6. MRM Electronic System
6.6 Sensors
A second output terminal on the ATmega 32L board was also connected to the motor driver to switch the direction of motor actuation (refer to diagrams in Appendix E.2).
Onboard H-Bridge
The H-Bridge motor driver, indicated in Figure 6.5, performed PWM on a 12 volt power source.
The unit is capable of supplying up to 10 amps continuously to servo drive motors. At the maximum current rating of 10 amp, a constant load of 9 Nm can be sustained at the output shaft of the motors. In linear axes the motors could theoretically maintain a constant actuation force of up to 1500 N at a current of 10 amp.
Collision Detection on the ATmega 32L Board
The external interrupt pins on the ATmega 32L were connected to limit switches for collision detection. When a collision occurs a limit switch closes a circuit which applies a 5 V signal to an interrupt pin. The interrupt software routine ends the servo control routine and the operation of the motor. An appropriate status message is then transmitted to the host PC via the servo communication module.
6.6 Sensors
6.6.1 Process Modules
a. b.
Figure 6.6: ADXL 204 dual axis accelerometer [69]
a. ADXL 204 sensor and circuit board b. ADXL 204 function block diagram
MRM process modules each contained the ADXL dual axis accelerometer illustrated in Figure 6.6.a. The accelerometer was used to measure the vibrations generated by the cutting process and hence introduce the characteristic of diagnosability into the system. The mechanical vibrations generated during the interaction of the tool with the work piece can provide diagnostic data for the analyses of the stability of machining operations. The data obtained from an accelerometer can also be used to detect the phenomenon of regenerative chatter (second order vibrations). Regenerative chatter causes volatility in the cutting process which leads to the degradation of machined parts and possibly tool breakage.
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6.6 Sensors
The ADXL 204 IC is capable of measuring accelerations up to 17 m/s2 with a bandwidth of 0.5 Hz up to 2.5 kHz. The bandwidth may be manipulated by changing the capacitors CY and CX
labelled in Figure 6.6.b. This option is available to narrow the bandwidth and increases the resolution. The commercial board of Figure 6.6.a has factory installed capacitors of 100 nF, setting the bandwidth to 50 Hz and resolution to 2 mg (g = gravity). The current setting enabled vibrations to be measured at spindle speeds up to 3000 rev/min on the basis of a single excitation per revolution. Other significant features of the ADXL 204 included a low current requirement of 700 µA when operated at 5 volt. The sensor unit can withstand temperatures of -40 oC to 125 oC and has a shock survival of 3500 g. Further details on the ADXL 204 may be found in [69].
The ADXL 204 interfaces with the servo control module via two of the eight ADC channels present on the ATmega 32L chip (see Section 6.3). The Xout and Yout channels indicated in Figure 6.6.b, output a signal between 0 to 5 volt for ADC. The nominal voltage on the device is 2.5 volt, which is the voltage when the device is sensing zero acceleration along an axis. Voltages less than 2.5 volt indicate acceleration in the negative direction of an axis while voltages greater than 2.5 volt indicate a positive acceleration. The 10-bit ADC on the ATmega 32L allowed a measurement resolution of 47 mg.
6.6.2 Motion Modules
Optical Encoders
a. b.
Figure 6.7: The HEDS-5540 Optical Encoder [70]
a. HEDS-5540 optical encoder
b. HEDS-5540 function block diagram
The HEDS-5540 optical encoder was integrated into MRM motion modules to create a servo drive system (see Section 5.5.3). The function block diagram for this device is illustrated in Figure 6.7.b; indicated on this diagram is the pin configuration for the unit. The HEDS-5540 is capable of producing two square waves in quadrature using channels A and B (CH A and CH B), with a third indexing pulse generated once per revolution on channel I (CH I).
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6.6 Sensors
The HEDS-5540 is capable of a resolution of 512 pulses per revolution on a single channel, or 1024 pulses per revolution in quadrature. Further features on the HEDS-5540 include a maximum velocity rating of 30000 rpm and acceleration of 250000 rad/s2. The encoders were also suitably rated at a maximum vibration of 20 g at 1000 Hz, and a maximum temperature of 100 oC [70].
Although the encoders possess three channels, only one channel (CH A) was used in the MRM design. The device was selected based on cost rather than its level of functionality and proved more cost effective than other single channel encoders. The direction of motion on an axis is predetermined by servo control modules and the decoding of a quadrature signal was not necessary. Channel „A‟ producing 512 pulses per revolution enabled resolutions of up to 0.70o on rotary axes and 0.003 mm on linear axes. The channel is connected directly to the interrupt terminal on an associated servo control module. The device is also operated on 5 volt DC and derives its power from the regulated power supply unit on servo control modules.
Limit Switches
Figure 6.8: Limit Switch on Column Module
Limit switches were used in the MRM design for contact sensing in motion modules. These sensors were necessary to prevent damage to the modules through collisions between the internal mechanical components. A single module would usually contain two switches at either end of its axis. The limit switches were connected in a „normally open‟ configuration, and would close a circuit when contact is made between a moving component and the switch. The sensors are connected to an interrupt terminal on servo control modules and a 5 volt interrupt signal is generated when the circuit is closed. The interrupt software routine then ceases the generation of a PWM signal to the H-Bridge motor drivers, which in turn stops the operation of the motor.
5 V DC input to switch
5 V DC output to microcontroller
Rolling Contact Contact Switch
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