DIY Motion Platform Servo Drive System - 64SPU-1 Card

 

UPDATE March 2015, IMPORTANT - the 64SPU-1 has now been superseded with the NEW 64SPU-2 card - see here for more details.

 

UPDATE March 2013, for an excellent example of a drive system build using the 64SPU-1 and other cards please see Roland van Roy's write-up here -  http://simprojects.nl/electrical_drive_iv.htm

 

Roland has helpfully provided advice and his technical wiring diagrams for his system on the page.

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The motion platform servo drive system developed for the DIY platform projects described on the web site has evolved through several versions to what is now a compact and effective low-cost DIY motion platform servo drive. The home-made Signal Processor Units I previously built on a PICAXE breadboard kit have now been replaced with a small standalone PCB that incorporates all the previous features of the SPU's and adds several important enhancements. The 64SPU-1 card is available to buy now.

 

The 64SPU-1 card works with the BFF PID Servo Controller software to provide a functioning servo control system.

 

Illustrative system wiring below: (for use with alternative Dimension Engineering Sabertooth / SyRen controllers see here...)

 

 

 

 

The main components that make up the DIY motion platform servo drive are now:

• The 64SPU-1 64MHz Signal Processor Unit card - available to buy now - see the order pages here.

• Devantech MD03 Motor Speed Controllers - 24V 20 Amp DC controllers delivering up to 480 Watts of electrical power per controller.

• 24V DC PM Motors to suit the DIY platform project. The MD03 controllers will support up to a nominal 20 Amps rating, sustained currents above this level will cause overheating and current limiting by the MD03's.

• Position feedback potentiometers (10 bit), or, recommended for smoother drives, the 12ADC-1 12bit ADC card or US Digital MA3 position encoders (12bit PWM). (Pots/encoders fitted to the drive actuators or elsewhere on the platform to report actual platform position.)

• DC power supplies for the drive motors and electronics.

• Actuator travel limit switches, fuses, cables and wiring.

PLEASE NOTE - I am not able to supply all of the above hardware, just the 64SPU-1 card, the BFF I2C Isolator card , 12ADC-1 12bit ADC card and motion software. If you want to build the servo drive for your own project then you will need to source the motors, controllers, encoders, switches etc etc yourself.

 

 

IMPORTANT - The 64SPU-1 card is part of a closed loop position feedback servo drive. The position feedback potentiometers/encoders shown on the wiring diagrams MUST be fitted to the drive motors/actuators to report platform position to the 64SPU-1 card. Without this the closed loop position control can NOT operate. If you are not sure what a closed loop position control servo drive is then I recommend you try and read-up a bit before attempting to use this hardware. It is very important to understand that this is not a "plug and play" system intended for consumer use - it is intended for use by informed DIY system builders. Read both the motion software user manual and the 64SPU-1 data sheet before using the hardware.

 

64SPU-1 Card - The new 64SPU-1 signal processor card is at the heart of the servo drive and acts as the communications bridge between the MD03 motor controllers, feedback pots/encoders and the BFF PID Servo Controller software running on the PC. It also acts as a system monitoring and information feedback unit - it monitors for communications interruptions and stops the motor drives when required and also feeds back MD03 current and temperature information to the Servo Controller software.

 

The 64SPU-1 is available to buy as a ready-to-connect card (80 mm x 80 mm) which removes the need for all the delicate breadboard wiring of the previous DIY versions.

 

For full details read the 64SPU-1 data sheet -

 

64SPU-1 Data Sheet

 

The main features of the 64SPU-1 are:

• I2C data output for use with Devantech MD03 motor speed controllers.

• 2 or 3 channel output for 2 or 3 DOF motion systems.

• Selectable Potentiometer (10 bit) or MA3 Digital Encoder (12 bit) actuator position feedback. Potentiometer feedback can be upgraded to 12bit resolution using the 12ADC-1 card.

• Built-in fast smoothing on the potentiometer feedback to reduce signal noise. Use with the 12bit 12ADC-1 card for best potentiometer feedback smoothness.

• Built-in “Pause Output” or "Drive Enable" trip loop.

• 64MHz operation giving up to 50Hz control loop refresh speeds (up to 70Hz when used with the 12ADC-1 pot feedback card).

• Direct connection to PC USB or Serial port (physical serial port recommended for fastest speeds). 115200 baud serial comms.

• LED status indicators, screw connectors.

• On-board 20X2 microcontroller re-programmable in-situ for firmware upgrade or customised output programming.

The card is available to buy on its own or bundled with the V2 Motion Driver software - see the order page here.

 

(NOTE - Data Sheet for superceded 40SPU-1 card can be found here...)

 

 

SERVO SYSTEM OPERATION

 

The servo system hardware works in conjunction with the BFF PID Servo Control software running on the PC.

 

The servo software receives platform position demand information from the BFF Motion Driver and also receives actual platform position feedback data from the position feedback pots/encoders connected to the 64SPU-1 card. Using this data it calculates motor speed demand outputs which control the direction and speed of the actuator drive motors to drive the platform to the demanded positions. The motor control commands are sent to the MD03 motor controllers via the 64SPU-1 signal processor card.

 

The servo control loop can run at up to 50Hz (PC and connection dependent) - ie the speed and direction of each motor is adjusted by the system up to 50 times each second and this makes for an effective system response under most platform motions. For the best performance it is better to run the motion software on a LAN PC separate from the flight sim PC. If the 12ADC-1 12bit potentiometer feedback converter card is used the servo loop can run at up to 70Hz.

 

The servo software uses a PID (proportional - integral - derivative) control algorithm and the PID parameters that define the control response can be set through the BFF Servo Controller GUI. This makes tuning the servo system easier as small changes can be made to the control algorithm and the resulting effects on the motion assessed immediately.

 

Actuator limit switches and an Emergency Stop switch should be wired in series in the 5V supply line from the 64SPU-1 to the MD03 motor controllers to cut the drive to the motors to prevent damage if the actuators over-stroke. The 64SPU-1 also includes a simple trip loop into which a "Pause Output" or "Drive Enable" switch can be wired.

 

Motor controller "live" current and temperature data is continuously fed back by the 64SPU-1 to be displayed on the PC by the BFF Servo Controller software. This makes for interesting viewing, however it can also be of significant assistance during platform set-up work as it provides a direct indication of the system electrical loading. The servo software also includes a virtual oscilloscope to view demanded and actual actuator positions and motor speed demand.

 

For the best performance of the servo drive I find that 12bit position feedback (using either MA3 absolute position encoders or pot feedback via the 12bit 12ADC-1 card) should be used for the position feedback, and the 64SPU-1 should be connected to the driving PC through a physical Serial (COM) port. If you are using the PICAXE USB Serial cable then it also can give good results by reducing the "Latency Timer" setting to 1 ms in the relevant COM port's advanced settings.

 

Overall the 64SPU-1 and associated servo software can provide an effective servo drive for DIY motion platforms at a seriously low price. The 64SPU-1 is however a relatively simple card and although it contains a number of safety features such as comms time-out monitoring, some feedback error detection and feedback limit detection the price does not allow the types of system monitoring capabilities that can be found on much more expensive commercial motion controllers. You must take care to ensure the platform drive is properly protected by over-travel limit switches and an accessible Emergency Stop button.

 

Although there are some electrical noise isolation features on the card in particularly electrically noisy systems the on-board 20X2 microcontroller can be affected by voltage spikes coming through on the 5V and ground lines. The source of such electrical noise is usually the drive motors. For added reliability the BFF I2C Isolator card can be used with the 64SPU-1 to fully isolate it from wiring bourn noise propagating from the electric drive motors through the MD03 logic ground connections. The isolator card allows the 64SPU-1 to be powered from its own independent clean 5V power supply.

 

If you wish to buy the 64SPU-1 please read the data sheet carefully and follow the installation and use recommendations.

 

 

 

 

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