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Electrical actuators



Electrical motors

Types of electrical motors with drivers

The following table is far from being exhausting, it only contains a few example that have high rating among DIY communities and other types of users.


Motor type

Controllers

Pros

Cons

Stepper motors


Designed to provide step motion, high repeatability. They are also optimized to hold torque in one position (work well in open loop drive operations). Operate at lower speed.


A stepper can be operated like a BLDC and vice versa.




uStepper


See Landshine closed source equivalent

Stand alone closed-loop motion control (no missing steps). With uStepper every motor has everything it needs to function: Arduino compatible atmega microcontroller, voltage regulator, motor driver. Haptic mode implemented already and demonstrated on robotic arm.

12 to 24V


Mechaduino


See Landshine closed source equivalent

Used in HangPrinter!

Stand alone closed-loop motion control (no missing steps).

Encoder feedback.

Arduino compatible.

8 to 25V

MicroUSB interface


NEMA17 MKS SERVO42 developed by Makerbase


Looks like a Chinese knockoff of mechaduinos

to verify!

Brushless servo motors

Very similar for stepper motors.


Designed to provide smooth motion, lower repeatability than stepper motors, better control over speed, acceleration and torque.


A BDLC can be operated like a stepper and vice versa.


Generate less heat, lower maintenance, powerful, efficient, and silent, smart/versatile motors.  

Higher cost than brushed motors

oDrive Robotics


Motore choice table


See Landshine closed source equivalent

High quality, open source, good community behind it



Parallax

Canned solution

Low torque

Brushed motor

Designed to provide smooth motion


Very affordable

Higher maintenance and generates more heat than broshless. Dumb motors.

Tarocco

Open source. Project documentation: Visit hackaday.io or github for more information.

Doesn’t seem to be very advanced or complex applications development.

See electrical motors pros and cons





Motor Selection



In this section we consider two types of electrical actuators.


Stepper motors and Brushless motors


In the past, there have been many discussions and articles regarding the benefits of brushless motors over steppers and vice versa. Upon inspection it becomes clear however that this is more of an apples to oranges comparison and not a true technology benchmark. Of course both motor types can be utilized as actuators in a motion control application. But if the motor type is properly selected based on the application requirements, there should be no doubt whether one or the other could be a better choice.

In the last few years there has been an increase of available brushless motors specifically designed for gimbals. One point that needs to be considered early on is that other than the physical motors themselves, the actual drive (or amplifier) electronics that are used with each motor type play a very large role in how both are compared, because steppers use an open loop system of control and brushless DC motors need a closed loop for positioning.


Description with pros and cons


Stepper Motor

Brushless Gimbal Motor

PROS:

  • Easier to control

  • Fixed steps

  • Less expensive

  • Widely available

  • Micro-stepping possible for minute angles, below 1o

PROS:

  • Latest technology

  • Full torque throughout RPM range

  • Infinite steps

  • Does not require gearing to achieve proper accuracy (direct drive)

CONS:

  • Old school technology

  • Widely used standard comes with only 1.8 or 0.9 degree rotation increments (if halfstepped)

  • Requires gears to achieve better precision and accuracy

  • Steep torque rolloff at higher RPM

CONS:

  • More expensive

  • Requires more complex electronics

  • Custom closed loop system requires extra feedback mechanism

  • Will require continuous power supply to hold in one position.


CONTROL:

  • Open loop system requiring no feedback for position. Incremental steps for each new position

CONTROL:

  • Requires custom closed loop system for precise encoding of rotational position


Stepper motor and BrushLess DC motor simulation







More on Stepper Motors



What are stepper motors good for?

  • Positioning – Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC, camera platforms and X,Y plotters.

  • Speed Control – Precise increments of movement also allow for excellent control of rotational speed for process automation and robotics.

  • Low Speed Torque - Normal DC motors don't have very much torque at low speeds. A Stepper motor has maximum torque at low speeds, so they are a good choice for applications requiring low speed with high precision.

What are their limitations?

  • Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.

  • Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. This particular application is low speed. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.

  • No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Coils and Phases

A stepper motor may have any number of coils connected in groups called phases. All of the coils in a phase are energized together.


Unipolar vs Bipolar:

A two phase bipolar motor has two groups of coils. A four phase unipolar motor has four. A two phase bipolar motor will have four wires - two for each phase. Some motors come with flexible wiring that allows you to run the motor as either bipolar or unipolar.


Bipolar drivers use H-bridge circuitry to actually reverse the current flow through the phases. By energizing the phases with alternating the polarity, all the coils can be put to work turning the motor.

Unipolar drivers always energize the phases in the same way. One lead, the "common" lead, will always be negative. The other lead will always be positive. Unipolar drivers can be implemented with simple transistor circuitry. The disadvantage is that there is less available torque because only half of the coils can be energized at a time.



More about microstepping


This table dramatically quantifies the significant impact of the incremental torque per microstep as a function of the number of microsteps per full step.

Microsteps/Full step

1

2

4

8

16

32

64

128

256

Holding Torque/ Microstep

100.00%

70.71%

38.27%

19.51%

9.80%

4.91%

2.45%

1.23%

0.63%



Choosing a stepper motor



There are many types of stepper motors  but we should focus on the ones that can be driven by commonly available drivers, mainly 2-phase bipolar or 4-phase unipolar models.

Motor size

Considering the work that the motor needs to do, torque rating has to be considered. The size or NEMA numbers define standard faceplate dimensions for mounting the motor. They do not define the other characteristics of a motor. Two different NEMA 17 (these are commonly used in 3D printing) motors may have entirely different electrical or mechanical specifications and are not necessarily interchangeable.

Step count

Commonly available step counts are 24, 48 and 200. Resolution is commonly expressed as degrees per step: A 1.8° motor is the same as a 200 step per revolution motor. The tradeoff is higher steps equal to less torque and rotational speed.


Gearing

Gearing is another way to increase resolution and torque. Tradeoff is of course speed. A 32:1 gear-train applied to the output of an 8-steps/revolution motor will result in a 512 step motor.

Adding gear reduction units increases the torque, but also adds losses due to friction in bearings and teeth. Gears also add "slop" (or takeup), and loss of precision due to wear over time. There are ways to reduce the slop but it usually involves pre-loading, which causes more torque-stealing friction and resultant increased wear. Belt drives are also an option that require no lubrication. However, you'll still have the friction losses, and some "slop" to deal with. And then there is the factor of 3D printing the parts of such a system and the problems they introduce.


Shaft style

Something to consider depending on how the motor interfaces with the Gimbal’s drive system (ie: direct drive, screw drive, etc…)

  • Round or D shaft - Many sizes and couplings are available, D-shafts have a flattened side to prevent slippage (good for higher torque applications)

  • Geared shaft - Shafts with gears milled right into them to mate with modular gear trains.

  • Lead-screw shafts - Used to build linear actuators.


Since the PV gimbal requires less than 1° of precision, to achieve this with a standard 1.8° per step motor we have two options:


Use a gearbox (3d printed or not) with an input to output ratio of 1.8::1 or 0.9::1 and inherit the problems mentioned above.

Here is an example of a ready made solution from STEPPERONLINE:

Gear Ratio 9:1 Spur Gearbox High Torque Nema 34 Stepper Motor 34HS38-4004D-SG9

$128.28

The resolution reach of the final output drive is a 0.2° step angle

Use a microstepping driver that PWM-modulates the current to the motor coils in a ratio that allows the rotor to stabilize in a position between the poles.



Sources and stepper motor comparison charts

StepperOnline (great chart),

Pololu (higher torque selection only),

Robot Shop





Open loop vs closed loop motion control


Open-loop systems require more rigid structures, good precision and repetitivity. These constraints can be relaxed in closed-loop systems, where a feedback signal is used to position the system at an absolute position and hold it in place. Closed-loop systems can reduce the costs of the device and can relax the need of expensive (highly precise) parts.


See Wikipedia page on Linear Encoders


A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller.

The encoder can be either incremental or absolute. Motion can be determined by change in position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current. Optical technologies include shadow, self imaging and interferometric. Linear encoders are used in metrology instruments, motion systems and high precision machining tools ranging from digital calipers and coordinate measuring machines to stages, CNC Mills, manufacturing gantry tables and semiconductor steppers.


See Wikipedia page on Rotary Encoders


A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.

There are two main types: absolute and incremental (relative). The output of absolute encoders indicates the current position of the shaft, making them angle transducers. The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position.

Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses,[1] computer input devices (such as optomechanical mice and trackballs), controlled stress rheometers, and rotating radar platforms.



Types of Rotary Encoders


There are two main types of Encoders : Incremental and Absolute. For detail see Wikipedia article about rotary encoders



pros

cons

Incremental Encoder

Position is determined through incremental movement. Positions are divided but not unique, giving feedback only relative to the last position.

Simple to operate and interface.


Can be used for very high speed rotations


Ideal for measuring speed and distance


Can easily be adapted for multi-turn setup.


Can be very high resolution

Require realignment or homing in case of power failure or for each new instance.


No absolute positional data available. Requires external elements to keep track of movement and relative position.



Absolute Encoder

The position is read in absolute terms by using special codes that denote each unique position of the shaft separately

Absolute positional data available even in case of power failure.


Changes in position while there is no power is immediately read and reported when system powers up.


Requires only initial alignment for proper functioning

Requires specialized interfaces to transfer and read positional codes.


Requires special encoders/decoders devices or software to relay information.


Can not do ultra high speeds.


Expensive and specialized

(compared to incremental encoders)




Commercially available encoders



Here are some links to commercially available encoders. Due to the different types of technology used and many different configurations available, we are only including samples that will broadly fit our need. However, for future reference and re-evaluation, we are including links to the manufacturers for more options.


Avagotech AEAT 9000 Absolute Encoder

17-Bit absolute rotary encoder enabling 131072 absolute positions over 360°. Uses optical technology. The modular package enables the encoder to be directly integrated to the motor, unlike conventional encoders. Interface output will be SSI (2wire SSI / 3-wire SSI). On-chip interpolation and code correction compensate for mounting tolerance

Pros

  • Contactless application. Can be mounted on the motor

  • Simple two or three wire interface.

  • In-built correction system for mounting alignment

  • Multiple models available with different resolutions, interfacing, input and power options.

  • Optical Technology for very high resolution, precision and accuracy

Cons

  • Proprietary solution that may be hard to source

  • Expensive (around $50 each)

  • Will require external decoding/encoding mechanism for reading sensor data.

  • Optical technology has limited usability in dust prone environments.

Allen-Bradley 845D absolute rotary encoder

Optical Encoder that uses NEMA 4 mount. Output in BCD, Gray Code or Binary. 8 to 12 bit resolution option (256 to 10,000 positions per rotation)

  • High resolution using optical technology.

  • Available in various mounting options.

  • Available for different outputs and resolutions.

  • Sturdy, weight bearing design.

  • ready to be mounted off-the-shelf and available in standardized sizes.

  • Works by contacting the motor. May have issues with interfacing with the motor shaft.

  • Outputs require specialized reading/conversion and perhaps a communication interface.

  • Somewhat specialized equipment. May not be easily available in many parts of the world.


From Piher (contactless motion sensors)

From Gill (contactless rotation sensor)

From BEI Sensors

From Bourns

From Rockwell Automation

From Digikey



AHRS and IMU as feedback or closed loop motor control



An Inertial Measurement Unit consists of sensors on three axes that provide attitude information including heading, pitch, and yaw. A basic IMU provides raw sensor data while an Attitude and Heading Reference System takes this data one step further: using complex algorithms and code, it converts the raw data into heading or direction in degrees and standard units like feet or meters, etc. This abstraction layer from the raw data is called

Sensor Fusion.




pros

cons

Adafruit 9-DOF IMU Breakout

$19.95

Adafruit's 9DOF (9 Degrees of Freedom) breakout board allows you to capture nine distinct types of motion or orientation related data: 3 degrees each of acceleration, magnetic orientation, and angular velocity.

PDF, Guide

This combines the LSM303DLHC(accelerometer and magnetometer) and the L3GD20 (gyroscope).

Sensor fusion is provided by the Adafruit Unified Sensor Driver

Inexpensive

Easily available

Requires dedicated arduino or compatible

Adafruit 9-DOF Absolute Orientation IMU Fusion Breakout - BNO055

$34.95


This board from Adafruit uses Bosch’s BNO055 SIP and combines the sensors and an ARM Cortex-M0 processors on a single die with the sensor fusion built in! Basically you don’t need an external microcontroller running the libraries that turn the raw data into usable output.

PDF, Guide


Abstracts sensor data


Handles computation


Easily available

Moderately Expensive

X-io AHRS & IMU (hybrid Open Source)

£249.00

The x-IMU was designed to be the most versatile Inertial Measurement Unit (IMU) and Attitude Heading Reference System (AHRS) platform available.


Open Source libraries

Abstracts sensor data


Handles computation


Data rate up to 512Hz


Open source Sensor Fusion and resources


Expensive


Overkill for PV application


Schematic and board not Open Source





Video: General info about closed loop servo motor control

Video: Comparing motor and motor drivers pairs



Other types of actuators to be considered



Pros

Cons

Miniature Hydraulic Actuator


Carry high loads


Require complex controls for precise movements


Specialized equipment for hydraulic fluid


Not easily available


Hard to replace or repair


Will require complex system of gears for converting linear extension to rotational motion


Compressed Air Actuators


Carry high loads


All of the above plus

change in temperature affects compressed air performance adversely

Magnetic Actuator built in to the PV carrier plate itself

Brief Description :The edges of the plate that carries the PV are magnets. There are coils on all sides of the plate that can be energized to rotate and tilt the PV plate. Essentially, the PV plate becomes the shaft of a motor.


Custom enclosed system that matched the requirements exactly


Removes need of gears, reducers and gimbals




Closed system, replacement parts need special development


Reinventing the wheel.

Not easy to fabricate and would require custom controllers


Would require external or internal encoding mechanism for position sensing

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