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Testing and Modification of Slip Rings for Behavioral Animal Experiments Mary La Dine

CHEM 395-047 INTRODUCTION

The Wightman laboratory at UNC monitors dopamine levels in rats’ brains. To do this they use fast-scan cyclic voltammetry. Because this cyclic voltammetry is conducted while the rat is behaving, electrodes have been surgically implanted in the rats’ brains and the processing

electronics are attached to the rat’s heads. The rat is placed in a cage and the signals from the electronics are sent to the equipment outside of the cages via a multi-wired cable. To provide the

rat with freedom of motion and to prevent the cables from becoming tangled, a multi-contact slip ring is essential to this process.

The slip ring currently used in the Wightman laboratory is the Crist SwivElectra, which costs around

five thousand dollars per unit. It contains 24 contacts and a tube for which liquids and gases may be passed through. There have been complaints about noise levels

with the SwivElectra during use. These high noise levels could be due to many different things, and the source had

never been explicitly identified or quantified. This paper describes the process developed to test the background noise in the SwivElectra and various

alternative slip ring devices.

MATERIALS AND METHODS

Four different models of slip rings were tested to obtain a diverse collection of data: (1)

The MED Associates PHY-101A, a slip ring that is currently being phased out in the lab and is no longer in production. (2) The Christ SwivElectrica, what is currently being used in the lab.

(3) The ZSR022-06A from www.zlintel.com ring which does not have enough contacts for actual implementation into the lab. And (4) the Orbex 512-1800.

Torque

The torque required to rotate the slip ring was taken into

consideration for fear that it might restrict the rat’s freedom of motion. Although this was originally a concern, it soon became apparent that the MED Associates slip ring, which is what the rat is used to, had by far the

highest torque values. The torque was approximated by placing the slip ring’s axis

horizontal and hanging weights attached to dental floss from the rotational part (Fig-2). The weight required to initiate rotation was recorded. To convert this weight measurement into torque, it was

multiplied by gravity to convert it into a mass. This was then crossed with the perpendicular radius to produce the equation .

Figure 1

Fig-1 Experimental setup

Fig-2 Torque setup

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Noise

Experimental setup:

As shown in Fig-3 Two contacts on the slip ring were shorted together with a connecting wire. One was then connected to the positive output of an HP E3616A power supply in constant

current mode at 0.050 Amps and the other connected to the negative output. These signals were then amplified through a Tektronix TM 3012B 200 times. The output from the amplifier was put into Tektronix Oscilloscope, TDS 3012B channel 1. To rotate the slip ring a 12V motor was used

(HOSSEN Mini 12V DC 60 RMP High Torque Gear Box Electric Motor).

To determine the noise caused by the motion of the slip ring, the background noise measured when it was not rotating was subtracted from the noise measured when it was rotating. First, the background noise was measured by turning on everything except for the motor. This

noise level was recorded with the Oscilloscope. Initial settings:

DC power supply (HP E2616A): Constant current mode, 0.060Amps. Amplifier (TM 503): Low Pass: 10kHz, High Pass: 0.1 Hz, Gain: 200.

Fig-3 Circuit Diagram

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Fig-5. Rotational testing of Zlintel slip ring

The motor was then turned on. The noise generated by the slip ring when rotating was then measured.

This recording was triggered on the signal from a slotted interrupter, Optek OPB933W55Z, which marked the rotations of the motor. The marking of the rotations was to

ensure that a full rotation would be considered when looking at the noise. The slotted interrupter operates as shown in the schematic, Fig-4. (Sample photos are found in Appendix B.)

Methods

Zlintel The Zlintel slip ring is an inexpensive ring acquired solely for testing purposes. It

contains six contacts, not enough for the experiments being done in the lab. It also does not have a place for a tube to be passed through. Due to its inexpensive price, it comes with no guaranties

from the company. In operating this slip ring large

amounts of axial run out were observed. This indicates that when it is loaded there will be considerably increased

friction in the slip ring which will lead to the rat having to exert more torque to turn the slip ring. This slip ring was top mounted during testing so as

to attempt to reduce the axial run out and get a clean reading. The slip ring was mounted into the top of a

plastic tube. The tube acts as a spacer to remove it from the motor’s changing magnetic field. The motor is placed under the box. The tube and the half of the slip ring

mounted inside rotate while the top hat-like piece remains stationary.

Fig-4. Optical Interrupter Trigger Circuit

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MED Associates The MED Associates slip ring has

been used in the lab for some time. It has twenty-four contacts and is fully

functional. In operating this slip, difficulty in rotating it became a concern because the belt (see Fig-6)

would not drive it. Both the slip ring shaft and the tube were wrapped with

gauze to provide more traction so that the belt would be able to rotate the slip ring. The DB-25F connector on the

rotating half is connected by a flexible cable which allows it to bend and move

independently of the rest of the slip ring. This model contains a central tube.

To measure the noise, this slip ring is driven by a belt. This belt is looped

around the tube which was used in measuring the Zlintel slip ring. It is then looped around the rotating shaft of

the MED Associates slip wring. When the motor is turned on, the belt drives

the rotation of the slip ring. Again, the slip ring is located at a distance from the motor to prevent induced currents

(Fig-6).

Crist

The Crist slip ring is a sleek looking device but is unnecessarily bulky. It has very

little axial run out. It contains a center tube. When measuring noise it was driven by a belt, just as for the MED Associates slip ring.

Fig-6. Rotational testing of the MED Associates slip ring

Fig-7. Rotational testing of the Crist slip ring

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Orbex The Orbex model is compact but extremely basic. It comes with raw wires and no DB-

25F connectors. In order to configure this slip ring to be used in the lab, modifications would be needed. It has a large, one-half inch, shaft which passes through its center. Even with this large

shaft, there is no discernable axial run out. When measured, it was also driven by a belt. The belt is looped around a one-half inch dowel which is secured inside of the one-half inch hollow shaft through the center of the slip ring. The belt rotates the dowel, which in turn causes the slip

ring to rotate, enabling data collection. The wires were taped up out of the way to ensure that they did not tangle the operation.

Understanding the Data

From the Tektronix Oscilloscope, TDS 3012B, an excel file containing the time and the voltage at that time was exported. This was done for both scenarios, when the slip ring was in motion and when the slip ring was stationary. The voltage was then divided by the gain, 200

(column C). The offset in the oscilloscope was then accounted for by finding the average voltage (J4) and subtracting it from column C. This accounted for the offset, because with

random noise the average should be zero. To find the root mean square voltage, the square root of the sum of the squared voltage divided by the number of voltage data points was found. This was then divided by the current to find the root mean square resistance, which measures the

noise. The peak to peak voltage was found by subtracting the minimum voltage from the maximum voltage. These calculations were done for both when the slip ring was rotating and

when it was stationary. The stationary values were subtracted from the rotational values to find the additional noise the slip ring created.

Fig-8. Rotational testing of the Orbex slip ring

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Results

Cost (USD) Torque (Nm)

Vpp Signal (mV)

Vrms Signal (µV)

Rrms signal (mΩ)

Zlintel 12 0.001 0.620 9.50 0.190

MED Associates Out of production 0.024 2.27 74.2 1.48

Crist 5,000 0.001 70.5 2,850 57.1

Orbex <1,000* 0.008 0.070 2.40 0.048

This Table displays the price, torque, peak-to-peak voltage, root mean square voltage,

and root mean square resistance of each slip ring. The Zlintel was the least expensive by far and had the lowest value for torque. The Orbex’s torque value, while higher than the Crist’s, is still considerably smaller than the MED Associates value, which indicates that it should not constrain

the rat. The next three columns are indicators of the noise values. In all three of these categories the Orbex had orders of magnitude smaller results than the other slip rings. This indicates that

the Orbex, while not having the lowest torque, is far by best at minimizing the noise in the slip ring.

Table-2. Results of the slip ring tests.

*The total cost includes the material and labor to modify into something of use in a lab.

Table-1. Example data sheet, Orbex

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Design To make the Orbex 512-1800 into a

functional device, an arm had to be designed. This arm needed to lower the DB 25F connector

to where it had been on the Crist. It was decided to change the connection point from the center to a cantilevered model for structural

stability and ease of assembly. A 5.5 inch long one 0.5 inch diameter aluminum tube was

placed into the shaft of the slip ring to shield the signal wires and to secure the arm to the slip ring. This was then secured by set screws. Into

this tube three slits were cut to enable the wires to pass inside of it for shielding purposes

(Appendix A). This tube then was secured to slipring_elbow via a set screw.

Figure 10 shows a joiner made out of

ABS plastic which has a one-half inch hole on the top face and a 0.377 inch sized hole on the

front face. These two holes connect to allow the wires to turn the corner. It also has a smaller hole, 0.206 inches, which continues from the 0.377 inch

hole through to the bottom face to allow the tube for liquids to pass through. (Appendix A)

The wires were then passed through slipring_elbow and into a 3/8 inch diameter 3.5 inch long aluminum tube. This pipe was secured at one end by set screw to

slipring_elbow and at the other end to block (Appendix A). The 0.19 inch diameter tube was then passed through

the top of the slip ring through the hole in the bottom of slipring_elbow.

Fig-10 Slipring_elbow

Fig-9 Slip ring arm sketch

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The second block is slightly wider at the bottom than at the top to enable the pipe intersect the aluminum

box, Pamona 2400, perpendicularly. This box allows the wires to be connected to the DB-25F connector while

remaining shielded. The wires are then soldered to a DB-25F connector which is secured through the lid of the Newark box.

A flange had to be designed so as to allow the slip ring to rest on top of the cage. This Falange has a

1.25 inch diameter hole cut out in the center with four , 0.094 inch, screw holes to secure it to the slip ring and three, 0.190 inch screw holes to secure it to the cage

(Appendix A). The wires from the top of the slip ring are passed through a hole in the bottom of the

Hammond box, 1590LB, and are soldered to a DB-25F connector which is secured in the lid.

Conclusion

Although the Orbex slip ring needs some work to make it ready to use, it is the best

qualified replacement for the Crist slip ring which is currently being used in the lab. The Orbex with its arm will cost around $4,000 less per device, and although it will have 8 times higher

torque, the noise it will add to the experimental system is three orders of magnitude smaller than the Crist. This will lead to better experimental results overall.

Fig-11. block

Fig-12. flange

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Appendix A

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Appendix B

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Orbex slip ring

Operating noise:

Background noise:

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Control setup: Appendix B