X-ray chest system with master-slave drive control
for image scanning drum

Philips Medizin Systeme in Hamburg have developed a new type of digital X-ray chest system that produces the X-ray image negative without the traditional chemical photographic process. The X-ray image is formed by the electrostatic principle on the surface of a rotating drum. The image is applied by a scanner that scans the radiographic information and stores it electrostatically on the drum. The read/write head continuously scans, in the µm-range, the information and deposits it in the circumferential direction on the cylinder surface and in a vertical direction for read-out with the same precision. This requires two drives operating in precise synchronism with each other. One drive turns the drum while the other drives the read/write head in synchronism in a vertical direction. Moreover, positioning is time optimized without giving rise to any torsion vibrations between drum and drive.

CAN-bus software and hardware made the chest system network compatible so that the digital information of the X-ray image can be digitally processed. The image information can be produced and called at a remote point from the electrostatic store. 

IAM GmbH in Brunswick was commissioned with the development of the drive control by means of a control card and CAN interfaces. The concept was implemented in such a manner that all tasks in the field of control technology of multi-axis units (e.g. machine tools, robotics) can be fulfilled. The new feature in this context is the possibility to operate any number of servodrives with the utmost precision as a master drive. The slaves can be coupled "on the fly" separately from the master. The control synchronizes the slave with the master via a parameterizable transient process. The hardware and software for resolvers, six-step transmitters and encoders, or appropriate combinations of these transmitters, were designed to be configurable with each other to ensure that the most favourable transmitter solution on the market can be selected, even in the future.

The master-slave drive unit can be controlled with the following commands:
   * Fast stop without brake ramp (emergency stop) 
   * Stop with braking ramp (regular stop) 
   * Approach a target position with the trajectory generator (TG) 
   * Approach a target position relative to the current position with the trajectory generator (TG)  
   * Travel to an end position with the trajectory generator (TG)  
   * Travel at a constant speed and start ramp for rotating axes 
   * Travel at constant speed and start ramp for linear axes up to maximum to an end position  
   * The drive follows a master position 
   * The drive follows a master speed  
   * Search for, and travel to, the reference point.   
Control was divided between an 8-bit micro-controller (80537) and a signal processor (DSP56001)
(Fig. 1)


Fig. 1: System configuration

The micro-controller handles the functions that are not time critical:
   * Switch on self-test 
   * Operation of the CAN interface  
   * Initializing and parameterizing 
   * Scaling user units  
   * Reference formation 
   * Sequence control  
   * Monitoring and fault treatment  
   * Periphery operation   
  

The signal processor takes over the following control-relevant functions: 
   * Controller cascade with position, speed, torque and stator-flux control 
   * Co-ordinates transformations  
   * Read-in of position and current actual values 
   * Generation of the PWM switching times 
   * Reference variable generator  
   * Master-slave coupling

The contracting party developed a matching ASIC parallel to the development of the hardware and software control, and then transferred it to production and testing. In addition to the generation of the PWM signals, the ASIC also evaluates two incremental transmitters (own position and master position). The times at which counting pulses arise are stored for high-resolution speed calculation (24 bits) above a minimum speed. Suitable software algorithms guarantee adequately accurate speed control within the range standstill and minimum speed. In practice the drive remains within this range for only a very brief period so that the problem of stationary accuracy is not relevant in this area. 

Control structure

The master drive of the drum and the slave drive of the read/write head is a permanent-field non-salient pole synchronous machine. Figure 2 shows the control structure that is required for this purpose. The controllers for TORQUE and stator flux (ST-FLUX) are located where the current controller is normally located. As the stator currents ISD and ISQ are proportional to the stator flux and drive torque with the permanent-field non-salient pole synchronous machine, these control sections correspond with the current control circuits. The stator flux is controlled at 0 because it is not necessary to change the motor flux in deviation from the permanent flux. The reference variable generator is given a target position from which it calculates a time-optimized set-point profile for position, speed and torque while maintaining the speed limitation as well as the acceleration and braking torque specifications. The reference variable generator is only activated for the master drive. Another feature is that the time when braking commences is calculated in such a manner that the drive stops precisely at the target position when changing from speed-controlled mode to position-controlled mode. 


Fig. 2: Control for the synchronous motor in the DSP

Master-slave coupling

The slave receives its position target value from the actual position of the master (POS2) and instantly compares its actual position (POS1) with the master position when it receives the coupling command. The position ramp for the position value of the master is then calculated for an adjustable compensation speed so that it can enter into synchronism with the master with any measure of softness (fig. 3). The speed of the master is calculated from the master position and supplied to the output of the position controller (fig. 2). This pre-control eliminates the following error otherwise associated with master movement. Figure 3 shows that the compensation time t3 - t1 only depends on the position difference DPOS0 and the adjusted compensation speed. Movement of the master during this compensation process does not change this time. In practice a transmission ratio between the master and slave position is taken into account (not described in this context).


Fig. 3: Master-slave position compensation

Damping the torsional vibrations

The torsion vibrations generated by time-optimized movement of the image drum (approx. 80 cm diameter) with a low-power motor can be successfully attenuated by the principal of differential speed application. The speeds for the drum and drive are available for this purpose.

Implementation

The control algorithms were implemented on the DSP56001 by Motorola. The control is operated at a switching frequency of 20 kHz and a scanning frequency of 20 kHz.  Customer benefits When the development work has been completed customers will have at their disposal a high-dynamic and flexible servo-drive control that will not only permit digital control of the X-ray chest system, but also the integration in the local data processing network of a hospital. All control parameters and status variables can be activated or read within the framework of the remote control or remote diagnosis. In the subsequent years this has resulted in a high quality product that is well ahead of its time in the entire field of x-ray diagnostics.

A detailed description of this development is given in the following joint publications with Philips Medizin Systeme: 

Ehrenberg, J.; Heins, E-J; Leymann, P; Schumacher, W.:
CAN-Bus mit OSI-Schicht 7 öffnet Tor zur Fabrik
Teil 1: Voll digitale Motorregelung in Medizinsystemen.
Elektronik 1991, No. 22, pages 70...80. 

Ehrenberg, J.; Heins, E-J; Leymann, P; Schumacher, W.:
CAN-Bus mit OSI-Schicht 7 öffnet Tor zur Fabrik
Teil 2: Voll digitaler Motorregler für universellen Einsatz.
Elektronik 1991, No. 23, pages 62...70.