Novasinergia 2022, 6(1), 95-104. https://doi.org/10.37135/ns.01.11.06 http://novasinergia.unach.edu.ec
Research Article
Implementation and performance evaluation of a library to improve the
communication with Maxon Motor's
Implementación y evaluación de rendimiento de una librería para mejorar la comunicación
con los motores de Maxon
Hernan Morales , Andres Cordova , Ismael Minchala , Fabian Astudillo-Salinas*
Departamento de Eléctrica, Electrónica y Telecomunicaciones, Facultad de Ingeniería, Universidad de Cuenca, Cuenca, Ecuador,
010103; hernan.morales@ucuenca.edu.ec, andresf.cordovac@ucuenca.edu.ec, ismael.minchala@ucuenca.edu.ec.
*Correspondencia: fabian.astudillos@ucuenca.edu.ec
Citación: Morales, H., Cordova, A.,
Minchala, I., & Astudillo-Salinas,
F., (2023). Implementation and
performance evaluation of a
library to improve the
communication with Maxon
Motor's. Novasinergia. 6(1). 95-104.
https://doi.org/10.37135/ns.01.
11.06
Recibido: 09 diciembre 2022
Aceptado: 14 enero 2023
Publicación: 16 enero 2023
Novasinergia
ISSN: 2631-2654
Abstract: The development of robotic devices like exoskeletons
involves using several components, such as actuators and sensors.
These components exchange information with the main device or
between them using a communication system; this allows data transfer
during the device's operation. In addition, a communication system
with good performance is the basis for the correct operation of the
control system. In this context, selecting an appropriate methodology
to meet design specifications during software development is
necessary. This research presents the development of ALLEX CAN
software, so named because it is part of the communication system of
the ALLEX-2 (ALLEX version 2) prototype and uses the Controller
Area Network (CAN) protocol. The system was created using the
SocketCAN library as a base, which provides a range of tools to work
with CAN interfaces. C++ was the programming language selected to
develop this solution for its lower execution time. The performance of
this software is compared with the ready-to-use functions provided by
the manufacturer Maxon Motor. Experimental tests show the superior
performance of our developed software.
Keywords: CAN protocol, communication system, Maxon Motor,
software.
Copyright: 2023 derechos
otorgados por los autores a
Novasinergia.
Este es un artículo de acceso abierto
distribuido bajo los términos y
condiciones de una licencia de
Creative Commons Attribution
(CC BY NC).
(http://creativecommons.org/licens
es/by/4.0/).
Resumen: El desarrollo de dispositivos robóticos como los exoesqueletos implica
el uso de varios componentes como actuadores y sensores. Estos componentes
intercambian información con el dispositivo principal o entre ellos mediante un
sistema de comunicación, esto permite la transferencia de datos durante el
funcionamiento del dispositivo. Además, un sistema de comunicación con buen
desempeño es la base para el correcto funcionamiento del sistema de control. En
este contexto, es necesario seleccionar una metodología apropiada para cumplir
con las especificaciones de diseño durante el desarrollo del software. Esta
investigación presenta el desarrollo del software ALLEX (Autonomous Lower
Limb Exoskeleton) CAN (Controller Area Network) software, llamado así
porque forma parte del sistema de comunicación del prototipo ALLEX-2
(ALLEX versión 2) y usa el protocolo CAN. El sistema fue creado usando la
librería SocketCAN por la gran cantidad de herramientas. C++ fue el lenguaje
de programación elegido para el desarrollo de esta solución debido a su bajo
tiempo de ejecución. El desempeño de este software se lo compara con las
funciones proporcionadas por el fabricante Maxon Motor. Las pruebas
experimentales muestran el rendimiento superior del software desarrollado
frente al proporcionado por el fabricante.
Palabras claves: Protocolo CAN, sistema de comunicación, motor Maxon,
software
Novasinergia 2022, 6(1), 95-104 96
1. Introduction
The CAN protocol is widely used in several applications, from industrial, medical, and
robotics (Di Natale et al. 2012; Seoane et al. 2021). The CAN topology is bus-type and provides a
high-speed serial interface of up to 1 Mbit/s. This protocol allows sending information under the
multi-master and peer-to-peer configuration. CAN is a low-layer protocol, so it is generally used
with the CANopen protocol to add higher-layer functions (CAN in Automation (CiA) e. V. 2011).
The popularity of the CAN protocol is mainly because of its: simplicity, reliability, low cost, and
real-time performance (Bosch 1991; Farsi, Ratcliff, and Barbosa 1999).
Currently, some manufacturers provide ready-to-use communications protocols for diverse
applications; for example, Maxon Motor produces high-precision motors, gear heads, and
controllers for different purposes (Maxon 2022) and provides software with different functions for
CAN communication. The ready-to-use software is great for speeding up the development process
and reducing implementation time. However, some applications demand specific requirements
from the control and communications system. In this case, a better approach is to develop the
software according to the application's needs.
Several up-to-date research papers focus on exoskeleton development and use hardware from
Maxon Motors and CAN protocol for communications. The authors of (Lu et al. 2014) developed a
single-leg exoskeleton with four DOFs (Degrees of Freedom). Its joints implement brushless Maxon
DC motors (EC45), while its software uses CAN and CANopen protocols. The authors of (Pan et al.
2018) present an exoskeleton for lower limbs with four joints: hip and knee. This device uses Maxon
motors and, through CAN, communicates the master controller with the slave controllers.
Furthermore, the work reported in (Yuan et al. 2019) presents an exoskeleton for lower extremities
with eight DOFs: four actives (using Maxon motors) and four passives. Its communication system
allows the monitoring, the analysis of data, and the adjustment of operating parameters in real-time.
This research compares two versions of a communication system based on the CAN protocol for a
lower limb exoskeleton called ALLEX-2, which results from several research projects at the
University of Cuenca. The first version of the communication system used only the CAN functions
provided by Maxon Motor, resulting in a system with acceptable performance, although not
sufficient for the requirements of the exoskeleton. On the other hand, the second version involved
the development of proprietary software to achieve performance as close to real-time as possible.
With this latest version, the communication system showed a substantial improvement in the times
for reading and writing tasks. This it was possible to meet the needs of the robotic device.
This document is organized as follows: Section 2 presents the methodology of this study. Section 3
presents experimental results and a discussion of the main findings. And finally, Section 4 presents
the conclusions.
2. Methodology
This work presents the development of the base functions for the software for the
communication system of the ALLEX-2 prototype. Figure 1 shows the CAN architecture of the
ALLEX-2 prototype. Seven nodes are connected to the CAN bus, one master node with a Raspberry
Pi 4, and six secondary nodes with the EPOS4 50/8 (Easy Positioning System) devices. The initial
node of the bus is node 3, and the final one is node 6. This architecture allows the communication of
a distributed control system, where the master node generates the movement trajectories of each
joint and sends the commands to the secondary nodes. The main objective of the CAN
Novasinergia 2022, 6(1), 95-104 97
communication system is to guarantee that the processing and transmission times of the commands
are as short as possible.
The transmission time is reduced by implementing a bus with less than 25 m of length, which allows
a maximum transfer rate of 1 Mbps (CIA 2005). It is also possible to decrease the processing time by
using functions that accomplish their goal with as few internal processes as possible. This work
focuses on the writing and reading processes of CAN frames. The former ones are present in the last
part of the processes implemented in the master node. These processes are responsible for taking
the resulting data from the control calculations, assembling the CAN frames, and sending the
information to the channel. The latter refers to the processing of the measurements sent by the
secondary nodes.
Figure 1: Layout of the CAN topology in the exoskeleton (Cordova et al. 2023).
The need to create new CAN write and read functions arises from the existing functions' poor
performance. As mentioned, the prototype uses devices from the manufacturer Maxon, for which it
Novasinergia 2022, 6(1), 95-104 98
was decided, in the first instance, to use the software provided by the same manufacturer. The
following subsection shows the Maxon function description.
2.1 Maxon Motor functions for CAN
The EposCmd library implemented by Maxon Motors allows communication management
in a master node, where the user and the EPOS4 applications are executed. This library has functions
for both network management and CAN communication. The administration functions perform the
necessary configurations to open a port to send and receive CAN frames and to prepare the EPOS4
for movement. In this research, these functions are not replaced. For CAN communication, this
library has low-layer functions for writing and reading called VCS_SendCANFrame (MAXON_W)
and VCS_ReadCANFrame (MAXON_R), respectively. The functions of Maxon Motors are not open
source, so their internal operation is a black-box. Algorithm 1 presents a summary of the described
functions based on an inference of the required parameters. It can be seen that there is a system for
error handling and data conversion to the CAN frame format. In addition, the reading function
includes a timeout or maximum waiting time. The EposCmd library implemented by Maxon Motors
allows communication management in a master node, where the user and the EPOS4 applications
are executed. This library has functions for both network management and CAN communication.
The administration functions perform the configurations to open a port to send and receive CAN
frames and to prepare the EPOS4 for movement. In this research, these functions are not replaced.
For CAN communication, this library has low-layer functions for writing and reading called:
VCS_SendCANFrame (MAXON_W) and VCS_ReadCANFrame (MAXON_R), respectively. The
functions of Maxon Motors are not open source, so their internal operation is a black-box. Algorithm
1 presents a summary of the described functions based on an inference of the required parameters.
It can be seen that there is a system for error handling and data conversion to the CAN frame format.
In addition, the reading function includes a timeout or maximum waiting time.
Algorithm 1: MAXON's CAN writing and reading functions
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
function MAXON_W(handle,
COBID, data lenght,
data, error pointer)
process data
send can frame
if everthing ok return True
else return False
function MAXON_R(handle,
COBID, data length,
timeout, errorCode)
Wait for CAN frame
if everything ok
return true and CAN frame
else
return false and error code
2.2 Implemented writing and reading functions
The purpose of these functions (ALLEX_W and ALLEX_R) is to improve the performance of
the communication system by reducing processing time. To achieve this, functions with more
efficient control over the subprocesses necessary to send or receive CAN frames are implemented.
These new tools use the SocketCAN (Community. n.d.) library as a base. This library is designed to
be similar to the TCP/IP protocols. Such an approach makes it easier for programmers to develop
CAN applications.
Novasinergia 2022, 6(1), 95-104 99
Algorithm 2 shows the structure of the writing function called ALLEX_W. This function takes the
data to send and transforms it into a frame object. This object serves as an input to the writing
method of the SocketCAN library. The input parameters of this function are communication object
identifier (COBID (CAN in Automation (CiA) e. V. 2011)), data, data length, and a socket. The latter
is the link to the CAN interface and contains the necessary information for communication.
Algorithm 2: ALLEX's CAN writing function.
function ALLEX_W(socket,
COBid, data length, data)
for byte in data:
frame[i]= byte
nbytes = write(socket,frame)
if nbytes == data length:
return true
else
return false
The reading function called ALLEX_R uses another socket object. A socket must be used for writing
and a different one for reading to avoid communication conflicts (Community. n.d.). The function
calls the SocketCAN reading method to listen to the channel waiting for a CAN frame. If the process
is successful, the function returns the received frame. Unlike the MAXON_R function, the reading
function does not have an internal timeout; the time control is executed in a higher process where
the read function is called 12 times (2 times for each joint). Algorithm 3 presents the structure of
ALLEX_R.
Algorithm 3: ALLEX CAN reading function
struct can_frame ALLEX_R(socket)
define nbytes, frame
call read method
if nbytes < 0
return error
else if nbytes < size of frame
return error, incomplete frame
else
return frame
Maxon and ALLEX functions to write and read data on the CAN channel are compared using the
same software flow. Figure 2 indicates, in a general way, the mentioned process flow of the
exoskeleton. In this diagram, it is possible to see the reading and writing processes together with the
device's other processes. The MAXON_W and ALLEX_W functions were used in the writing
process, while MAXON_R and ALLEX_R were used in the reading process. The algorithm is
executed using one of the writes and reads options to measure the execution time, achieved through
the methodology explained in the following subsection.
Novasinergia 2022, 6(1), 95-104 100
Figure 2: General flow chart.
2.3 Processing time comparison of writing functions
The communication process between the nodes is performed according to the writing
methods described above. Before sending a CAN frame, the central node prepares the data to be
sent. Once this process is complete, the writing method is called to assemble and send the CAN
frame. To measure processing times, a ``timer'' object is used. Algorithm 4 describes the structure of
the chronometer. It starts the measurement through its constructor method and ends when the object
is destroyed to avoid undesired measurements. To measure the execution time is enough to create
an object of the timekeeper class. The measured time is appended to a vector; this avoids saving data
directly to disk to reduce timer processing. The storage process works at the end of the experiment.
The experimentation process comprises the execution of a gait cycle. This cycle lasts 8 seconds; a
sampling and transmission period of 20 ms is used. So, there are 400 writing processes and 800
reading processes per joint (one frame is written and two are read per joint). This experiment was
repeated until the data shown in the result section was obtained.
Algorithm 4: High resolution chronometer
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struct Timekeeper{
Timekeeper //Constructor
start = high resolution clock now
~Timekeeper //Destructor
Stop = high resolution clock now
duration = stop-start
save duration
3. Results
To obtain sufficient data to compare the Maxon and ALLEX functions, the algorithm of
Figure 2 was executed while the times of interest were measured. The times are measured in the
read-and-write loop (circled in the green box). In the prototype's context, the experiment monitored
a gait trajectory; this monitoring was carried out only with three nodes. In each execution of the
algorithm, 400 frames per node were written, that is, 1200 frames per execution. When writing a
frame, we have a sample of the execution time of the writing function. In total, 6000 measurements
were collected for each writing method.
Table 1 shows the results of 6000 executions of the writing method, five experiments for three joints.
This table indicates that the method developed in this project (ALLEX_W), on average, performs
5.76 times faster than the MAXON_W method. Also, the reduction in standard deviation means that
the ALLEX_W function is more stable than MAXON_W.
Table 1: Measurement result of writing execution times.
MAXON_W (ms)
ALLEX_W (ms)
Mean
1.8875
0.3275
Standard deviation
0.3135
0.1492
Max
8.6899
8.5573
Min
1.5451
0.2258
Figure 3 shows that 99.96% of the execution times for the ALLEX_W function are under 2 ms. On
the other hand, 22.37% of MAXON_W execution times are between 2 and 9 ms, while only 0.033%
of ALLEX_W function times are over this interval.
Figure 3: Execution time histogram for CAN writing methods.
During the experimentation, exchanging the reading methods, it was found that the internal
structure of the MAXON_R function was not designed for a 20 ms sampling period. This function
caused the system to stop, and the reading processes were not executed. Instead, the function
Novasinergia 2022, 6(1), 95-104 102
developed in this research performs reading processes at a time lower than the mentioned period.
The results show that 99.75% of the times the reading takes less than 2 ms when measuring the
processing time to read six frames (2000 measures of six frames reading time). The communications
system requires reading two frames per join every 20 ms, and the results show that the implemented
reading function is better than expected.
This communication system is an essential part of the development of the ALLEX-2. The trajectory
references generated by the central processor through this system are transmitted to the distributed
controllers. Then, proof of the efficiency of the ALLEX-2 CAN functions can be obtained by
analyzing the result of the trajectory tracking in a system where the functions developed in this work
are the base of the communication. Figure 4 shows that the root-mean-square tracking error is low
in the device performance tests with and without load; this proves that the communication system
meets the design requirements. More details about the control system result can be found (Cordova
et al. 2023). The source code of the ALLEX-2 project is available in Github
(https://github.com/Hiyperion/ALLEX2_SOFT.git).
Figure 4: Tracking error for a gait cycle, right lower limb (Cordova et al. 2023).
4. Conclusions
The library developed in this research has better performance features compared to the one
provided by Maxon for both writing and reading tasks. In the case of writing, the ALLEX_W
function executes 5.76 times faster than the function provided by the Maxon Motor. Also, this
improvement implies that the control system can send more operation commands to the
exoskeleton.
In this work, the ALLEX_R reading function is significantly better than that provided by Maxon
Motor. The function developed in this work can be used in similar applications that require sampling
times between 2 ms and 20 ms. This is supported by the results, which show that 99.75% of the
Novasinergia 2022, 6(1), 95-104 103
reading times for 6 frames is less than 2 ms. Even if fewer frames were used, this time could be less
than 2 ms, allowing other processes to be carried out until the sampling time is completed.
Although manufacturers provide tools and software for the fast development of applications. In the
case of robotic devices such as exoskeletons, where high performance is required, it is better to
develop software that allows the device's requirements to be satisfied. In this research, the developed
functions allow the communication system of the ALLEX-2 exoskeleton to comply with the times
required for its operation.
Author contributions
En concordancia con la taxonomía establecida internacionalmente para la asignación de
créditos a autores de artículos científicos (https://casrai.org/credit/). Los autores declaran sus aportes
en la siguiente matriz de contribuciones:
Morales, H..
Cordova, A..
Minchala, I..
Satudillo-Salinas,
F..
Conceptualización
Análisis formal
Investigación
Metodología
Recursos
Validación
Redacción – revisión y edición
Conflicts of Interest
The authors declare that there are no conflicts of interest in any nature.
Acknowledgment
The authors are grateful for the support provided by the Research vice-rectorate of the
University of Cuenca, under the financing of the project: ``Robotic exoskeleton for functional
assistance in walking patients with incomplete spinal cord injuries: design and initial application".
References
Bosch. (1991). “CAN Specification.”. Obtenido de http://esd.cs.ucr.edu/webres/can20.pdf
CAN in Automation (CiA) e. V. 2011. “CANopen Application Layer and Communication Profile.”.
Obtenido de file:///mnt/429874b3-0f62-46ad-98c6-
f1dd11bed9c1/Descargas/301v04020006_cor3.pdf
CIA. (2005). “CAN in Automation (CiA): CANopen.” (January):1–26.
Community., Kernel development. (n.d). “SocketCAN - Controller Area Network — The Linux
Kernel Documentation.”. Obtenido de https://docs.kernel.org/networking/can.html
Novasinergia 2022, 6(1), 95-104 104
Cordova, A. F., Morales, H., Astudillo-Salinas, F., Zhang, H., & Minchala, L. I. (2023). “Deployment
of a High-Speed Communication Network to Enable Real-Time Control of a Lower Limb
Robotic Exoskeleton. International Journal of Innovative Computing, Information and Control.
DOI: DOI: 10.24507/ijicic.19.01.181
Farsi, M., Ratcliff, K., & Barbosa, M. (1999). An overview of controller area network. Computing &
Control Engineering Journal, 10(3), 113-120. DOI: 10.1049/cce:19990304
Lu, R., Li, Z., Su, C. Y., & Xue, A. (2013). Development and learning control of a human limb with a
rehabilitation exoskeleton. IEEE Transactions on Industrial Electronics, 61(7), 3776-3785. DOI:
10.1109/TIE.2013.2275903
Maxon. 2022. “Discover the Maxon’s World of Drive Technology | Maxon Group.” Obtenido de
https://www.maxongroup.com/maxon/view/content/index
Di Natale, M., Zeng, H., Giusto, P., & Ghosal, A. (2012). Understanding and using the controller area
network communication protocol: theory and practice. Springer Science & Business Media.
DOI: 10.1007/978-1-4614-0314-2
Pan, C. T., Chang, C. C., Sun, P. Y., Lee, C. L., Lin, T. C., Yen, C. K., & Yang, Y. S. (2019). Development
of multi-axis motor control systems for lower limb robotic exoskeleton. Journal of Medical and
Biological Engineering, 39(5), 752-763. DOI: https://doi.org/10.1007/s40846-018-0449-z
Seoane, L., Díaz, C., Zafra, J., Ibarmia, S., Quintana, C., Canora, C. P., ... & Araujo, A. (2018). CAN
implementation and performance for Raman Laser Spectrometer (RLS) Instrument on
Exomars 2020 Mission. IEEE Transactions on Emerging Topics in Computing, 9(1), 67-77. DOI:
10.1109/TETC.2018.2874643
Yuan, Y., Li, Z., Zhao, T., & Gan, D. (2019). DMP-based motion generation for a walking exoskeleton
robot using reinforcement learning. IEEE Transactions on Industrial Electronics, 67(5), 3830-
3839. DOI: 10.1109/TIE.2019.2916396