This is the embedded chiller from an AMS Greenlight HPS surgical laser system, also known as a Laserscope. A small embedded chiller is contained within the bottom section of the frame. This chiller maintains the temperature of the water loop that cools the laser diode stack, diode driver, and q-switch crystal baseplate. All of these components generate a significant amount of heat during operation, which is most efficiently removed using water cooling. The chiller uses a small evaporator and fan to dump that heat into the ambient environment. Two heat exchangers are used to move heat from the laser's cooling water loop to the refrigerant, and then from the refrigerant to the ambient air. The chiller has a pre-programmed setpoint of around 70-72 degrees Fahrenheit for the water loop. It's not a deep cool chiller, and is only intended to maintain a suitable temperature for the sensitive components. The chiller is controlled and monitored by the Greenlight HPS rear panel board and digital control circuitry over RS-485. The laser systems turns on the chiller and constantly monitors its status, checks for alarms, and keeps an eye on the loop temperature. If the chiller malfunctions or the loop temperature falls too far out of range, the laser will shut down and display an error message related to the chiller. The laser will not enable if the chiller fails to start or is disconnected. Conveniently, the chiller can be easily removed from the Greenlight HPS frame after removing the electrical connections, quick disconnect water fittings, and two large bolts below the yellow fan shroud and filter cartridge.
The chiller weighs about as much as a medium-sized window air conditioner or portable A/C unit and can be removed and replaced as a module or serviced outside of the laser system. It's very compact and includes several critical components such as the compressor, evaporator coil, evaporator fan, water pump, sensors, water reservoir, and digital control board.
Components from left to right: Evaporator coil assembly and filter, evaporator fan (under yellow shroud), compressor and thermal expansion valve assembly, power supply and distribution, digital control board, water reservoir, and pump (under digital control board tray).
On this side, the water pump and VFD (variable frequency drive) are visible. The pump and VFD are manufactured by Fluid-o-Tech. The pump is a TMFR1 integrated pump-motor unit, and is paired with a TMFE1 VFD. The pump is a high-reliability, magnetically-driven unit where the motor has no moving parts. The chiller board controls the AC input power to the VFD but only has the ability to turn it on or off. The VFD does have the capability for external speed control and monitoring, but that is not implemented on this system. It just runs the pump at a constant, pre-programmed speed as defined by the DIP switches.
On the other side are multiple filters for the water loop which can be easily replaced using the CPC quick-disconnect fittings. Also visible is an inline flow switch which is constantly monitored by the control board. The controller will stop the pump if flow is not detected or if the reservoir level is too low, to prevent damage to the pump. Also installed are high and low pressure refrigerant sensors, which inform the controller of the status of the coolant loop. Buried deeper is a water temperature sensor as well. According to the Greenlight HPS/XPS service manual, this chiller has a solenoid valve (lower left corner) that can be activated in order to allow return water to flow through a resin, de-ionizing filter cartridge in order to keep the water resistivity around 0.25 megohm-cm. This is known as D.I. polishing. Presumably, this means there is also a sensor to determine the resistivity of the water, but we have not yet identified such a sensor.
In this section, the flow switch is visible in the lower left corner. Towards the top are the capped refrigerant fill and evacuation ports on the low and high side. A pressure sensor is also visible (orange and yellow wires). A refrigerant sight glass is provided to monitor refrigerant flow and also includes a chemical moisture indicator. The indicator in this chiller appears to be purple, which means caution based on the key on the sight glass. Perhaps the refrigerant circuit has accumulated a bit of moisture. Luckily, the chiller works fine and this has not been an issue as of yet.
This section includes some relays and a contactor responsible for switching the high-current components such as the compressor, fan, and pump. The relays are commanded by the digital controller. The transformer labeled CT1 is the control transformer that steps down the 240 VAC input to around 24V for use by the low-voltage control electronics. This is further rectified and reduced on the digital board.
This is the Lydall Chiller Board (REV 1.3 / 2005). This serves as the chiller's full-authority digital control and monitoring system. It's based around a PIC microcontroller and constantly monitors all inputs and sensors and subsequently controls the chiller via a series of relay outputs. Once powered, the chiller is started by the REMOTE input. Supplying around 7V DC equates to a chiller run request and will start the chiller and pump. It will continue to run as long as no faults are detected and power to the REMOTE input is maintained. The status of the chiller can be monitored over RS-485 by a proprietary protocol that we decoded (more information provided at the bottom of this page). Although we have not tested this, we presume the setpoint is also adjustable over the RS-485 interface. The chiller stores the setpoint and does not require any input over RS-485 to run. It will periodically cycle the compressor on and off as required to maintain the setpoint. The logic contained within the firmware of this board is capable of protecting the system from damage in the event of faults such as insufficient flow, loop temperature limit violations, low water reservoir level, and compressor overload. It will also protect the system from short-cycling of the compressor. When we first worked with this chiller, it powered on briefly and then shut down. It refused to run for any subsequent attempts. After checking the coolant reservoir and status LEDs, we determined that it was low on water. Adding water and then restarting it solved this problem immediately and allowed the chiller to run without issues.
This is a small additional board mounted to the side of the electronics tray. It's labeled "pump inrush board" and appears to serve that exact function. This helps prevent disruptions to the electronics and other components from the large inrush current when the pump starts.
We're not entirely sure of the purpose of this board, but it includes a relay, connections to the main chiller board, LM324 operational amplifier, voltage regulator, and the connection for the status LED. It may just be a simple status monitoring board. A bi-color led is connected to a 2-pin header on this board and can be used to quickly tell whether the system is powered on and if the compressor is currently running. Green indicates compressor not running, but loop temperature within range. Red indicates loop temperature not within range.
The electronics tray can be removed from the top of the chiller after undoing four nuts with lock washers, two of which are directly beside the control transformer. The usage of electrical connectors allows the chiller electronics to be swapped out without having to disassemble the entire unit.
Here are the majority of the electrical connections from the electronics and power distribution tray that lead to all the individual sensors and controls. The drain line (blue hose with silver CPC fitting) is also visible. The usage of quick-disconnect fittings and electrical plugs makes this a very serviceable unit.
Here is a view of the top of the chiller with the control electronics tray removed and placed off to the side. The water reservoir is visible on the left with a green host coming from the top. This is for the external fill port. To the right of the reservoir is the pump and VFD. Common chiller components such as the compressor and coolant lines are visible above the reservoir and pump. Specific sections of the copper coolant lines are wrapped in a sticky insulating material to prevent condensation buildup.
This side view of the Fluid-O-Tech TMFE1 VFD shows the fins for heat dissipation and DIP switches for configuration. As presently configured, this VFD will spin the pump at around 1750-2000 RPM without any external control or monitoring. We are basing this off the TMFR2/TMFE2 manual as we were unable to locate the exact manual for this pump/driver combination. Assuming the DIP switch configuration is the same across models, it should be around 1750-2000 RPM.
Communicating with this chiller was a learning experience. Luckily, the system designed to control and monitor it (AMS Greenlight HPS) was in working condition, so we captured and analyzed the RS-485 communication between the laser control system and the chiller control board. From startup to shutdown, the laser polls the chiller board but does not attempt to write/adjust any parameters. It only reads a variety of status registers. The chiller board is a slave and will not transmit any data unless polled by a master. We wrote a Python program that monitors the chiller over RS-485 using an RS-485 to USB adapter. The program can operate in passive mode to listen in on the conversation between the laser controller and the chiller board. The program can also operate in active mode to poll a standalone chiller being operated outside of the context of the original laser system. We don't recommend polling the chiller while the laser controller is also polling it, as this could cause issues.
The serial parameters are: 38400 baud, 8 data bits, no parity, 1 stop bit. Half-duplex RS-485. Below is the poll/reply frame syntax. Every byte in the frame, including the checksum, sums to 0x00 mod 256. To validate any received frame, sum all bytes. The low byte must be 00.
- poll: 40 42 30 [len] [ASCII COMMAND] [checksum]
- reply: 06 [len] [big-endian data] [checksum]
Below is an abbreviated list of commands with the checksums included. These are register requests, and do not change any chiller parameters or attempt to control the device. These can be sent directly to the Lydall chiller over RS-485 to verify functionality and ensure proper communication.
| Hex Data (send) | Register | Response |
|---|---|---|
| 40 42 30 02 43 30 D9 | C0 | Status bitfield — run / compressor / SPARE / lockout |
| 40 42 30 02 42 31 D9 | B1 | Status / fault word (0 = healthy) |
| 40 42 30 02 41 30 DB | A0 | Config / limit constant (unknown) |
| 40 42 30 02 48 30 D4 | H0 | Temperature setpoint (raw counts) |
| 40 42 30 02 48 31 D3 | H1 | Process temperature (raw counts, live PV) |
| 40 42 30 02 48 32 D2 | H2 | Live analog data (unknown) |
| 40 42 30 02 48 33 D1 | H3 | Live analog data (unknown) |
| 40 42 30 01 49 04 | I | 16-byte device ID / config block (static) |
| 40 42 30 01 5A F3 | Z | Scan-boundary keepalive |
The chiller board has a variety of green status LEDs that are somewhat useful for monitoring the chiller manually. We say somewhat because the silkscreen labels are ambiguous for some of the LEDs. Monitoring the LED behavior as the chiller starts up, runs, and cycles, does provide useful insight, but still leaves some unanswered questions. Here is a table of the LEDs along with a description of the purpose based on our observations.
A table of LED descriptions has been provided below. We made our best effort to determine the purpose of each LED, despite the ambiguity.
| LED Silkscreen Marking | I/O | Description |
|---|---|---|
| +24V | - | 24V power OK |
| VCC | - | Processor low-voltage DC power OK |
| HB | - | Processor heartbeat (1 Hz) |
| SSR | O | Solid state relay status (function unknown, periodically activates during operation) |
| TEMP (D19) | O | Potentially temperature alarm, never active during normal operation |
| MAIN | O | Main chiller electronics status (fan, etc. always illuminated while chiller is running) |
| REMOTE | I | Remote control input status (illuminated when remote control is active) |
| FLOW (D10) | O | Flow sensor status (ambiguous, periodically cycles on and off, potentially related to SPARE) |
| LEVEL 1 | O | Ambiguous (illuminated when compressor is running, off when setpoint reached) |
| TEMP (D2) | O | Unknown |
| SPARE | I | SPARE input status, laser controller periodically drives the input (ambiguous, potentially related to FLOW relay and LED) |
| SET | I | Never active during normal operation, potentially used to enable changing setpoint |
| RUN | I | Unknown, never active during normal operation |
| OVLOAD | I | Overload, illuminated = normal |
| PHASE | I | Phase-change compressor cycle, illuminated = compressor running |
| LO PRES | I | Low pressure switch status, illuminated = normal |
| HI PRES | I | High pressure switch status, illuminated = normal |
| LEVEL | I | Level sensor, illuminated during normal operation |
| FLOW | I | Flow sensor/switch status, illuminated during normal operation |
Below are some useful resources for learning about chillers in general. There is no public documentation for this model of chiller so we included as much relevant information as possible, but most applies to different variations/models. Lastly, the Python program for monitoring this chiller over RS-485 can be downloaded at the link below.
- Lydall Live Monitor Python Program
- Laserscope Greenlight XPS Basic Service Manual
- LG - How Does a (HVAC) Chiller Work
- Applied Membranes - Some information about D.I. polishing filters
- Fluid-O-Tech Manual for a similar, but different pump and driver combination (TMFE2)
- Lydall Affinity Custom Chiller User Manual (different model)