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Artemis Basic Components/Principles www.rdstec.com. Single Motor Schematic. Dual Motor Schematic. Why CAN bus?. By utilising CAN-bus technology additional modules can be used when extra inputs/outputs are required.
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Why CAN bus? By utilising CAN-bus technology additional modules can be used when extra inputs/outputs are required. This provides a ‘modular’ solution to easily cover a wide range of possible build configurations. Wiring is also simpler between instrument and implement.
What is CAN-bus • CAN bus (for controller area network) is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle. • CAN bus is a message-based protocol, designed specifically for automotive applications but now also used in other areas such as industrial automation and medical equipment. • Development of CAN bus started originally in 1983 at Robert Bosch GmbH. The protocol was officially released in 1986 at the Society of Automotive Engineers (SAE) congress in Detroit, Michigan. The first CAN controller chips, produced by Intel and Philips, came on the market in 1987. Bosch published the CAN 2.0 specification in 1991. • CAN is a multi-master broadcast serial bus standard for connecting electronic control units (ECUs). • Each module/node/ECU is able to send and receive messages, but not simultaneously. A message consists primarily of an ID (identifier), which represents the priority of the message, and up to eight data bytes. It is transmitted serially onto the bus. This signal pattern is encoded in non-return-to-zero (NRZ) and is sensed by all nodes. • The devices that are connected by a CAN network are typically sensors, actuators, and other control devices. These devices are not connected directly to the bus, but through a host processor and a CAN controller (CAN Module).
Motor Control Module This module located on the drill is responsible for controlling the motor speed (metering shaft speed) proportional to forward speed. It controls the motor speed with a PWM signal and monitors the actual speed from the encoder on the motor. If the load on the motor increases the MCM will see the speed reduce and will automatically increase the PWM signal operating the motor to achieve the correct motor speed again. When the load is removed and the motor is rotating faster than it should be the MCM reduces the PWM signal to achieve the correct speed. This process is occurring continually, response is so quick it cannot be seen by eye. The MCM also controls the motor speed proportional to forward speed. When forward speed increases then the MCM increases motor speed. If forward speed halves then motor speed halves. When the forward speed is 0km/h the MCM stops the motor.The relationship required between motor speed and forward speed is achieved by a product calibration.
What is PWM • Pulse-width modulation (PWM) is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches. • The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is. • The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings are tens of kHz for a motor drive i.e. 10 thousand times a second. • The term ‘duty cycle’ describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on. • The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero.
What is PWM With the MCM this is all happening at 20kHz
Motor/Gearbox/Encoder The standard motor is a 400watt unit with sufficient power and torque for most types of conventional or pneumatic metering units. The gearbox ratio is matched to achieve the maximum metering shaft speed at the highest forward speed of the mechanical drive system i.e. Accord metering unit is 96rpm @ 18km/h. The encoder provides 100 pulses per revolution of the motor so that the system can immediately monitor and control motor speed (metering shaft speed).
Radar For the metering unit (motor) to respond immediately to changes in forward speed then we need a very high data rate from the forward speed sensor. The radar sensor provides a pulse every 7.8mm. It is ideally mounted on the drill to ensure the system remains self-contained.
PSi Instrument This is the operator interface used for controlling and monitoring the drill/seeder via CAN-bus. It is used to set the required application rate, provide the product calibration routine and send the multiplier to the MCM for the relationship between motor speed and forward speed. It also contains the normal tramline control, fan speed and hopper level monitoring, area accumulation etc. The PSi instrument is the new version of the well proven Pro-Series platform and it also now contains an integral SD/MMC card reader for data logging/Precision Farming functions.The software can also be configured for dual motors i.e. left & right metering units or seed & fertiliser products.
Power Cable The Artemis system is designed to be powered directly from the tractors battery. The cable is rated at 80amps but this is done to minimise voltage drop between tractor and motor. It is not due to the current requirement of the motor. Typcially when running with no load a single Artemis motor requires 8 – 10amps. If the motor required 20amps through 20amp cable, then over a distance of 10m (from battery to motor) the voltage drop would be 2.9volts. 13volts at the battery becomes 10.1volts at the motor and this will compromise its operation (torque and maximum speed). For installations with long distances between tractor battery and motor, and dual motor capabilities we provide a standard heavy duty power cable to avoid problems.
Reminders • If the forward speed doubles then the motor speed will double – the MCM maintains the ‘electronic ratio’ to forward speed. • If the operator increases the application rate by 10% (275kg/ha) the ‘electronic ratio’ will increase by 10% automatically. • If the calibration factor is halved then the ‘electronic ratio’ between motor speed and forward speed will be doubled to achieve the correct amount of revolutions per hectare. • If the gearbox ratio is ever changed the ‘electronic ratio’ between motor speed and forward speed will be changed to achieve the correct amount of revolutions per hectare. • If the forward speed factor (SSF) is wrong by 10% then the application rate will be wrong by 10%. • The system is only as accurate as the information it is given to work with!
