This µModule replaces the built-in electronics of many standard R/C servos. It adds the same features to a servo that the µModule H-Bridge module offers. It is capable to drive up to 1.5A servo motors and uses the built-in potentiometer for position-feedback. For modified continuous-rotation servos it offers the same back-EMF based speed-control than it’s larger cousin offers. It operates from 5V. and communicates using the TWI interface that’s common among all µModules. It also has an optional logic-level RS-232 interface. Due to size constrains the module does not use the standard µModule interface headers but is electrically compatible with all other modules.
- Standard µModule TWI with optional RS-232 interface
- 1.5A current limit
- Over-current protection
- High efficiency complementary MOSFET bridge
- Rich speed-feedback options for closed-loop operation
- Back-EMF feedback for continuous rotation servos
- PID control loop
- Acceleration and deceleration limits for continuous rotation servos
- Traveled distance calculation for continuous rotation servos
- ‘Go-to-distance’ support with trapezoid speed-profile for continuous rotation servos
- Braking and free-wheeling support
- Parameters can be stored in permanent EEPROM storage
- Duty-cycle throttling to limit torque
- Per-cycle programmable current-limit
This document and all the accompanying design documentation (for example schematic and PCB files) are covered by the H-Storm Non-Commercial License (HSNCL).
H-Storm Non-Commercial License (HSNCL)
Copyright 2004-2007 Andras Tantos and Modular Circuits. All rights reserved.
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The bus interface
The TWI bus interface follows the µModule standards. There are six or four-pin connectors on the board, both with identical functionality. Two wires are used for the TWI signal transmission (clock and data) while the rest provide power and ground signals. The interface can be operated at a rate up to 400kHz. This module never initiates any transactions on the bus, it operates in slave-only mode. Status information can be acquired by polling and commands can be sent to the module at any time. The module implements the standard 8-bit register bank µModule communication protocol.
The module can source and sink power on this interface. The power to and from these sockets can be interrupted by an on-board jumper. With this there are three possible powering configurations with regards to the logic-level functions:
- The module is powered form the same power supply as the motor. The module provides power to other devices on the TWI bus
- The module is powered from the power supply of the motor however it does not power other devices on the TWI bus
- The module is powered from the TWI bus
The third powering mode has to be used if 5V operation is required.
The H-bridge is comprised from tow n-channel and two p-channel power MOSFETs. These transistors have extremely low on-resistance, leading to high-efficiency and low heat-generation even at high power levels. The high-side transistors are driven through a HW over-current protection circuit. All four MOSFETs can independently be turned on and off allowing all possible valid operating modes of the bridge to be used:
- Forward drive
- Backward drive
A wide range of feedback options are available on the module to provide additional health-monitoring and establishing closed-loop speed-control functionality. Both voltage (relative to the ground) on the motor connectors can be monitored as well as the voltage across the current-sense resistor which is proportional to the current flowing through the motor. Monitoring these values with a 10-bit resolution A/D converter precise knowledge of the current system status can be established. These measurements can be used to calculate the back-EMF response of the driven motor, which is proportional to the rotation speed. This value in turn can be used to close the control loop and create a true speed-controlled H-bridge. For traditional servo control, the module uses the R/C servos integrated potentiometer as feedback.
The module on the top of the standard TWI interface, that is common among all µModules also contains a (logical level) RS-232 interface as well. This interface can be used to connect the module to other microcontroller modules or (after level-shifting) to a PC which doesn’t have a TWI interface.
The TWI interface
The TWI interface adheres to the standard µModule communication protocol. It implements 8-bit register-bank addressing, and defines 32 registers. Each register corresponds to a single channel.
|0||int16_t||RequestValue||R/W||Position or speed control request signal R/W (Scaled between -0x3fff and 0x3fff). 0x4000 is freewheeling and 0x4001 is braking.|
|2||int16_t||IFactor||R/W||PID control loop integrator value|
|4||int16_t||PFactor||R/W||PID control loop proportional value|
|6||int16_t||DFactor||R/W||PID control loop differentiator value|
|8||int16_t||PFFactor||R/W||Control bypass proportional value|
|10||int16_t||SampleOffset||R/W||Control loop input bias|
|12||int16_t||MaxPositiveChange||R/W||Maximum single-step change in speed in the positive direction. If a bigger change is requested in
|14||int16_t||MaxNegativeChange||R/W||Maximum single-step change in speed in the negative direction. If a bigger change is requested in
|16||int32_t||Distance||R/W||Integrated actual speed value. An approximate measurement of the travelled distance.|
|20||int32_t||FwDistanceLimit||R/W||Maximum distance allowed in the positive direction. When
|24||int32_t||BwDistanceLimit||R/W||Maximum distance allowed in the negative direction. When
|28||int32_t||DistanceToStop||R/O||Current estimated distance required to stop|
|32||int16_t||CurrentRequest||R/O||Actual current speed request value. This in general is equal to
|34||int16_t||Command||R/O||Command given to the H-bridge. This is the output of the control loop|
|36||int16_t||IValue||R/O||PID loop working set|
|42||int16_t||VoltageSample||R/O||This is the measured position or speed, and the input of the control loop|
|44||int16_t||BaseValue||R/O||Back-EMF sampling code working set|
|56||int16_t||OriginalRequestValue||R/O||Ramp-generation code working set|
|58||uint8_t||SampleState||R/O||Back-EMF sampling code working set|
|59||uint8_t||DutyCycleThrottle||R/W||Maximum duty cycle allowed on the H-bridge. This limits the maximum (average) voltage that can be applied to the motor|
|60||uint8_t||NewData||R/O||Used for debug outputs|
|61||bool8_t||IsForward||R/O||Set to true if the motor is energized in the forward direction|
|62||uint16_t||CurrentMax||R/O||Peak current drawn by the motor|
|64||uint16_t||CurrentDelta||R/O||Current draw detection code working set|
|70||uint8_t||ADBufferEnable||R/O||When set to 0, AD sampling is in progress|
|71||bool8_t||ADBufferEnableHost||R/W||When set to true, host request a new set AD sampling. It will happen in the next control cycle. When sampling is done, it is reset to false|
|72||uint16_t||CurrentLimit||R/W||Maximum current draw allowed. If during the ‘on’ part of the cycle, the current draw reaches above this level, the MOSFETs are switched off and the ‘on’ part of the cycle is terminated|
|74||uint8_t||OperatingMode||R/W||Set to 0 for speed-controller, and 1 for servo-controller mode|
|76||uint16_t*80||ADBuffer||R/W||AD sampling buffer|
- Signed 8-bit integer
- Signed 16-bit integer
- Signed 32-bit integer
- Unsigned 8-bit integer
- Unsigned 16-bit integer
- Unsigned 32-bit integer
- 8-bit boolean value (possible values are 0 for false, and non-zero for true)
- Register has read/write access
- Register has read-only access