The most important elements of a robot

A beginner roboticist should learn the basics of a few type of technologies and components before embarking himself in the design and construction of his own robot. I would say that the main areas one should be familiar with are:

  • Electric motors: it is important to distinguish the types and to know the basics of operation of the most used motors, e.g. DC brushed, DC brushless, PMSM, steppers, servos, etc. In a typical project, very frequently, one has to select the proper motor for an application and possibly the combination of motor and gearbox. I have published a very useful guide to select an electric motor for a simple robotic application. (https://enriquedelsol.com/2017/11/19/motor-selection-for-robots-i/).

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    Brushless motor

  • Motor controllers: the motors need to be powered and controlled with the right hardware, typically based on a H bridge made of transistors. It is important to understand the very basics of motor control to avoid spending a considerable amount of money in unnecessary hardware.
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H bridge

  • Sensors: in robotics, it is a must to know the different sensors that can provide positional feedback, such as encoders, resolvers, potentiometers. Some of them will be needed depending on the motor we select for our application. In the simpler scenario of choosing servomotors, position sensors will only be needed as a backup sensing method. Other types of sensors very common are: temperature sensors, force sensors, velocity sensors (tachometers), linear displacement, light sensors, etc. It is very important to measure distance, in order to map the environment or to avoid obstacles. The most common sensors to measure distance are: infrared sensors (relatively inexpensive), ultrasonic sensors (inexpensive as well), lasers and depth cameras. These can measure the distance on a large interval to which they have been calibrated for. For an accurate distance detection, there are switches based on capacitive and inductive technology, very reliable but only available for fixed distances. Other common sensors that are used during the electronic control are current sensors and voltage meters.

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Posted in General, Robotics

The most common places to shop for robotics components

This time I would like to review the most used online shops to buy components for a robotic design. Some of them are more focused on the professional market and some others pay attention to the hobbyist.

The two main companies that distribute components for the robotics industry and other engineering projects are RS (https://uk.rs-online.com/web/) and Farnell (https://www.farnell.com/). They are generalist companies oriented to the professional market with a very good client service, lead time and stock. Their search engines are very powerful and one can practically find everything they need. They are very well known in the European market and also established in the American market under other names: https://americas.rsdelivers.com/ , and https://www.newark.com/ . They usually offer free or very competitively priced next day delivery.

More focused on the hobbyist market, one can find RobotShop (https://www.robotshop.com) which has a deep and very well priced stock. It sells internationally. In this shop, the robotics enthusiast can find everything, from ready to assemble kits, to motors, sensors, cables, fixings, electronics, etc.

Another important reference in the hobbyist market is SparkFun (https://www.sparkfun.com/). Almost everything you may need can be found in this online shop, with special focus on electronic components.

If we need specialised hardware like wheels, structural elements, aluminium profiles, etc, a good online shop to buy from is Active Robots (https://www.active-robots.com/)

For an additional stock in electronics and mechanical components, I recommend to check Pololu (https://www.pololu.com), with a good range of components at a very competitive price.

Mouser (https://www.mouser.com/) and DigiKey (https://www.digikey.com/ ) are also two reference shops that work internationally, delivering plenty of products specially for the professional or the engineer. They are more focused in electronic components.

If we are looking for servos or ready to mount kits, we should check out Trossen Robotics, with a wide catalogue for the robotics enthusiast (https://www.trossenrobotics.com).

In addition to previous websites, the engineer or enthusiast should not forget the official distributor of Arduino (https://www.arduino.cc/) and the RaspberryPI official website (https://www.raspberrypi.org/), with tons of information about how to program their widely used development boards.

Depending on the desired component we are after, we can also review the website of the main manufacturers. For example, if there is plenty of budget available and we are looking for a motor, I would recommend checking out Maxon motors (https://www.maxongroup.com), with almost an infinite catalogue of electric motors of all types, electric drives and gearboxes. Other famous brands for the professional markets are Infranor (http://www.infranor.com/) and Yaskawa (https://www.yaskawa.com).

 

And this is all for now. Enjoy your shopping!

 

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Posted in Robotics

Best blogs and podcasts to stay tuned on the Robotics Industry

ABB robot with a closed loop.

Hi, I would like to come back to my blog by commenting on very useful resources for the roboticist.

  • IEEE Spectrum: https://spectrum.ieee.org/ .The first one on the list. The IEEE is the world largest technical professional organization. They organise conferences, publish standards, and serve as a repository for an enormous amount of technical papers. Normally, when a researcher or just a curious person tries to access the papers in its brother website (ieee.org), they need to either pay a monthly/annual fee, or one-off payment to access the information. If the need is for professional use, your research centre or company should cover the expenses of the consultation or subscription. However, the IEEE Spectrum website can be enjoyed for free and it contains probably the best summary of new discoveries, advances and new trends in robotics and related industries. It is however, biased towards the American research.
  • Robohub: https://robohub.org/ .Robohub is a non-profit organisation that as its name says, it is a hub for sharing stories, news, research, articles and interviews about robotics. It brings together experts in robotics to make sure the contents meet the highest editorial standards. They claim that their information is original and does not appear anywhere else. It is a good source of educational resources. It also has a very good podcast at: https://robohub.org/podcast-episodes/.
  • org: The Robotic Industries Association posts daily news, case studies, technical papers, job openings and much more. It has a very good section about news in emerging markets and industry standards ( https://www.robotics.org/).
  • Robotics Business Review: The Robotics Business Review claims to be “The Largest, Most Comprehensive Online Robotics News and Information Resource“. It regularly posts about all aspects of the business of robotics. However, most of its posts are only short abstracts and you must pay a yearly membership fee to access the complete article ( https://www.roboticsbusinessreview.com/).
  • The Robot Report: The Robot Report is written by roboticist, Frank Tobe, cofounder of Robo-Stox, a tracking index for the robotics industry, now RoboGlobal. Through the blog he aims to follow the business of robotics and regularly reports on developments from all areas of the robotics industry.( https://www.therobotreport.com/)
  • Machine Design: (machinedesign.com/). The technical quality of this blog / website is impressive. They provide a general knowledge of different fields in robotics, automation and engineering. It is a good source of technical articles and also news about the industry. If anyone is looking to explore more in-depth the technical issues behind robotics this is the place to go.

For the people interested in attending to conferences, workshops or looking for jobs in the robotics industry, there are two mail lists where every event is published. These are:⅞

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Posted in News, Robotics

Magnetic Encoders VS Optical Encoders

Encoders, whether rotary or linear, absolute or incremental, typically use one of two measuring principles—optical or magnetic. While optical encoders were, in the past, the primary choice for high resolution applications, improvements in magnetic encoder technology now allow them to achieve resolutions down to one micron, competing with optical technology in many applications. Magnetic technology is also, in many ways, more robust than optical technology, making magnetic encoders a popular choice in industrial environments.

Parameter Optical Sensor Characteristics Magnetic Hall Sensor Characteristics
Principle  coded disc/scale, through beam arrangement  magnet/tape/polewheel opposed to sensor
Incremental accuracy of target  100 nm – 1 μm

(lithography process)

 5 μm – 30 μm

(magnetisation process)

Energising by external LED (20 mW)  by target (Br>220 mT)
Signal Frequency  > 1 MHz possible  < 50 kHz
Benefits  high code density, high code accuracy  robust
Disadvantages  sensitive to contamination, high alignment requirements  raw code density, medium code accuracy

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Complementary Encoder Signals to mitigate the electrical noise

Electrical noise is a common problem that occurs during the transmission of an incremental encoder’s signal to the receiving electronics, especially when the cable lengths are very long. Stray electromagnetic fields or currents induce unwanted voltages into the signal. These voltages can cause the receiver to make false counts, producing errors in the position or velocity feedback.

The primary way to alleviate electrical encoder noise is to use TTL output – also known as differential line driver output. This output format provides not only the standard A and B square wave signals and a Z reference signal, it also includes their complementary signals, /A, /B, /Z (sometimes written as A’, B’, and Z’). These complementary signals are produced by splitting the output of each channel (A, B, and Z) into two signals that are 180 degrees out of phase (complements) with each other. In other words, when the A signal is high (logic state 1) the A’ signal will be low (logic state 0). The receiving electronics take the state of that channel as the difference between the two signals.

encoder noise

In order for the complementary signals to be read, however, the receiving electronics must have a circuit that is designed for differential input – known as line receiver input. In addition, the wires for each channel (A and A’, for example) should be a twisted pair. In this twisted pair of wires, any electrical encoder noise that is induced will be the same on each signal. The receiving electronics recognise only the difference between the two signals, and because the signals are complements (equal in magnitude, with 180 degree phase lag) but the noise is common mode (equal on each signal, with no phase lag), the noise is cancelled out on the receiving end.

Image result for Incremental optical encoder quadrature operation

Incremental quadrature encoder for noise rejection

 

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Posted in Actuators, Robotics, Sensors

Optical Encoders in Brushless Servo Motors

The trend towards wide scale use of Brushless Motors is being driven by manufacturing
cost reductions, improved efficiencies, greater reliability, availability of improved drive
electronics, and the availability of improved sensors for motor control.

The brushless motors, independently whether they are PMSM or BLDC motors, require to know the relative position of the windings with respect the motor poles. To do this, a set of Hall Effect sensors are typically employed and placed between windings. An alternative option is to use the positional feedback from more accurate sensors to determine the presence of the magnetic field without using Hall Effect sensors. However, an intelligent drive is required to perform this count and issue the signal that conmutates the phases. This action is performed automatically with the bipolar Hall-Effect sensors.

Image result for hall effect sensors

Hall Effect sensor

For the majority of applications in the US and Japan, the trend in brushless motor sensor designs is moving away from Hall boards and feedback elements to integrated devices. For resolver applications, it can be handled by adding a dedicated set of 2, 3, or 4 speed windings for commutation, or it can be handled with a single speed winding and an intelligent drive.

The elimination of the Hall sensors from the BLDC motor eliminates many of the potential problems which can occur in a motor application. Hall devices are sensitive to acoustic noise, current spikes, temperature, EM fields, and can be difficult to align, which results in torque ripple. When a BLDC motor is used in a servo application with a high resolution feedback sensor, Hall sensors are redundant and consume space. They also add to motor length, assembly costs, cable harnessing complexity, and decrease overall reliability. The use of an encoder or resolver to eliminate Hall sensors in this situation is not only cost effective, but also improves the overall system performance.

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Drive with Hall board, Encoder or Resolver for Commutation and Feedback a caption

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Drive with Commutating Encoder

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Encoder Types.

When an encoder is used as the feedback element, there are a variety of types to choose from. The following is a short summary of the predominant types currently available.

1. Incremental, (TTL)
Readily available from a wide variety of Suppliers. Almost unlimited line count availability up to 5000 cycles per revolution. Special line counts and output options
are easily obtained.

2. Incremental with Commutation, (TTL)
Becoming more common in the US and Japan, availability is somewhat constrained by lack of industry standards. Mounting configuration, signal conditioning, and power supplies vary widely. Available in line counts up to 8000 for 2, 4, 6, and 8 pole motors. They are being developed in both hollow-shaft and modular versions by a variety of encoder suppliers.

3. Incremental with Commutation, (Sine wave)
More common in Europe, this type of encoder generally has sinusoidal quadrature
outputs, with a 1 volt pk-pk amplitude. Commutation is accomplished using a quadrature one cycle per revolution output.

4. Absolute Single Turn, (TTL/Parallel)
Less common for drive applications, these are usually found in 10 to 12 bit versions. Larger word sizes are available, but costs become a real issue and make them unsuitable for all but the most specialized applications.

5. Absolute Multi-turn, (Sine wave Incremental, Serial Absolute)
These encoders are generally based upon a 12 or 13 bit single turn absolute encoder, with a 12 bit turn counter yielding 24 or 25 bits of position information. Although these have been available for some time, they have been too costly for widespread applications. Recent developments in Europe, however, are making these more available, and costs are starting to come down. These encoders contain an incremental output with A, B, and Reference pulse, a serial absolute interface, and commutation outputs. Commutation output is derived from the MSB of the single-turn absolute. The Incremental tracks are derived from the LSB of the absolute encoder, and generally result in a 2048 or 4096 cycles per revolution incremental signal that is suitable for use in high-speed servo controls.

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Posted in Actuators, Robotics

A good robotic blog

One of my mates is developing several projects related to robotics which are worth mentioning.  In his blog,  one of his last projects is the construction of an humanoid robot based on steppers.

Main Assembly v114 Spine

Posted in Robotics
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