Proceedings of the 2002 IEEE International Conference on Robotics 8 Automation

Washington, DC May 2002

One-Wire Smart Motors Communicating over the DC Power Bus-Line with Application to Endless Rotary Joints

Eric Wade and H. Hany Asada dkbeloff Laboratory for Information Systems and Technology

Department of Mechanical Engineering Massachusetts Institute of Technology

Cambridge, MA 02139, USA

ABSTRACT A system of multi-axis networked smart motors is developed to reduce complex cable harnesses. Motor control signals are sent over the DCpower bus line and received by motors with integrated power amplifers and controllers. Unlike traditional AC power line communication, the noise level and signal attenuation are much lower for the DCpower bus communication. This allows for high fidelity, broadband communication among many smart motors without bulky, costly cables. A modem is designed to exploit the features of the DCpower bus communication. A power-signal coupling circuit with an impedance trap filter is designed for superimposing control signals over the DC power bus line with a maximum of signal transmissibility and reduced line noise, An experimental setup was developed to identifL the noise spectrum and signal transmissibility of the consolidated power-signal line. Optimal carrier frequency and filter parameters are then obtained based on the experiments. A simple protocol for bi-directional communication between the motors and the central controller is designed and tested. Finally, the design of a joint undergoing continuous rotation is proposed utilizing the consolidated power-signal cable connected to endlessly spinning smart motors.

1. Introduction Complex cabling places a limitation on servo

system design. The precision actuators in servo systems have multiple connections with other motors, a centralized controller, a power source, etc. For servo systems, housing and guidance become problems during the design phase. It is important that the cabling not restrict the bending and twisting connections. Thus, the cable guidance can become so complicated that automated installation is not possible. Such complex designs become problematic when maintenance is required as it may be inconvenient to repair cables without taking the entire machine off-line. Cabling contributes to higher material, manufacturing, and maintenance costs as well as increased weight and bulk. The nuiiber of problems associated with cabling varies

directly with the increasing number of axes in an apparatus.

In this paper a method of reducing the amount of cabling in these servo systems is presented. We would like to propose a system where information and power are combined onto a single consolidated power-signal (CPS) line. For applications that require numerous actuators, the ability to consolidate many lines into one is very advantageous. This can be done by combining two separate concepts: power line communication and smart motors.

2. One-Wire Architecture for Dual Transmission of Power and Signals Issues on Power Line Communication

Power line communication (PLC), i.e. technology for sending data using already existing power lines, allows us to reduce cable requirements, and has been shown to be a useful and economically viable technology in recent years [4][7]. The bulk of PLC research to this point has been done with regards to in- home networks. Current PLC techniques allow signal speeds of up to lkbps and are expect to reach up to 1 Mbps, and cover communication methods such as phase- locked loop, spread spectrum, and digital modulation techniques [2][7][9]. However, there are a nuniber of limitations to PLC. The large power transformers that are used to step down power from outdoor power lines to individual houses attenuate high frequency signals required to send reliable information [7]. Also, residential loads such as dimmers, switching power supplies, and other communication media often contribute noise in ranges anywhere between 100 Hz and 1 MHz [2]. Depending on what loads are connected to the line, the line characteristics of impedance, noise, and signal attenuation vary in a wide range and are highly complex and non-linear.

Another problem for PLC is bandwidth limitations. In most countries, transmissions on the commercial power line are allocated to the range between 3 Hz and about 500 kHz. For instance, the FCC in the United States allows power line transmissions in the 10 -

0-7803-7272-7/02/$17.00 0 2002 IEEE 2369

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450 Wiz range, while European authority CENELEC currently allows for transmissions in the 3 - 148.5 kHz range [lO][ll]. Communication above these ranges is shared by many other operations, including AM radio and amateur broadcasters [12]. Therefore, the current bit transmission rate for PLC is severely limited due to these restrictions and regulations. DC Power Bus Line Communication among Integrated Motors

new PLC architecture for multi-axis servo systems is developed in this paper. Figure 1 shows the basic architecture of the system.

To avoid these difficulties of traditional PLC, a

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Figure 1: Schematic of System

In lieu of the AC power line we use a DC power bus line to supply voltage to the system. We combine the concept of integrated motors, or smart motors, with the DC power bus communication. Specifically, a smart motor is a type of integrated motor having a localized microprocessor, localized sensors, and other components required to create a localized control loop [l]. This architecture of DC power bus communicating with smart motors has the following salient features: High transmissibility with known load conditions

Since the DC power line is isolated from external loads buffered by the DC power supply, the line's characteristics do not depend on those loads beyond the DC power supply. Furthermore, since no large transformers (which act as large inductances) are directly connected to the line, the line impedance at high frequencies is much higher than that of the traditional AC power line. Another important advantage of DC power bus communication is that the load conditions are known or at least predictable, since the power bus line is connected only to a known set of smart motors that can be quantified prior to operation. Modems for transmitting signals can be designed based on the known characteristics of loads and noise sources. This would allow for broadband, reliable communications needed for multi-axis servomotor systems.

Lower bandwidth requirements for outer-loop control communications

In high performance motion control applications, the bandwidth requirements for the current loop or the innermost feedback loop are very high; the sampling interval is between 50 ps and 500 ps in most cases. Power line communication is not feasible for such time-critical wideband servo controls. In our DC power bus architecture, however, the innermost feedback loop is closed locally inside the integrated motor. The control signals going into the integrated motor are reference inputs to the local feedback controller, whose sampling rate is typically 10 times slower than that of the feedback loop. Sensor signals coming out of the integrated motor are to be used mostly for outer loop controls to update the reference inputs. They may be used for monitoring the motor, but the sampling rate is substantially lower than the time-critical inner loop control. Low Electro Magnetic Interference

Traditional PWM amplifiers driving servomotors often create serious EMI and switching noise problems. The problems are alleviated for integrated motors, since the cables connecting the PWM amplifier and the motor is eliminated, and the noise sources are confined within the motor body. This reduced EMI and PWM switching noise are desirable not only for the motor control system and other equipment but also for the power line communication.

In the following sections we will exploit these features of the DC power bus communication combined with the integrated motor architecture.

3. Modem Circuit Design A modem is needed for communication over the

power line. Although a number of previous publications address modem design issues, they are for AC power line communication and are substantially different from our DC power bus communication.

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Figure 2: Power-sibmal coupling circuitry

Figure 2 shows the basic power-signal coupling circuit for superimposing control signals onto a DC power bus line. A control transformer and a capacitor steps down operating voltages and isolates the signal source from the DC power supply and other power sources. There are a number of references that discuss this function in greater detail [3]. A valuable feature of this circuit is that the

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attenuation of superimposed signals can be minimized with an optimal combination of the capacitor and inductance values. When the reactance of the resonant elements balances the impedance of the load, the signal attenuation is minimized [8]. In other words, we tune the resonant elements are tuned so that their reactive impedances cancel at the transmission frequency. Thus, the circuitry couples the signal and power through the transformer and provides some filtering effect depending on the values of the resonant elements.

Modeling the transmission side is critical to designing the modem. The actual power-signal coupling circuit exhibits some parasitic dynamics at high frequencies. The transformer inevitably has some leak of magnetic flux and possesses some capacitance. Figure 3 depicts the model of the modem circuit including the parasitic dynamics.

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Figure 3: Equivalent circuit of the modem including the parasitic dynamics.

Capacitance C, represents the total capacitance at the windings of the transformer, Le the leakage inductance in the transformer, and L, is its magnetizing inductance. With a transformer tums ration 'n', we denote the effective capacitance of the coupling capacitor by Cl'=n2 C, and the effective resistance of the load by RI'=n2 RI. The transfer knction from input voltage V, to the AC component of the output voltage superimposed on the power bus line V, is given by

vo - S 2 -- V, a0s4 + up3 + a2s2 + a3s +a4

Rs a4 = R'l C'I L,

Figure 4 illustrates the transfer function. It exhibits a type of band pass filter characteristic with a peak at a resonant frequency. If the camer frequency of signal transmission

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is selected around the resonance frequency, the signal attenuation is minimized.

In the model of Figure 3, an equivalent resistance was used to represent the overall loads connected to the power bus line. These loads include the DC power supply, the power amplifiers of individual motors, and other modems connected to the same power bus line. Obviously, their characteristics are not purely resistive, but are complex impedances that vary depending on the number of motors simultaneously driven and other states of the loads. Therefore it is difficult to predict the exact transfer characteristics of the power bus line. To cope with this we devise an additional 'filter' that would not significantly depend on the states of the loads but would attain the highest transmissibility at a given carrier frequency. This filter would suppress disturbances and noise generated by switching transistors of the DC power supply, power amplifiers of the motors, and other noise sources.

Figure 4 shows the additional components, which make up what we will call the distinct frequency pass filter (DFPF). This filter is a modified version of the impedance trap filter, which is tuned to a certain carrier frequency. The upper capacitor C,, is simply to shut down the DC current. The LC circuit created by the coil L, and the capacitor C, has a resonant frequency. The LC circuit has a peak in impedance for currents at its resonant frequency, but very little impedance for all others. This resonant frequency is tuned to the carrier frequency, so that its impedance across the parallel circuit Z, becomes infinite at the carrier frequency. Currents with harmonics of other frequencies (e.g. noise) see the low impedance of the filter and become 'trapped', resonating back and forth between the inductor and capacitor.

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Figure 4: Distinct-Frequency Pass Filter

Verification of Transformer Model The transformer model of Figure 3 was verified

by placing a 100 R resistor across the output of the actual transformer. Experimental values of vi and V, were measured and compared with the frequency response of the derived transfer function. The results can be seen in Figure 5.

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Figure 5 : Experimental and Theoretical Transfer Functions

4. Line Transmissibility and Noise Characteristics The modem circuit designed in the previous

section must be tuned. At this point, it is most important to characterize the DC power bus line through experimentation. A simplified apparatus was created such that data concerning the signal transmission and the line noise could be gathered. In the experiments, the circuit depicted in Figure 2 was constructed. A low-voltage sinusoidal signal was superimposed onto the positive voltage supply line of a power amplifier. This amplifier was in turn being used to run a DC servomotor using the circuitry shown in Figure 6.

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Figure 6: Experimental setup for line impedance and noise measurement: Receiver end

Line Impedance The transmission circuitry of Figure 2 was used with the DC power supply placed across V, to represent the entire load. The amplitude of the signal injected by a function generator at the signal transmission node Vi was measured and compared V,.

We found that the gain is uniform at -5dB until it reaches a maximum peak value of 5dB at 20 MHz, after which it decreases rapidly. Ideally, signals would be sent using a carrier frequency as close to this value as possible in order to minimize the current required to send the signal.

Noise Characteristics To acquire noise spectrum data, the FFT function

of an oscilloscope with a sampling rate of 25 GS/s was used. It is assumed that the primary sources of noise must be active elements, i.e. the switching transistors in the motors and the DC power supply. Therefore, using the experimental setup of Figures 2 and 6, the noise level in the CPS line was monitored and recorded over a finite range. To get detailed information, the noise level was measured for four scenarios: no motors running, one motor running, two motors running, and three motors running.

0 0 5 1 1 5 2 2.5 3 35 4 4.5 5 Freq [MHz]

Figure 7: FFT data for DC power bus line with three motors running

All four experiments produced similar data. The noise spectrums for all four experiments are presented on one plot in Figure 7 for comparison purposes. While not evident from this plot, the most noticeable differences in results were between the scenario in which no motors were on, and the other three scenarios. A number of low fkequency components between 100 Hz and 2 kHz show up as soon as any motors are turned on. These components range in size from 0.5 to 1.5 x 10 v-S (compared to 0.2 x 10 Vms for the random noise). This agrees with the claim by Downey and Sutterlin that such motors often cause impulse noise, which can be found over a wide bandwidth [2]. It is also known that the oscillator of a switching power supply creates noise between 20 kHz and 1 MHz. This is likely the cause of the two large spikes that occur below 1 MHz in Figure 7.

5. Tuning and Optimization Carrier Frequency

Based on the experimental results, we now have the information necessary to find an optimal carrier frequency and filter parameters. Unfortunately, noise spikes occur at some of the same frequencies at which signal transmissibility is a maximum. Recall that it is

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acceptable to use most frequencies below 20 MHz, due to the uniformity of attenuation in that range. The noise characteristics reveal that the range between 900 kHz and 1.7 MHz as well as that of 2.2 MHz to 2.8 MHz are relatively noise free. Therefore the carrier frequency should be selected from within these ranges. Impedance Trap Filter

Based on the noise characteristics present in the line, an impedance trap filter was built to increase signal transmissibility at the selected carrier frequency and decrease noise at all other fiequencies. As mentioned, the capacitor and inductor of the filter are tuned so that the resonant frequency corresponds with the transmission frequency. If necessary, for a frequency-shift keying application for example, filters can be 'stacked' such that they respond at different frequencies. We optimized the values of the circuit to those given in Table 1.

Table 1 : Component Values

6. Protocol Design A protocol is needed for transmitting control

signals. Since communication over the DC power bus line is performed with only one wire the protocol needed for the DC power bus communication is therefore similar to the ones for optical links and wireless communication. It must provide various types of functionality in a single stream of data bits including synchronization of senders and receivers, addressing, sendreceive mode selection, information transmission, parity check, and termination of transmission. Figure 9 shows one example of the bit pattern used for the DC power bus communication.

Component

Source Resistance R,

Rima7 Inductance 22.17 PH

Secondary Inductance 5.47 pH Coupling Capacitance C,

Trap Inductance L, 4.1 pH

Trap Capacitance C,

Verificution Experiments were conducted to verify the

modem design. Figure 8 shows the waveforms of superimposed signals over the DC power bus line using of the impedance trap filter. At 600 kHz, the attenuation of signal is significant. At the optimal carrier frequency we have selected, 1.9 MHz, there is a clear signal with large transmissibility and low noise. At a higher frequency, attenuation is evident.

1 15 5 10

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5 10 14.6

15

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Figure 8: Carriers of 600 kHz, 1.9 MHz, and 3.3 MHz

The preamble sequence synchronizes the receiver-side clock, followed by an m-bit address signal to select one of the smart motors or other devices hooked up to the line. Next is a sequence that tells the receiver to either send or receive information. After completing data transmission along with parity check, the sender and the receiver terminate the transmission with the stop bits, which change the state of the token to allow other transmissions to occur.

A simplified version of this protocol with only eight bits of data was implemented and tested. Modulation and signal processing were done using circuit board mounted components. Prototype smart motors, each containing an analog amplifier and position controller, were hooked to the DC power supply line. Using this protocol, a series of position commands were sent to the motors, and the motors were driven to the commanded positions with the local feedback controllers. Thus, the desired scheme has been achieved.

As an idealized, scaled up version of the experiment, one could create a system in which we are able to transmit at 1 Mbps of information based on the carrier frequency of 5 MHz. Let the information be sent and received every 10 ms. If this is a 16 bit signal, a total of 3.2 kbps is required for each send and receive. 1 Mbps divided by 10 kbps allows for 100 actuators being simultaneously controlled in one system.

7. Application to endless rotation joints There are many applications that become

realizable with the system proposed in this paper, one of which is presented here. One can imagine a servo system with multiple axes of rotation, such as the head of a

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machine tool. In such a system, three actuators are required for the three axes of rotation (Figure 10). Typically, slip rings might be used to transmit the power and information to the extreme actuators of the machine tool. Unfortunately, slip rings are known to have problems with wear and friction.

Figure 10: Three-axis joint needing endless rotation

However, with the apparatus proposed in this paper, the same tool can be manufactured much more efficiently. Figure 11 demonstrates a method of transferring power and signals through an axis of rotation.

Figure 1 1 : Slip ring free rotary joint

In the figure, the shaft rotates with respect to the frame of the machine. With any type of rotation there is no linear velocity at the center. Using this premise, we can imagine an insulated CPS line being run through each shaft. The point of contact between the two will undergo a minimum amount of rotation. A small amount of conductive lubricant such as carbon or silver grease is held in place by capillarity between the two CPS lines. This way the power and signal are transmitted through a rotating joint with minimal fiction and minimal use of space. A further benefit is evident if we desire infinite rotation. With traditional cabling, there is only a certain amount of time before all the cabling has been wound up. At this point, the tool must come off the piece and rewind itself causing alignment problems and flash on the piece.

With our design, such rotation would be infinite, and there would never be any need to unwind the tool. This is just one of many possible situations in which this apparatus is advantageous.

8. Conclusion In this paper, a servo system based on a

consolidated signal-power line was proposed. The noise

and attenuation characteristics of the line were measured to help find the optimal carrier frequency. It was found that this optimal carrier frequency is between 2 and 5 MHz based on the current experimental setup. Using standard amplitude modulation, this allows for a signal transmission rate of above 200 kbps. It has been demonstrated that this will work for communication with multiple actuators connected to the line.

9. References [l] Bigler, Robert A. and Bigler, Punita P. Integrated

DC Servo Motor and Controller, US. Patent, Patent Number: 591241, June 1999. [2] Downey, W. and Sutterlin, P. Power Line Communication Technology Update, Echelon Corporation Presentation. [3] Herman, Stephen L. Electrical Transformers and Rotating Machines, International Thomson Publishing Company, 1999. [4] Hines, David. Unlocking the Potential of Power . Distribution Networks, Powerline Communications, April 2000. [5] Liu, Chun-Hung, Wade, Eric and Asada, H. Harry. Reduced-Cable Smart Motors Using DC Power Line Communication, Proceedings of the 2001 IEEE International Conference on Robotics & Automation, May 200 1. [6] Propp, Michael. The Use of Existing Electrical Powerlines for High Speed Communications to the Home, Haward Information Infrastructure Project, June 2001. [7] Strassberg, Dan. Powerline Communication: Wireless Technology, EDN Magazine, June 1996. [8] Sutterlin, Philip H. Powerline Coupling Network, US. Patent, Patent Number: 5485040, Jan 1996. [9] Tanaka, Masaoki. High Frequency Noise Power Spectrum, Impedance and Transmission Loss of Power Line in Japan on Intra-building Power Line Communications, IEEE Transactions on Consumer Electronics, Vol. 34, No. 2, May 1998. [ 101 Newbury, John E. Communications Services using the Low Voltage Power Distribution Network, Transmission and Distribution Converence and Exposition, 2001 IEEEPES, P. 638-640, Vol. 2. Nov. 2001. [l 11 Chen, Yi-Fu, and Chieuh, Tzi-Dar. A 100-Kbps Power-Line Modem for Household Applications, International Symposium on VLSI Technology, Systems, and Applications. June 1999. [ 121 FCC Bandwidth Allocation Chart, http://www .ntia.doc.gov/osmhome/allochrt.pdf [ 131 Bansai, Rajeev. Doing Double Duty: Power-Line Communications. IEEE Antennas and Propagation Magazine Vol. 43, No. 5, October 2001.

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