maintain its target position even when external forces are applied to the shaft.

When the servo is physically blocked or commanded to a position it cannot reach, it enters a stall condition. The official FTC Docs explain that at stall, the servo draws maximum electrical power to continuously apply torque against the force or obstruction. This continuous application of power and torque by the internal motor to correct the position error is what physically manifests as a buzzing sound.

If holding power is not required and the buzzing needs to be stopped, the PWM signal can be disabled. Game Manual 0 notes that casting a servo to ServoImplEx allows the use of setPwmDisable(), which turns off the PWM signal and causes certain servos (such as goBILDA and REV Smart Robot Servos) to de-energize and stop holding their position.

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SDK Servos

Game Manual 0·CC BY-NC 4.0

The SDK offers several methods of controlling and communicating with servos, as well as a couple of hidden methods that can be easily accessed.

Servos are controlled via a PWM signal. PWM signal is one where the signal turns on for a number of microseconds, then off for a number of microseconds. Servos are controlled by sending variable length pulses every 20 ms, with the length of the pulses dictating what angle the servo should move to (or with CR servos, what speed it should move at). This length of the pulse, expressed in microseconds, is called the PWM pulse width. By default, the sdk generates signals from 600 to 2400 microseconds (with SDK 0 being 600 microseconds, and SDK 1 being 2400 microseconds). However, the expansion hub can actually generate between 500 and 2500 microseconds if the range is set manually.

Open Game Manual 0

Calculating Motor and Servo Power > Electrical Power and Mechanical Power

FTC Docs·BSD 3-Clause

The fundamental concept we need to understand is power. We are generally concerned with two similar but different kinds of power, so let's look at the two most common types. In a motor, electrical power is the energy supplied by the electrical current flowing through the motor's windings. This electrical energy is transformed into mechanical power, which is the rate at which the motor performs work by rotating a shaft. Both kinds of power are measuring different aspects of the motor; electrical power deals with the movement of electrical charges, and mechanical power deals with the movement of objects due to forces. Both of these measurements are expressed in the same unit, Watts (W), because power, in general, is defined as the rate of energy transfer or work done. No matter the form of energy (electrical, mechanical, thermal, etc.) the fundamental concept of power remains the same. Even though these two power measurements carry the same unit, they are calculated differently and cannot be used interchangeably (or together!).

Motors and servos are constructed similarly - both are electromechanical devices that convert electrical energy into mechanical energy - but there are big differences in how they're used. Motors are often used in applications requiring continuous power, such as pumps, fans, and conveyor systems. Motors are typically rated for continuous power output, meaning they can sustain that power level indefinitely without overheating. Servos are commonly used in robotics and precision positioning systems, where controlled movement and precise positioning are essential. Servos are designed for intermittent operation - typically cycling through on/off periods to control movement - and are often rated for their stall torque and no-load speed reflecting their ability to hold a position against a force and how fast they move when unloaded. While electrical power is calculated generally the same for both types of devices, these design and use differences have an impact on how mechanical power is determined.

Both motors and servos calculate electrical power the same, using the standard electrical power formula:

Electrical Power(W) = volts(V) x amps(A)

Open FTC Docs

Controlling a Servo Blocks

FTC Docs·BSD 3-Clause

In the section titled Creating an Op Mode with Blocks you learned how to use the Blocks Programming Tool to write an op mode that controls a 12V DC motor. In this section, you will learn how to write an op mode that controls a servo motor.

A servo motor is a special type of motor that is designed for precise motion. A typical servo motor has a limited range of motion.

In the figure below, a "standard scale" 180-degree servo is shown. This type of servo is popular with hobbyists and with FIRST Tech Challenge teams. This servo motor can rotate its shaft through a range of 180 degrees. Using an electronic module known as a servo controller you can write an op mode that will move a servo motor to a specific position. Once the motor reaches this target position, it will hold the position, even if external forces are applied to the shaft of the servo.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

Open FTC Docs

SDK Servos > Hidden Methods

Game Manual 0·CC BY-NC 4.0

All REV hub servos are instances of ServoImplEx, which exposes some more methods to the user, such as setting the servo PWM range.

Note: There is no downside to using ServoImplEx, in order to convert a Servo to a ServoImplEx the user simply just needs to cast the Servo returned by the hardwareMap to a ServoImplEx.

Tips:

You can use setPwmRange() to increase the servo range to a maximum of 500 to 2500 microseconds. This changes the SDK mapping, with 0 becoming the minimum microseconds and 1 becoming the maximum microseconds. Increasing the range to 500-2500 can give more range when controlling servos that use 500-2500 PWM Range, such as goBILDA servos and REV Smart Robot Servo
setPwmDisable() and setPwmEnable() can be used to turn on and off the PWM signal to a servo. On some servos, this will cause the servo to de-energize and "turn off" with no holding power. Other servos may stay on and hold their position. Behavior will vary from servo manufacturer to servo manufacturer. It will also disable 5V power if the servo is the only one in its group of ports.

Tip: Both REV Smart Robot Servos and goBILDA servos will stop holding their position when setPwmDisable is called, regardless of if the 5v power is still on or not.

Open Game Manual 0

Calculating Motor and Servo Power > Servo Mechanical Power Calculator

FTC Docs·BSD 3-Clause

One of the most difficult parts of calculating Servo Mechanical Power is working with unit conversions, especially since servo manufacturers use lots of different units. In order to calculate servo mechanical power correctly the speed unit MUST be converted to radians-per-second and the max stall torque unit MUST be converted to Newton-meters. Below is a handy calculator that you can use to automatically perform the necessary conversions and calculate Servo Mechanical Power (Thank you to Orion DeYoe for providing this tool).

Tip: - For Speed, use the radio button to choose the unit type that the manufacturer has provided - for most servos this will be listed in a period of time per 60 degrees (such as with the REV Smart Servo example) or perhaps the manufacturer may provide an angular velocity, such as rotations-per-minute (RPM). Enter the no-load speed value and unit as the manufacturer has provided.

For stall torque, provide the value and select the unit as specified by

the manufacturer. If the manufacturer merely provides kg, assume kg*cm.

The calculator automatically recalculates on any changes, there is no button to press in order to trigger a calculation.

Open FTC Docs

Controlling a Servo AS

FTC Docs·BSD 3-Clause

In this section, you will modify your op mode to control a servo motor with the buttons of the gamepad.

A servo motor is a special type of motor. A servo motor is designed for precise motion. A typical servo motor has a limited range of motion.

In the figure below, "standard scale" 180-degree servo is shown. This type of servo is popular with hobbyists and with FIRST Tech Challenge teams. This servo motor can rotate its shaft through a range of 180 degrees. Using an electronic module known as a servo controller you can write an op mode that will move a servo motor to a specific position. Once the motor reaches this target position, it will hold the position, even if external forces are applied to the shaft of the servo.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

Open FTC Docs

Controlling a Servo OBJ

FTC Docs·BSD 3-Clause

In this section, you will modify your op mode to control a servo motor with the buttons of the gamepad.

A servo motor is a special type of motor. A servo motor is designed for precise motion. A typical servo motor has a limited range of motion.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

Open FTC Docs

Reading and Writing to Hardware > Examples of Using Common Hardware Components > Servo

Game Manual 0·CC BY-NC 4.0


Servo relicServo = hardwareMap.get(Servo.class, "Release Servo");

After instantiating a Servo, there are two main functions that can be called: setPosition() and getPosition().:


releaseServo.setPosition(0.75);
telemetry.addData("Release Servo Target", releaseServo.getPosition());

setPosition() sets the position of the servo. The SDK will use a built-in control loop with the servo's potentiometer to drive the servo to that position and hold that position. setPosition() takes in a double between 0 and 1, where 0 is the servo's lower limit of rotation and 1 is the servo's upper limit of rotation. Everything between is directly proportional, so 0.5 is the middle, 0.75 is 3/4 the way up, etc.

getPosition() does not return the servo's current position, rather its current target position. If a variable for the servo's current target position is stored properly, this function should never be needed.

Open Game Manual 0

SDK Servos > General Explanation > 5V Power

Game Manual 0·CC BY-NC 4.0

All servos require a minimum of 5V to operate, and so the expansion hub generates 5V to power the servos. The 5V power is shared between pairs of ports (0-1, 2-3, 4-5). By default, this 5V power is OFF, and doesn't provide power. However, doing any servo operation, such as setting the position of a servo, will turn ON the 5V power to both shared ports. Calling disable PWM seems to turn OFF shared power, however if another servo is used or is active the 5V power will remain ON instead.

(Figure: An example of servo PWM waves. NOTE: Servo angles are arbitrarily selected and will not be accurate to all servos)

Warning: Every pair of two ports, (0-1, 2-3, 4-5) share the SAME 2 amp limitation, so care should be taken that pairs of servos do not pull more than 2 amps. Putting servos in every other port is recommended when possible.

Open Game Manual 0

Controlling a Servo Blocks > Modifying Your Op Mode to Control a Servo

FTC Docs·BSD 3-Clause

Let's modify your op mode to add the logic required to control a servo motor. For this example, you will use the buttons on the Logitech F310 gamepad to control the position of the servo motor.

With a typical servo, you can specify a target position for the servo. The servo will turn its motor shaft to move to the target position, and then maintain that position, even if moderate forces are applied to try and disturb its position.

For the blocks Program & Manage server, you can specify a target position that ranges from 0 to 1 for a servo. A target position of 0 corresponds to zero degrees of rotation and a target position of 1 corresponds to 180 degrees of rotation for a typical servo motor.

In this example, you will use the colored buttons on the right side of the F310 controller to control the position of the servo. Initially, the op mode will move the servo to the midway position (90 degrees of its 180-degree range). Pushing the yellow "Y" button will move the servo to the zero-degree position. Pushing the blue "X" button or the red "B" button will move the servo to the 90-degree position. Pushing the green "A" button will move the servo to the 180-degree position.

Open FTC Docs

Calculating Motor and Servo Power > Electrical Power and Mechanical Power

FTC Docs·BSD 3-Clause

For example, a typical REV Smart Servo is supplied with 6V when used with a REV Servo Power Module (SPM) or 5V when used with a Control or Expansion Hub. Per the servo's specs, at 6V the servo will pull at most 2A at stall (when the servo cannot physically move to the position it's being commanded to). This means the maximum electrical power the servo will consume is 12Watts of power when plugged into the REV SPM and being commanded to a position it cannot reach. The REV SPM supplies 90W of maximum electrical power, so the maximum number of fully-stalled REV Smart Servos the SPM can supply full power to is 7 (90W divided by 12W, ignoring the remainder).

Motors and servos also generally calculate mechanical power similarly.

Mechanical Power(W) = torque (N-m) x angular speed (rad/s)

Mechanical Power for a DC motor generally follows a very specific curve, based on its efficiency, stall current, stall torque, speed, and a bunch of other factors. The general performance curve of a DC motor can be seen in Figure 1.

(Figure: Figure 1: General DC Motor Performance Curve)

From this we can see that the Peak Power is found at the intersection of 1/2 Stall Torque and 1/2 Speed. Even though a servo is used different than a generic motor, this approximation is still good for calculating the maximum mechanical power of a servo. Simplified, we can use this formula:

Servo Max Mechanical Power(W) = 0.25 x stall torque(N-m) x no-load speed(rad/s)

Using this approximation the REV Smart Servo, when being provided 6V, produces a maximum Stall Torque of 13.5kg-cm (1.33N-m) and a time of 0.14s per 60 degrees of travel (7.48rad/s) yielding an approximate max servo mechanical power of 2.48W.

Tip: It's important to point out that a high speed motor or servo that is loaded past its maximum power point will actually do worse than a slower motor or servo with the same load. It's all about getting the maximum mechanical power by operating the motor at the max power point.

Open FTC Docs

Robot Wiring Guide > Common Problems and Troubleshooting > Hub and Phone Communication Issues

FTC Docs·BSD 3-Clause

The signals that pass between the Android phone and the controllers are sensitive to interference. If a motor power wire or servo wire is routed next to a USB cable, it is possible to induce a stray signal that can lead to intermittent problems.

Open FTC Docs

Choosing a Servo > Servo Range

Game Manual 0·CC BY-NC 4.0

The angle over which a non-continuous servo can rotate while retaining position feedback is called the range. When choosing a servo, it is important to make sure you have enough range for the movement you need.

By default, the FTC control system outputs 600-2400 microsecond signals. However, this range can be expanded to 500-2500 microseconds. When choosing a servo, it is important to make sure that its range will be usable for your application inside of 500-2500 microseconds.

Note: The default 600-2400 range of the FTC control system when selecting the "Servo" configuration limits the range of many servos to less than the advertised range. Make sure you select the "Full Range Servo" configuration instead to get the full 500-2500 range.

Open Game Manual 0

Servo Usage Tips

Game Manual 0·CC BY-NC 4.0

goBILDA 6V Servo Power Injector — A Servo Power Injector (SPI) is a device made by goBILDA that boosts the voltage that the Expansion Hub provides to a servo. The Control/Expansion Hub's output for servos is 5V at 6 amps, and the SPI boosts the voltage to 6V and up to 24 amps total output. It works similarly to the original REV Servo Power Module, but with significant improvements to maximum output power and overcurrent handling to prevent failure.

Open Game Manual 0

Servos

FTC Docs·BSD 3-Clause

REV Smart Robot Servo (SRS)

^^^

(Figure: REV-41-1097)

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REV-41-1097

goBILDA Dual Mode Servo

^^^

(Figure: 2000-0025-0504)

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GB 2000-0025-0504

Hitec Conventional Servo

^^^

(Figure: W39197)

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W39197

A servo is a type of device that takes a Pulse-Width Modulated (PWM) signal as an input and, with the help of an embedded controller, produces linear or rotational movement based upon the input signal. Servos may take an input signal generated by a REV Hub (either by a Control Hub or Expansion Hub) which itself provides 5V of power and a limited amount of current (see REV Documentation for more information). Servo power injectors such as the REV Servo Power Module (SPM) may be utilized to boost the power provided to servos up to 6V for up to 6 servos per device. Robots in FIRST Tech Challenge are subject to limitations on servos, including quantity and mechanical power. Please see the Competition Manual for details.

Open FTC Docs

*FIRST* Tech Challenge Tech Tips > Control System Hardware

FTC Docs·BSD 3-Clause

The "Calculating Motor and Servo Power" Tech Tip, including the Servo Mechanical Power Calculator, now lives on its own page: Calculating Motor and Servo Power.

Open FTC Docs